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MP3: The Definitive Guide

MP3: The Definitive Guide

By Scot Hacker
1st Edition March 2000
1-56592-661-7, Order Number: 6617
400 pages, $29.95

Sample Chapter 2:
How MP3 Works: Inside the Codec

In this chapter:

So what's the trick? How does the MP3 format accomplish its radical feats of compression and decompression, while still managing to maintain an acceptable level of fidelity to the original source material? The process may seem like magic, but it isn't. The entire MP3 phenomenon is made possible by the confluence of several distinct but interrelated elements: A few simple insights into the nature of human psychoacoustics, a whole lot of number crunching, and conformance to a tightly specified format for encoding and decoding audio into compact bitstreams. In this chapter, we'll take a look at these elements in detail in order to understand exactly what's going on behind the scenes of MP3 encoding and decoding software, as well as some of the chicanery that takes place between your ears.

Note that this chapter goes fairly deeply behind the scenes of MP3, and is somewhat technical in nature. You can skip this chapter if you're not interested in learning how MP3 works. If you just want to get started creating and playing MP3 audio, you can skip ahead to Chapters 3, 4, and 5.

A "Perceptual" Codec

Well-encoded MP3 files can sound pretty darn good, considering how small they are. As mentioned in Chapter 1, The Nuts and Bolts of MP3, your typical MP3 file is around one-tenth the size of the corresponding uncompressed audio source. How is this accomplished? That's a somewhat complex topic, so we've devoted this entire chapter to explaining the process.

MPEG Audio Compression in a Nutshell

Uncompressed audio, such as that found on CDs, stores more data than your brain can actually process. For example, if two notes are very similar and very close together, your brain may perceive only one of them. If two sounds are very different but one is much louder than the other, your brain may never perceive the quieter signal. And of course your ears are more sensitive to some frequencies than others. The study of these auditory phenomena is called psychoacoustics, and quite a lot is known about the process; so much so that it can be quite accurately described in tables and charts, and in mathematical models representing human hearing patterns.

MP3 encoding tools (see Chapter 5, Ripping and Encoding: Creating MP3 Files, for examples and usage details) analyze incoming source signal, break it down into mathematical patterns, and compare these patterns to psychoacoustic models stored in the encoder itself. The encoder can then discard most of the data that doesn't match the stored models, keeping that which does. The person doing the encoding can specify how many bits should be allotted to storing each second of music, which in effect sets a " tolerance" level-the lower the data storage allotment, the more data will be discarded, and the worse the resulting music will sound. The process is actually quite a bit more complex than that, and we'll go into more detail later on. This kind of compression is called [_Fi_] lossy, because data is lost in the process. However, a second compression run is also made, which shrinks the remaining data even more via more traditional means (similar to the familiar "zip" compression process).

MP3 files are composed of a series of very short frames, one after another, much like a filmstrip. Each frame of data is preceded by a header that contains extra information about the data to come. In some encodings, these frames may interact with one another. For example, if one frame has leftover storage space and the next frame doesn't have enough, they may team up for optimal results.

At the beginning or end of an MP3 file, extra information about the file itself, such as the name of the artist, the track title, the name of the album from which the track came, the recording year, genre, and personal comments may be stored. This is called " ID3" data, and will become increasingly useful as your collection grows. We'll look at the structure of MP3 files and their ID3 tags in this chapter, and the process of creating and using ID3 tags in Chapter 4, Playlists, Tags, and Skins: MP3 Options. Let's zoom in for a closer look at the entire process.

NOTE

Always remember to set your encoder to store ID3 data during the encode process, if possible-doing so will save you a lot of work down the road.

Waveforms and Psychoacoustics

Everything is vibration. The universe is made of waves, and all waves oscillate at different lengths (a wavelength is defined as the distance between the peak of one wave and the peak of the next). Waves vibrating at different frequencies manifest themselves differently, all the way from the astronomically slow pulsations of the universe itself to the inconceivably fast vibration of matter (and beyond). Somewhere in between these extremes are wavelengths that are perceptible to human beings as light and sound. Just beyond the realms of light and sound are sub- and ultrasonic vibration, the infrared and ultraviolet light spectra, and zillions of other frequencies imperceptible to humans (such as radio and microwave). Our sense organs are tuned only to very narrow bandwidths of vibration in the overall picture. In fact, even our own musical instruments create many vibrational frequencies that are imperceptible to our ears. Frequencies are typically described in units called Hertz (Hz), which translates simply as "cycles per second." In general, humans cannot hear frequencies below 20Hz (20 cycles per second), nor above 20kHz (20,000 cycles per second), as shown in Figure 2-1.[1] While hearing capacities vary from one individual to the next, it's generally true that humans perceive midrange frequencies more strongly than high and low frequencies,[2] and that sensitivity to higher frequencies diminishes with age and prolonged exposure to loud volumes. In fact, by the time we're adults, most of us can't hear much of anything above 16kHz (although women tend to preserve the ability to hear higher frequencies later into life than do men). The most sensitive range of hearing for most people hovers between 2kHz to 4kHz, a level probably evolutionarily related to the normal range of the human voice, which runs roughly from 500Hz to 2kHz.

Figure 2-1: While vibratory frequencies extend both above and below, human hearing is pretty much limited to the range between 20Hz and 20kHz

These are simple and well-established empirical observations on the human hearing mechanism. However, there's a second piece to this puzzle, which involves the mind itself. Some have postulated[3] that the sane mind functions as a sort of "reducing valve," systematically bringing important information to the fore and sublimating or ignoring superfluous or irrelevant data.[4] In fact, it's been estimated that we really only process a billionth of the data available to our five senses at any given time. Clearly, one of the most important functions of the mind is to function as a sieve, sifting the most important information out of the incoming signal, leaving the conscious self to focus on the stuff that matters.

The basic principle of any perceptual codec is that there's little point in storing information that can't be perceived by humans anyway. As obvious as this may sound, you may be surprised to learn that a good recording stores a tremendous amount of audio data that you never even hear, because recording equipment (microphones, guitar pickups, and so on) is sensitive to a broader range of sounds and audio resolutions than is the human ear. After getting an overview of how perceptual codecs work in general, we'll take a closer look at exactly how the MP3 codec does its thing.

NOTE

The word " codec" is a foreshortening of the words "compress" and "decompress," and refers to any of a class of processes that allow for the systematic compression and decompression of data. While various codecs are fundamental to many file formats and transmission methods (for instance image and video compression formats have their own codecs, some of which are perceptual as well), it's the MP3 codec that concerns us here.

Breaking It Down

MP3 uses two compression techniques to achieve its size reduction ratios over uncompressed audio-one lossy and one lossless. First it throws away what humans can't hear anyway (or at least it makes acceptable compromises), and then it encodes the redundancies to achieve further compression. However, it's the first part of the process that does most of the grunt work, requires most of the complexity, and chiefly concerns us here.

Perceptual codecs are highly complex beasts, and all of them work a little differently. However, the general principles of perceptual coding remain the same from one codec to the next. In brief, the MP3 encoding process can be subdivided into a handful of discrete tasks (not necessarily in this order):

Along the way, many other factors enter into the equation, often as the result of options chosen prior to beginning the encoding (more on those in Chapter 5). In addition, algorithms for the encoding of an individual frame often rely on the results of an encoding for the frames that precede or follow it. The entire process usually includes some degree of simultaneity; the preceding steps are not necessarily run in order. We'll take a deeper look at much of this process in the sections that follow.

Notes on "Lossiness"

Compression formats, whether they operate on audio, video, images, or random collections of files, are either lossless or lossy. The distinction is simple: Lossless formats are identical to the original(s) after being decompressed, while lossy formats are not. A good example of a lossless compression format is the ubiquitous . zip archiving scheme. When you unpack a zip archive containing a backup of your system from last month, losing even a single byte is unacceptable. However, some types of data can withstand having information thrown away, on the grounds that either you'll never notice what's missing, or you're willing to make a compromise: Smaller files in exchange for missing but unimportant data.

A good example of a lossy compression format is JPEG, which banks on the fact that image files often store more information than necessary to display an image of acceptable quality. By throwing away some of the information, and by encoding redundant information with mathematical algorithms, excellent compression ratios can be achieved for images that don't need to be displayed at high resolutions.

While the JPEG analogy doesn't depict the MP3 compression process accurately, it does illustrate the concept of lossiness, and it's important to understand that all MP3 files, no matter how well-encoded, have discarded some of the information that was stored in the original, uncompressed signal.

Many lossy compression formats work by scanning for redundant data and reducing it to a mathematical depiction which can be "unpacked" later on. Think for a moment of a photograph depicting a clear blue sky, and below it a beach. If you were to scan and store this image on your hard drive, you could end up storing hundreds of thousands of pixels of perfect blue, all identical to one another, and therefore redundant. The secret of a photographic compression method like GIF is that this redundant information is reduced to a single description. Rather than store all the bits individually, they may be represented as the mathematical equivalent of "repeat blue pixel 273,000 times." When the part of the image depicting the sand is encountered, the sand is analyzed for redundancy and similar reductions can be achieved. This is why simple images can be stored as small files, while complex images don't compress as well-they contain less redundancy. On the other hand, JPEG compression works in accord with user-defined "tolerance thresholds"; determining how similar two adjacent pixels (or, more accurately, frequencies) have to be before they're considered redundant with one another is the key to determining the degree of lossiness. If JPEG compression is set high, light blue and medium blue pixels may be treated as being redundant with one another. If JPEG compression is set low, the codec will be more fussy about determining which pixels are redundant. The end result will be a clearer picture and a larger image file.

Masking Effects

Part of the process of mental filtering, described earlier in this chapter, which occurs unconsciously at every moment for all of us, involves a process called masking, and is of much interest to students of psychoacoustics: the study of the interrelation between the ear, the mind, and vibratory audio signal. Two separate masking effects come into play in MP3 encoding: auditory and temporal.

Simultaneous (auditory) masking

The simultaneous masking effect (sometimes referred to as "auditory masking") may be best described by analogy. Think of a bird flying in front of the sun. You see the bird flying in from the left, then it seems to disappear, because the sun's light is so strong in contrast. As it moves past the sun to the right, it becomes visible again. In more concrete audio terms, recall how you can sometimes hear an acoustic guitarist's fingers sliding over the ridged spirals of the guitar strings during quiet passages. Of course, you seldom if ever hear this effect during a full-on rock anthem, because the wall of sound surrounding the guitar all but completely drowns these subtle effects.

The MP3 codec, of course, is unconcerned with guitar strings; all it knows are relative frequencies and volume levels. So, to put simultaneous masking into more concrete terms, let's say you have an audio signal consisting of a perfect sine wave fluctuating at 1,000Hz. Now you introduce a second perfect sine wave, this one fluctuating at a pitch just slightly higher-let's make it 1,100Hz-but also much quieter-say, -10 db.[6] Most humans will not be able to detect the second pitch at all. However, the reason the second pitch is inaudible is not just because it's quieter; it's because its frequency is very close (similar) to that of the first. To illustrate this fact, we'll slowly change the frequency (pitch) of the second tone until it's fluctuating at, say, 4,000Hz. However, we'll leave its volume exactly as it was, at -10db. As the second pitch becomes more dissimilar from the first, it becomes more audible, until at a certain point, most humans will hear two distinct tones, one louder than the other, as illustrated in Figure 2-2. At Point A, Tone 2 is barely audible next to Tone 1. At Point B, Tone 2 is quite audible, even though its volume remains unchanged.

Figure 2-2: As two simultaneous tones become more dissimilar, they become recognizable as separate entities

What's going on here is a psychoacoustic phenomenon called "simultaneous masking," which demonstrates an important aspect of the mind's role in hearing: Any time frequencies are close to one another, we have difficulty perceiving them as unique, much as mountains on the distant horizon may appear to be evenly textured and similarly colored, even while the same mountains might be full of variation and rich flora if one were hiking in them. In effect, we have the aural equivalent of an optical illusion-a trick of our perceptual capacity that contributes to our brain's ability to filter out the less relevant and give focus to stronger elements.

Now consider for a moment the fact that an audio signal consisting of two sine waves-even if one is quieter-contains almost twice as much data as a signal containing a single wave. If you were to try and compress an audio signal containing two sine waves, you would want the ability to devote less disk storage space to the nearly inaudible signal, and more to the dominant signal. And, of course, this is precisely what the algorithms behind most audio compression formats do-they exploit certain aspects of human psychoacoustic phenomena to allocate storage space intelligently. Whereas a raw (waveform or PCM[7]) audio storage format will use just as much disk space to store a texturally constant passage in a symphonic work as it will for a dynamically textured one, an MP3 file will not. Thus, MP3 and similar audio compression formats are called "perceptual codecs" because they are, in a sense, mathematical descriptions of the limitations of human auditory perception. The MP3 codec is based on perceptual principles but also encapsulates many other factors, such as the number of bits per second allocated to storing the data and the number of channels being stored, i.e., mono, stereo, or in the case of other formats such as AAC or MP3 with MPEG-2 extensions, multi-channel audio.

Temporal masking

In addition to auditory masking, which is dependent on the relationship between frequencies and their relative volumes, there's a second sort of masking which also comes into play, based on time rather than on frequency. The idea behind temporal masking is that humans also have trouble hearing distinct sounds that are close to one another in time. For example, if a loud sound and a quiet sound are played simultaneously, you won't be able to hear the quiet sound. If, however, there is sufficient delay between the two sounds, you will hear the second, quieter sound. The key to the success of temporal masking is in determining (quantifying) the length of time between the two tones at which the second tone becomes audible, i.e., significant enough to keep it in the bitstream rather than throwing it away. This distance, or threshold, turns out to be around five milliseconds when working with pure tones, though it varies up and down in accordance with different audio passages.

Of course, this process also works in reverse-you may not hear a quiet tone if it comes directly before a louder one, so premasking and postmasking both occur, and are accounted for in the algorithm.

NOTE

For more information on psychoacoustics, read any of the excellent papers on the subject at www.cpl.umn.edu/auditory.htm.

Enter Bitrates, Stage Left

While MP3 users cannot control the degree of lossiness specifically, as they might do with a JPEG image, they can control the number of bits per second to be devoted to data storage, which has a similar net result.

In the process of coding, the "irrelevant" portions of the signal are mapped against two factors: a mathematical model of human psychoacoustics (i.e., the masking requirements), and the bitrate, which is established at the time of encoding (see Chapter 5). The bitrate simply refers to the number of bits per second that should be devoted to storing the final product-the higher the bitrate, the greater the audio resolution of the final product, as shown in Figure 2-3. An easy way to visualize the effect of bitrate on audio quality is to think of an old, turn-of-the-century film. Old movies appear herky-jerky to us because fewer frames per second are being displayed,[8] which means less data is distributed over a given time frame.

Figure 2-3: More bits per second means more audio resolution, pure and simple

For example, the current de facto standard is to encode MP3 at 128 kbps, or 128,000 bits per second. The codec takes the bitrate into consideration as it writes each frame to the bitstream. If the bitrate is low, the irrelevancy and redundancy criteria will be measured harshly, and more subtlety will be stripped out, resulting in a lower-quality product. If the bitrate is high, the codec will be applied with leniency, and the end result will sound better. Of course, the file size of the end product corresponds directly with the bitrate: If you want small files, you have to settle for less quality. If you don't mind larger files, you can go for higher bitrates.

NOTE

Bitrates refer to the total rate for all encoded channels. In other words, a 128 kbps stereo MP3 is equivalent in size and quality to two separate 64 kbps mono files. However, a 128 kbps stereo file will enjoy better quality than two separate 64 kbps mono files, since in a stereo file, bits will be allocated according to the complexity of the channels. In a given time, one channel may utilize 60% of the bits while the other uses only 40%. The cumulative size in bits will, however, remain constant.

CBR vs. VBR

Most of the information you'll read in this book and elsewhere assumes that the bitstream is being encoded at a constant bitrate (CBR). In other words, if you specify a 128 kbps encoding, then that's what you're going to get, start to finish. The drawback to CBR is that most music isn't structured with anything approaching a constant rate. Passages with many instruments or voices are succeeded by passages with few, simplicity follows complexity, and so on. The response to this situation has been the development of variable bitrate (VBR) encoders and decoders, which vary the bitrate in accordance with the dynamics of the signal flowing through each frame. VBR technology was first implemented by Xing, which is now owned by Real Networks, but is now supported by dozens, if not hundreds, of third-party products.

Rather than specifying a bitrate before encoding begins, the user specifies a threshold, or tolerance, when encoding with VBR. All notions of bits per second go right out the window, of course; instead, one selects VBR quality on a variable scale. Confusingly, this scale is represented differently in different encoders. While MusicMatch Jukebox gives you a scale of 1 to 100, the LAME command-line encoder lets you specify a quality of 0 to 9, where the scale represents a distortion ratio. Therefore, you can't just assume that higher numbers mean higher quality-see the documentation for your encoder before proceeding, or run the tests yourself. In any case, the scales are essentially arbitrary; think of them as though you were using a slider to control the overall quality versus file size ratio as you might with a JPEG editor.

While VBR files may achieve smaller file sizes than those encoded in CBR at a roughly equivalent fidelity, they present a number of drawbacks of their own. First, these files may not be playable in older-generation decoders, which had no notion of VBR concepts (although the ISO standard specifies that a player must handle VBR files if it's to be considered ISO-compliant). Second, VBR files may present timing difficulties for decoders. You may expect your MP3 player to display inaccurate timing readouts-or no timing information at all-when playing back VBR files. However, VBR techniques conveniently take some of the guess work out of trying to find an optimal bitrate for any given track-whereas you might have to encode a file several times with CBR to find the perfect balance, you can just set your encoder to use a relatively high quality level and let the computer figure out an optimal bitrate for each frame automatically.

NOTE

In general, the header data in most CBR files is same for each frame, while header data is necessarily different for each frame of a VBR file. However, VBR files don't incur more processing power, as all MP3 players read the header data for each frame regardless of whether they're playing a CBR or VBR file.

Bitrates vs. samplerates

Bitrates aren't quite the final arbiter of quality. The resolution of audio signal in general is in large part determined by the number of source samples per second stored in a given format. While bitrates are a measure of the amount of data stored for every second of audio, samplerates measure the frequency with which the signal is stored, and are measured in kiloHertz, or thousands of samples per second. The standard samplerate of CD audio is 44.1kHz, so this is the default samplerate used by most encoders, and found in most downloadable MP3 files. Audio professionals often work with 48kHz audio (and, more recently, 96kHz[9]). Digital audio storage of lectures and plain speech is sometimes recorded as low as 8kHz. Streamed MP3 audio is often sent out at half, or even a quarter of the CD rate in order to compensate for slow Internet connection speeds. If you need to minimize storage space, or are planning to run your own Internet radio station, and are willing to sacrifice some quality, you'll want to do some experimenting with various samplerates. More details can be found in Chapter 5.

NOTE

Note that nothing is ever actually played or heard during the encoding process-you can encode MP3 on a computer with no sound card or speakers, if you need to for some reason. In fact, this is exactly how things are done in some professional organizations, particularly those dedicated to Internet broadcasting (see Chapter 8, Webcasting and Servers: Internet Distribution). In such instances, one computer may be used for auditioning and selecting files, a second used for the actual encoding process, and a third dedicated to serving the files to the Internet. Of course, the beefiest machine available will always be used as the encoding machine in such a scenario.

Freedom of Implementation

Interestingly enough, the MP3 specification (ISO 11172-3) does not specify exactly how the encoding is to be accomplished. Rather, it outlines techniques and specifies a level of conformance; in other words, it tells developers that their resulting MP3 files must meet certain structural criteria.[10] This is necessary for the same reason that any standard exists: To allow for the proliferation of MP3 encoders and players by various vendors and developers. The specification only serves to guarantee a baseline consensus in the community regarding how certain things will operate. An encoder developed according to the MP3 specification will be capable of outputting a "compliant bitstream" that can be played successfully with any MP3-compliant decoder, just as you can create a JPEG file in any image editor under any operating system and expect it to display properly in any JPEG-compliant image viewer on any operating system.

It's important to maintain the distinction between the primary developers of the codec itself, The Fraunhofer Institute, and the committee that codified the work of Fraunhofer into the MPEG-I Layer 3 specification, the International Standards Organization (ISO). Standards are often created this way: A company produces a technology, other companies apply to become a part of the standards-creation process, and together they lay down the laws of implementation so that all vendors can compete around that technology. Note, however, that just because MP3 has been standardized by ISO does not mean that Fraunhofer (and their partners Thomson Multimedia) don't still hold the patent on the technology itself. As you'll see in Chapter 7, The Not-So-Fine-Print: Legal Bits and Pieces, Fraunhofer's patent is being aggressively exercised, making it difficult for small-time developers to affordably implement the ISO standard.

In any case, while the standard specifies exactly how decoding is to be accomplished, it only provides sample implementations (one simple and one complex) for encoding. As a result, there's a certain degree of headroom available for developers to make up some of the rules as they go along. In general, encoder developers work toward two goals: speed and quality. While there is some difference in the quality of audio files output by various encoders (as you'll see in Chapter 5), there are vast differences in the speed at which encoders operate. Sometimes, encoding speed comes at a distinct disadvantage to the quality of the resulting bitstream, though this is not necessarily the case.

A good example of the kind of freedom left to developers is the fact that the MP3 standard does not specify exactly how to treat the upper end of the spectrum, above 16kHz. Since human auditory perception begins to diminish greatly (with age and exposure to loud volumes) between 16kHz and 20kHz, some developers have historically chosen to simply chop off frequencies above 16kHz, which can be beneficial at low bitrates, since it leaves more bits available for encoding more audible frequencies. Xing, for example, did this with the first versions of their very fast codec. Later, they rewrote their codec to handle frequencies up to 20kHz (probably at the behest of the audiophile MP3 community).

NOTE

If you're curious about the upper and lower thresholds of your own hearing, download a sine wave generation program for your platform and run some tests. If you find a graphical program, you can simply turn the dial or drag the slider up the frequency spectrum until you can no longer hear it. If the program works from the command line, you can either generate sweep frequencies or generate a series of files at different frequencies at the upper end of the range and play them in sequence. BeOS and Linux users should check out a utility called sinus, while users of any platform can generate pure tones through any of the many simple synthesizer programs available at your favorite software library. The potential problem with running this kind of test lies in the fact that your playback hardware may itself not be capable of reproducing frequencies above, say, 17kHz. A test like this is best conducted on the highest quality equipment you can find.

Other Considerations

In addition to the general principles outlined in this chapter, the MP3 codec does a lot of additional work maintaining frequency tables, storing and allocating bits optimally, handling user options set at encode time, and the like. While we don't cover everything the encoder is responsible for exhaustively, here are a few of the more important additional chores the encoder must tackle.

Dipping into the reservoir

Because the bitrate is taken into consideration at every time frame, there will inevitably be certain frames of such complexity that they cannot be adequately coded to adhere to the limitations imposed by the chosen bitrate. In such a case, the MP3 spec allows for a "reservoir of bytes," which acts as a sort of overflow buffer when the desired amount of data cannot be stored in the given timeframe. In actual practice, this reservoir is not a separate storage space in the file, but rather the "empty space" left over in frames where the necessary information was encoded into the available space with room to spare. In other words, the byte reservoir is a portion of the algorithm designed to rob Peter and pay Paul.

While the CD and DAT audio formats typically offer 16 bits of resolution, the processing of a very complex musical passage may result in only four or six bits of resolution being encoded into the final bitstream,[11] since there isn't enough storage space allocated to handle the data needs of each frame. What can't be drawn from the reservoir will simply result in an audible degradation of the signal quality. Thus, the byte reservoir is only a partial solution to the loss of signal quality in complex passages. The only real solution to quality loss is to encode the signal at a higher bitrate.

The joint stereo effect

Most people have had an opportunity at some point to listen to a stereo system with a separate subwoofer attached (in fact, most better-quality computer speaker systems consist of two or four satellite speakers and a separate subwoofer). And as you may have noticed, the placement of satellite speakers is critical to high-quality audio reproduction, whereas the placement of the subwoofer is almost entirely irrelevant-people stuff subwoofers under desks, behind couches, or integrate them with other pieces of living room furniture. The reason it's possible to do this without affecting sound quality is because the human ear is largely insensitive to the location of the source of sounds at the very low and very high ends of the frequency spectrum.

The MP3 spec optionally exploits this aspect of human psychoacoustics as well. A file being encoded in stereo is by definition twice as large as a monophonic file. However, this file size doubling effect can be somewhat mitigated by combining high frequencies across the left and right tracks into a single track. This is done during the encoding phase by selecting the "joint stereo" option in the encoder's preferences, or by passing an appropriate command-line option to the encoder (there are actually several subtle differences between the various joint stereo encoding "modes"-more on that in Chapter 5). Since you might not be able to tell which speaker very high signals are emanating from anyway, there may be no point in storing that data twice.

NOTE

Some hard-core audiophile tweaks claim that bass sounds are not entirely nondirectional, only that they're less so than mid- and high-frequency sounds. Listeners with ears trained this well are probably not much interested in MP3 to begin with, but those listeners might be able to tell the difference in high-frequency spatialization when comparing MP3 to unencoded audio.

When the joint stereo option is enabled, a certain amount of "steering information" is added to the file so that these sounds can be placed spatially with some approximation of accuracy during playback. This becomes especially important at the upper edge of the bass spectrum, where the ear becomes more sensitive to the spatial location of bass signals. Joint stereo (in "Intensity" mode) really is a low-fi solution best reserved for situations where you need to keep file size at an absolute minimum.

WARNING

The joint stereo option can in some instances introduce audible compression artifacts which can't be removed by increasing the bitrate. The only way to find out whether this is a problem for you is to experiment. If you don't like the results, re-encode without joint stereo enabled. Remember: Your ears don't lie.


Side Information

If joint stereo is used in M/S (middle/side) mode, the left and right channels aren't encoded separately. Instead, a "middle" channel is encoded as the sum of the left and right channels, while a " side" channel is stored as the difference between the left and the right. During the decoding process, side information is read back out of the frame and applied to the bitstream so that the original signal can be reconstructed as accurately as possible. The side information is essentially a set of instructions on how the whole puzzle should be re-assembled on the other end.


Who Defines "Imperceptible?"

Before moving away from the topic of perceptual codecs, there's an important point to be made about the category as a whole: They all make baseline assumptions about the limitations of human perception, and about how closely the end result will be listened to. The fact of the matter is that all that stuff being stripped out adds up to something. While no recording format, whether it be vinyl, reel-to-reel, compact disk, or wax cylinder, can capture all of the overtones and subtle nuances of a live performance, nor can any playback equipment on the face of the earth reproduce the quality of a live performance. All compression formats-especially perceptual codecs-are capable of robbing the signal of subtleties. While certain frequencies may not be distinctly perceptible, their cumulative effect contributes to the overall "presence" and ambience of recorded music. Once a signal has been encoded, some of the "magic" of the original signal has been stripped away, and cannot be retrieved no matter how hard you listen or how good your playback equipment. As a result, MP3 files are sometimes described as sounding "hollow" in comparison to their uncompressed cousins. Of course, the higher the quality of the encoding, the less magic lost. You have to strike your own compromises.

Many feel that the current digital audio standard offers less resolution than the best analog recording, which is why many audiophile purists still swear by vinyl LPs. Digital audio introduced a host of distortions never before encountered with analog, but hasn't had analog's 50+ years of research and development to eradicate them. Compressing and further modifying "CD quality" audio with a lossy perceptual codec like MP3, some might say, adds insult to injury.

But then there's reality, and the reality right now is that the vast majority of us do not listen to music with the trained ears of a true audiophile, nor do most of us possess magnificent playback equipment. Most of us use middle-ground sound cards and PC speakers, most of us have limits to the amount of data we can store conveniently, and most of us connect to the Internet with relatively low-bandwidth modems. Reality dictates that we make compromises. Fortunately, the reality of our sound cards and speakers, the quality of which lags far behind the quality of decent home audio systems, also means that most of these compromises won't be perceived most of the time.

The bottom line is that the perceptual codec represents a "good enough" opportunity for us to have our cake and eat it too. As things stand now, it all comes down to a matter of file size if we want to store and transfer audio files with anything approaching a level of convenience. In a perfect world, we would all have unlimited storage and unlimited bandwidth. In such a world, the MP3 format may never have come to exist-it would have had no reason to. If necessity is the mother of invention, the invention would never have happened. Compression techniques and the perceptual codec represent a compromise we can live with until storage and bandwidth limitations vanish for good.

The Huffman Coding

At the end of the perceptual coding process, a second compression process is run. However, this second round is not a perceptual coding, but rather a more traditional compression of all the bits in the file, taken together as a whole. To use a loose analogy, you might think of this second run, called the " Huffman coding," as being similar to zip or other standard compression mechanisms (in other words, the Huffman run is completely lossless, unlike the perceptual coding techniques). Huffman coding is extremely fast, as it utilizes a look-up table for spotting possible bit substitutions. In other words, it doesn't have to "figure anything out" in order to do its job.

The chief benefit of the Huffman compression run is that it compensates for those areas where the perceptual masking is less efficient. For example, a passage of music that contains many sounds happening at once (i.e., a " polyphonous" passage) will benefit greatly from the masking filter. However, a musical phrase consisting only of a single, sustained note will not. However, this passage can be compressed very efficiently with more traditional means, due to its high level of redundancy. On average, an additional 20% of the total file size can be shaved during the Huffman coding.

Raw Power

If you've surmised from all of this that encoding and decoding MP3 must require a lot of CPU cycles, you're right. In fact, unless you're into raytracing or encryption cracking, encoding MP3 is one of the few things an average computer user can do on a regular basis that consumes all of the horsepower you can throw at it. Note, however, that the encoding process is far more intensive than decoding (playing). Since you're likely to be decoding much more frequently than you will be encoding, this is intentional, and is in fact one of the design precepts of the MP3 system (and even more so of next-generation formats such as AAC and VQF).

Creating an MP3 file, as previously described, is a hugely complex task, taking many disparate factors into consideration. The task is one of pure, intensive mathematics. While the computer industry is notorious for hawking more processing power to consumers than they really need, this is one area where you will definitely benefit from the fastest CPU (or CPUs) you can get your hands on, if you plan to do a lot of encoding.

It's impossible to recommend any particular processor speed, for several reasons:

In any case, it's easy enough to set up batch jobs with most encoders, so you can always let 'er rip while you go out to lunch, or even overnight. Unless you're really stuck with an old clunker of a machine (a CPU manufactured prior to 1996, for example) and your needs aren't intensive, don't even think about running out to get a new computer just to pump up your encoding speed. You'll be better off making sure you have an adequate complement of RAM, a fast and accurate DAE-capable[12] CD-ROM drive, a good sound card, and that you're using the most efficient encoder available for your platform (see Chapter 5).

Notes on Decoding

As noted earlier, the great bulk of the work in the MP3 system as a whole is placed on the encoding process. Since one typically plays files more frequently than one encodes them, this makes sense. Decoders do not need to store or work with a model of human psychoacoustic principles, nor do they require a bit allocation procedure. All the MP3 player has to worry about is examining the bitstream of header and data frames for spectral components and the side information stored alongside them, and then reconstructing this information to create an audio signal. The player is nothing but an (often) fancy interface onto your collection of MP3 files and playlists and your sound card, encapsulating the relatively straightforward rules of decoding the MP3 bitstream format.

While there are measurable differences in the efficiency-and audible differences in the quality-of various MP3 decoders, the differences are largely negligible on computer hardware manufactured in the last few years. That's not to say that decoders just sit in the background consuming no resources. In fact, on some machines and some operating systems you'll notice a slight (or even pronounced) sluggishness in other operations while your player is running. This is particularly true on operating systems that don't feature a finely grained threading model, such as MacOS and most versions of Windows. Linux and, to an even greater extent, BeOS are largely exempt from MP3 skipping problems, given decent hardware. And of course, if you're listening to MP3 audio streamed over the Internet, you'll get skipping problems if you don't have enough bandwidth to handle the bitrate/sampling frequency of the stream.

Some MP3 decoders chew up more CPU time than others, but the differences between them in terms of efficiency are not as great as the differences between their feature sets, or between the efficiency of various encoders. Choosing an MP3 player becomes a question of cost, extensibility, audio quality, and appearance. That's still a lot to consider, but at least you don't have to worry much about benchmarking the hundreds of players available on the market (unless you've got a really slow machine).

NOTE

If you're a stickler for audio quality, you've probably got a decent to excellent sound card already. However, if you've got an older sound card (such as a SoundBlaster 16) and a slower CPU (slower than a Pentium 133), be aware that the " look ahead" buffer in the MP3 player can easily become exhausted, which will result in an audible degradation of sound quality. However, sticking a better sound card (such as a SoundBlaster 64) in the same machine may eliminate these artifacts, since better sound cards perform more of the critical math in their own hardware, rather than burdening the computer's CPU with it.

While this situation won't affect many modern geeks, there's an easy way to test your equipment to determine if its lack of speed is affecting audio quality: Just pick a favorite MP3 file and decode it to a noncompressed format such as WAV, then listen to the MP3 and the WAV side-by-side. If the WAV version sounds better, you'll know that your machine isn't up to the MP3 playback task, since the uncompressed version requires very little processing power to play .

The Anatomy of an MP3 File

Aside from being familiar with the basic options available to the MP3 encoder, the typical user doesn't need to know how MP3 files are structured internally any more than she needs to know how JPEG images or Word documents are structured behind the scenes. For the morbidly curious, however, here's an x-ray view of the MP3 file format.

Inside the Header Frame

As mentioned earlier, MP3 files are segmented into zillions of frames, each containing a fraction of a second's worth of audio data, ready to be reconstructed by the decoder. Inserted at the beginning of every data frame is a "header frame," which stores 32 bits of meta-data related to the coming data frame (Figure 2-4). As illustrated in Figure 2-5,[13] the MP3 header begins with a " sync" block, consisting of 11 bits. The sync block allows players to search for and "lock onto" the first available occurrence of a valid frame, which is useful in MP3 broadcasting, for moving around quickly from one part of a track to another, and for skipping ID3 or other data that may be living at the start of the file. However, note that it's not enough for a player to simply find the sync block in any binary file and assume that it's a valid MP3 file, since the same pattern of 11 bits could theoretically be found in any random binary file. Thus, it's also necessary for the decoder to check for the validity of other header data as well, or for multiple valid frames in a row. Table 2-1 lists the total 32 bits of header data that are spread over 13 header positions.


Locking onto the Data Stream

One of the original design goals of MP3 was that it would be suitable for broadcasting. As a result, it becomes important that MP3 receivers be able to lock onto the signal at any point in the stream. This is one of the big reasons why a header frame is placed prior to each data frame, so that a receiver tuning in at any point in the broadcast can search for sync data and start playing almost immediately. Interestingly, this fact theoretically makes it possible to cut MPEG files into smaller pieces and play the pieces individually. However, this unfortunately is not possible with Layer III files (MP3) due to the fact that frames often depend on data contained in other frames (see "Dipping into the reservoir," earlier). Thus, you can't just open any old MP3 file in your favorite audio editor for editing or tweaking.


Figure 2-4: Data describing the structural factors of that frame; this data is called the frame's "header"

Figure 2-5: The MP3 frame header represented visually

Table 2-1: The Thirteen Header Files' Characteristics

Position Purpose Length (in Bits)
A Frame sync 11
B MPEG audio version (MPEG-1, 2, etc.) 2
C MPEG layer (Layer I, II, III, etc.) 2
D Protection (if on, then checksum follows header) 1
E Bitrate index (lookup table used to specify bitrate for this MPEG version and layer) 4
F Sampling rate frequency (44.1kHz, etc., determined by lookup table) 2
G Padding bit (on or off, compensates for unfilled frames) 1
H Private bit (on or off, allows for application-specific triggers) 1
I Channel mode (stereo, joint stereo, dual channel, single channel) 2
J Mode extension (used only with joint stereo, to conjoin channel data) 2
K Copyright (on or off) 1
L Original (off if copy of original, on if original) 1
M Emphasis (respects emphasis bit in the original recording; now largely obsolete) 2
32 total header bits

Following the sync block comes an ID bit, which specifies whether the frame has been encoded in MPEG-1 or MPEG-2. Two layer bits follow, determining whether the frame is Layer I, II, III, or not defined. If the protection bit is not set, a 16-bit checksum will be inserted prior to the beginning of the audio data.

The bitrate field, naturally, specifies the bitrate of the current frame (e.g., 128 kbps), which is followed by a specifier for the audio frequency (from 16,000Hz to 44,100Hz, depending on whether MPEG-1 or MPEG-2 is currently in use). The padding bit is used to make sure that each frame satisfies the bitrate requirements exactly. For example, a 128 kbps Layer II bitstream at 44.1kHz may end up with some frames of 417 bytes and some of 418. The 417-byte frames will have the padding bit set to "on" (1) to compensate for the discrepancy.

The mode field refers to the stereo/mono status of the frame, and allows for the setting of stereo, joint stereo, dual channel, and mono encoding options. If joint stereo effects have been enabled, the mode extension field tells the decoder exactly how to handle it, i.e, whether high frequencies have been combined across channels.

The copyright bit does not hold copyright information per se (obviously, since it's only one bit long), but rather mimics a similar copyright bit used on CDs and DATs. If this bit is set, it's officially illegal to copy the track (some ripping programs will report this information back to you if the copyright bit is found to be set). If the data is found on its original media, the home bit will be set. The " private" bit can be used by specific applications to trigger custom events.

The emphasis field is used as a flag, in case a corresponding emphasis bit was set in the original recording. Th emphasis bit is rarely used anymore, though some recordings do still use it.

Finally, the decoder moves on through the checksum (if it exists) and on to the actual audio data frame, and the process begins all over again, with thousands of frames per audio file.

NOTE

For more details on the structure of MP3 header frames, including the actual lookup tables necessary to derive certain details from the bit settings previously listed, see the Programmer's Corner section at www.mp3-tech.org/. If you want to go straight to the horse's mouth, start at www.iso.ch.

ID3 Space

http://www.id3.org/ Tacked to the beginning or end of an MP3 file, " ID3" tag information may be stored (see Chapter 4), possibly including artist and title, copyright information, terms of use, proof of ownership, an encapsulated thumbnail image, and comments. There are actually two variants of the ID3 specification: ID3v1 and ID3v2, and while the potential differences between them are great, virtually all modern MP3 players can handle files with tags in either format (though a few older players will have problems with ID3v2 tags). Not only are ID3v2 tags capable of storing a lot more information than ID3v1 tags, but they appear at the beginning of the bitstream, rather than at the end. The reason for this is simple: When an MP3 file is being broadcast or streamed rather than simply downloaded, the player needs to be able to display all of this information throughout the duration of the track, not at the end when it's too late.

It's unfortunate that ID3 tags ever ended up being tagged onto the end of MP3 files to begin with; we'd be much better off if all MP3 files stored their ID3 data at the beginning rather than at the end of the file. As it stands, some MP3 players will simply give up if actual audio data is not encountered within the first few frames. While players developed to the actual ISO MPEG specification will know how to handle either type, the specification itself is unfortunately vague on this point. It simply states that a player should look for a " sync header," without specifying exactly where seeking should start and stop. This laxness in the spec has caused some controversy among developers of ID3-enabled applications, who naturally don't want their applications seeking blindly through 1GB image files, should the user happen to hand one to the application. Fortunately, the ID3v2 spec is more specific on the matter.

One of the more interesting portions of the ID3 specification is the numerical categorization of types of audio, as shown in the Appendix. The numerical identifiers are stored in the ID3 tag, and typically mapped to the actual names via a picklist or another widget in the MP3 player or ID3 tool.

Frames per Second

Just as the movie industry has a standard that specifies the number of frames per second in a film in order to guarantee a constant rate of playback on any projector, the MP3 spec employs a similar standard. Regardless of the bitrate of the file, a frame in an MPEG-1 file lasts for 26ms (26/1000 of a second). This works out to around 38fps. If the bitrate is higher, the frame size is simply larger, and vice versa. In addition, the number of samples stored in an MP3 frame is constant, at 1,152 samples per frame.

The total size in bytes for any given frame can be calculated with the following formula: FrameSize = 144 * BitRate / (SampleRate + Padding).

Where the bitrate is measured in bits per second (remember to add the relevant number of zeros to convert from kbps to bps), SampleRate refers to the samplerate of the original input data, and padding refers to extra data added to the frame to fill it up completely in the event that the encoding process leaves unfilled space in the frame. For example, if you're encoding a file at 128 kbps, the original samplerate was 44.1kHz, and no padding bit has been set, the total size of each frame will be 417.96 bytes: 144 * 128000 / (44100 + 0) = 417.96 bytes .

Keeping in mind that each frame contains the header information described above, it would be easy to think that header data accounts for a lot of redundant information being stored and read back. However, keep in mind that each frame header is only 32 bits long. At 38fps, that means you get around 1,223 bits per second of header data, total. Since a file encoded at 128 kbps contains 128,000 bits every second; the total amount of header data is miniscule in comparison to the amount of audio data in the frame itself.


1 Figure 2-1 is based on a chart and data produced by Xing Technology Corporation ( www.xingtech.com ).

2 However, note that frequency-dependent sensitivity flattens out the louder the sound is. For more information, see the section on Fletcher-Munson curves in Chapter 4} .

3 Aldous Huxley, The Doors of Perception . First published by Chatto and Windus Ltd., 1954. Now available in numerous reprints.

4 Presumably, one of the distinguishing characteristics of insanity is a failure of the mind to perform its function as a reducing valve, allowing "irrelevant" information to take as much precedence as the relevant.

5 See the glossary for definition and more information.

6 Decibels are a unit of sound pressure, more commonly known as "volume." The measurement of decibels is always relative, so 10db does not imply something quieter than silence, only that the second tone is quieter than the first.

7 Pulse Code Modulation is the standard designator for the digitization of uncompressed audio, such as that found on audio CDs. PCM audio is sampled 8000 times per second at 8 bits, for a total storage consumption of 64 kbps.

8 And because old movie cameras did not operate at a constant rate, nor was the frame rate accurate with the passage of time in the real world.

9 Generally, stored audio frequencies cannot be higher than half the samplerate, so a 96kHz samplerate allows for storage of frequencies well outside the human 20kHz threshold.

10 Documentation on the MP3 specification can be ordered for around $150 from www.iso.ch .

11 This isn't necessarily a bad thing; it depends on the complexity of the passage in question.

12 DAE stands for Digital Audio Extraction, and refers to a CD-ROM drive's ability to grab audio data as raw bits from audio CDs, so you don't have to rip tracks via the sound card. More details on DAE can be found in {_XRef\>74323: ch05: Chap ter _} .

13 Figure 2-5 is based on a diagram produced by ID3.org ( www.id3.org/mp3frame.html ).

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