Sound — Music

Music Storage - the scale of the problem

A CD stores a song as digital information. The data on a CD uses an uncompressed, high-resolution format. Here's what happens when a CD is created:

Music is sampled 44,100 times per second. The samples are 2 bytes (16 bits) long.
Separate samples are taken for the left and right speakers in a stereo system.

So a CD stores a huge number of bits for each second of music:

44,100 samples/second * 16 bits/sample * 2 channels = 1,411,200 bits per second

Let's break that down: 1.4 million bits per second equals 176,000 bytes per second. If an average song is three minutes long, then the average song on a CD consumes about 32 million bytes of space. That's a lot of space for one song, and it's especially large when you consider that over a 56K modem, it would take close to two hours to download that one song.

MP3

The Left Bank (Format MP3, Filesize = 623 KB, Duration = 32 seconds - looped)

MPEG is the acronym for Moving Picture Experts Group. This group has developed compression systems used for video data. For example, DVD movies, HDTV broadcasts and DSS satellite systems use MPEG compression to fit video and movie data into smaller spaces. The MPEG compression system includes a subsystem to compress sound, called MPEG audio Layer-3. We know it by its abbreviation, MP3.

The MP3 format is a compression system for music. The MP3 format helps reduce the number of bytes in a song without hurting the quality of the song's sound. The goal of the MP3 format is to compress a CD-quality song by a factor of 10 to 14 without noticably affecting the CD-quality sound. With MP3, a 32-megabyte (MB) song on a CD compresses down to about 3 MB. This allows a song to be downloaded in minutes rather than hours.

We use compression algorithms for images all the time. For example, a GIF file is a compressed image. So is a JPG file. We create Zip files to compress text. So we are familiar with compression algorithms for images and words and we know they work. To make a good compression algorithm for sound, a technique called perceptual noise shaping is used. It is "perceptual" partly because the MP3 format uses characteristics of the human ear to design the compression algorithm. For example:

There are certain sounds that the human ear cannot hear.

There are certain sounds that the human ear hears much better than others.

If there are two sounds playing simultaneously, we hear the louder one but cannot hear the softer one.

Using facts like these, certain parts of a song can be eliminated without significantly hurting the quality of the song for the listener. Compressing the rest of the song with well-known compression techniques shrinks the song considerably - by a factor of 10 at least.

MIDI

The Left Bank (Format MIDI, Filesize = 12.4 KB, Duration = 2 minutes 39 seconds)

Unlike MP3, WAV, and other digital audio files, MIDI files don't really contain recorded music. Instead, the music is stored as a series of numbers which tells a synthesizer how the music is to be played back. Yes, a MIDI file must be played on a synthesizer. You may not know it, but the sound card in your computer also contains a MIDI synthesizer.

MIDI files aren't recorded, they are "sequenced." Most MIDI files are made by a musician playing on a synthesizer keyboard. Each instrument must be entered separately, but since MIDI can have multiple tracks, the resulting sequence sounds like the instruments are all playing simultaneously.

Here's a simplified explanation of how MIDI works. To reproduce the sound of a piano playing a C note, the MIDI file, or sequence, contains digital information that says, "this is a piano sound." Another number says, "a note has been played," and other numbers convey information such as, "the note is middle C," "the key was struck very softly," "the note has now stopped," etc. Musicians love MIDI because it's easy to edit the files.

Finally, since MIDI must be played on a synthesizer, you can't record vocals or other sounds that are not available on the synthesizer. That's why you can't convert an MP3 file of a pop song to MIDI.