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Recording: digital audio Digital Audio Tutorial: Bit rates and file formats

Digital audio tutorial
Introduction to digital audio
What sounds best?
Music compression
Common computer digital audio formats
Perceptual encoding schemes
Chart 1: bit depth and sampling rate chart
Chart 2: audio file formats
Chart 3: compression schemes
Glossary of common digital audio terms
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musiq.com digital audio
Computer digital audio formats have fairly well consolidated in recent years, mainly due to the proliferation of multi-track digital audio recording software, and the need to maintain some standard of compatibility between platforms and programs.

However, between platforms there is less of a standard. Because of the Windows Media Player, built into Windows 95, 98, and NT, most Windows machines can only read WAVE format and certain mp3 files, without needing to acquire external software. Macintoshes have no such limitation, and commonly work with AIFF, Sound Designer 2, WAV, and Sun Audio, mp2 and mp3, as well as more obscure formats. Most UNIX boxes set up for digital audio use Sun's proprietary format.

So what is the difference in formats? The basic differences are in the sophistication of header contents. By this, I mean that sophisticated headers are capable of more bit rates and sampling rates. The most sophisticated is .au, the Sun format, which supports 4, 8, 16, 24, and 32 bit digital audio at all sampling rates. At the bottom of the scale is Microsoft .wav, which only supports 8 or 16 bit soundfiles.

The Sound Designer 2 format (SDII), though limited in its bit depth, has two unique features which make it useful for many applications. First is its support for quadraphonic sound files. Second is the use of region data in the header. With regions, you can mark sections of a song inside 1 sound file, or make a single sample file with 20 "regions" which behave like unique sound files. Most software which recognizes regions also contains playlists -- if you're recording a song with 4 choruses and 4 verses, it's easy to come up with a "remix" by changing the playlist order for the 8 regions.

If you're just recording 16 bit audio and burning CDs, it's not too important what format you're using. They all sound the same. However, if you are sending files to a Windows computer, they may not be able to read anything other than .wav files. Also, most CD burner software can read one format in particular and NOT others. Check with your favorite software and record in the format you'll eventually output.

If you will be doing DSP processing on sound, you may need the capability of AIFF or AU which can make 24-32 bit sound files. If you are gating and mutating a sound file, some amplitudes may go above the peak in the original sound file -- you use an intermediate 24-32 bit sound file and then remove the "LSB's" (Least Significant Bits), resulting in a 16-bit sound file that is at maximum undistorted volume.

Finally, there is a new standard in audio established with the new DVD audio players. Though it will take years for DVD audio to phase out CD, at some point it may be apparent that 24 bits is the norm. Then again, it might not. Here' s a handy chart of file formats...

Other formats

.midi files (also, .med, .mid) don't actually contain any digital sound, but contain MIDI messages which work with software or external hardware to produce music. Basically, MIDI files store note on-off information, which uses very little memory. Long songs can take as little as 100k. However, the artist has no control on what the song will sound like, since the mapping of MIDI commands is up to the end user. Thus, the glorious gothic dirge string quartet could end up sounding like squawking ducks with an off-tempo beat box.

.rb and .rbm files (from Propellerheads/ Rebirth) are a subset of MIDI files which contain songs and instrument controls for the Rebirth RB 338 drum machine software. Since it is instrument specific, the composer has much more control.

Click to read about Compressed Formats..

Read about perceptual encoding....

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