by January 1, 2001 0 comments

Audio data is defined by certain parameters, which determine
its quality and format. The major ones are: the sampling rate, number of bits
per sample, and the total number of channels. However, before we take a look at
these, let’s first understand what they’re all about.

Digital audio files are created by taking an analog audio
stream and sampling it at frequent intervals. Sampling rates are specified in
kHz and are measured per channel. So audio data recorded in stereo at 16,000
samples/sec is actually 32,000 samples/sec, since stereo recording involves dual
channels. A higher sampling rate means more number of samples captured per
second, and hence better quality sound. For normal listening, the playback of
recorded sound at a slightly different rate doesn’t show significant
difference in hearing, but for professional musicians who do real-time editing
of audio, even slight differences matter.

The number of bits per sample defines the size of the sampled
data packets. This can be either 8 or 16 bits. Coming to channels, the number of
channels can be equated to the number of paths over which the complete sound is
split. This can either be on the basis of instruments or frequencies. In
professional music recordings, the sound for different instruments is recorded
on to different paths or tracks. This makes them easier to edit. For playback,
the hardware required to handle, say 16 channels, will turn out to be very
expensive. So stereo sound or a maximum of 5+1 channel Dolby surround is enough
for homes. Some movie theatres, however, do have a setup for giving you true
16-channel surround sound.

Making waves

The WAV or ‘wave’ format can be considered as the basic
uncompressed form of audio. Almost all primary recordings of audio or music are
done in WAV format. Since it’s uncompressed, all data remains intact, making
it easy to edit. A WAV file consists of uncompressed 8 or 16-bit sound samples
with a header preface and specifications on how the audio data is formatted in
the file. The sample rates used are typically 11,025 sample/sec for telephone
quality, 22,050 sample/sec for radio quality or 44,100 sample/sec for CD
quality. As for the number of channels, hold your breath, it’s possible to
encode up to 65,536 channels. Most other audio formats use some compression
algorithm or the other, which reduces their size making them unsuitable for
editing.

MIDI

Moving on from WAV, another old favorite in audio is the MIDI
format. MIDI stands for Musical Instrument Digital Interface. It’s basically a
standard for storing sounds from musical instruments like synthesizers and
keyboards. This format doesn’t have any actual sound data in it. Typically, an
MIDI file has instructions that include the note’s pitch, length and volume.
Plus, there are a few other parameters like envelopes, rise, attack and delay
for sound that can also be included in MIDI instructions.

These instructions then direct the digital-to-analog
converter in your sound card to equate them to actual sound samples from the
sound bank for the particular sound card. So, though not always, there’s a
possibility of hearing the same MIDI files differently with two different sound
cards. For professional musicians, this music format is a must as it’s easy to
edit and has lower levels of noise. Most good quality synthesizers in the market
have a MIDI interface that can be connected to PCs and used to create scores.
Software is also available that can convert MIDI sample to written music notes.
So you can simply print these notes and use them for your music class.

MP3

The audio format that’s making waves across geographical
boundaries–MP3–is perhaps the most popular and prevalent of music formats.
MP3 stands for MPEG-1 Layer 3. It’s a digital audio compression algorithm,
which gives you a maximum compression factor of about twelve while still
retaining superb sound quality. Due to this, it takes lesser hard disk real
estate, unlike other audio formats.

MP3 encoding is able to achieve effective compressions as it
works mostly on the range of frequencies perceivable to the human ear. It chucks
out all non-audible frequencies and keeps others for high compression. Ever
since International Organization for Standardization (ISO) formalized this
standard, a number of other high-compression formats have followed. WMA (Windows
Media Audio) is one such venture from Microsoft that’s pitted against the MP3
format. As an inherent part of Windows Me from Microsoft, WMA boasts of better
compressions. People worldwide seem to differ in opinions, some in its favor,
others against it.

AIFF format

AIFF (Audio Interchange File Format) was developed by Apple
Computers, and is the standard audio format on the Macintosh. It’s parallel to
the uncompressed WAV format on the PC, and is therefore meant for easy editing
of audio on the Mac. The AIFF format encodes audio data in 8-bit mono or stereo
waveforms. You can’t compress the data in AIFF, but another version of the
same called the AIFF-C let’s you compress data by a factor of around six.
Apple developed this for recording and storing high-quality sampled audio and
musical instrument information. Apart from the Mac, Silicon Graphics (SGI)
platform and several other professional audio packages use AIFF format for
playing with the sound.

RealAudio

When it comes to streaming audio, Real Audio is the first
thing that comes to mind. It’s a proprietary format of RealNetworks and is
widely used on the Web. The format can be used over the Web or a company
intranet. One advantage of RA is that its content can be customized depending
upon the transmission bandwidth available.

Hardware audio storage

When we talk of music for homes, we have audiotapes and audio CDs as the
storage media. In an audiocassette, data can be stored on the magnetic tape in
primarily two tracks (for A and B sides) and two channels (stereo). The inherent
disadvantage here is that data is accessible only sequentially as the tape
passes under the magnetic head. On the other hand, data on an audio CD is stored
in the form of crests and troughs via burning through a laser beam and accessed
in a similar manner (remember old gramophones). The file format on an audio CD
is raw WAV (CDA). Here, the advantage is that you can quickly jump from one
track to another, so there’s no waiting time.

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