What Is Digital Audio?
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Digital audio is created by 'sampling' an analog signal at a fixed rate and
converting each sample into a number that corresponds to the level of the file
at the sample point. (Sample Rate is the number of samples per second). The 
sample rate determines the highest frequency that can be captured from the 
analog signal, while the bit (binary digit) depth determines the volume levels
that can be accurately captured from the analog signal. 

The quality of a digital audio signal is determined by the sample rate and bit
depth. The digital audio bit depth determines how many different levels
of volume can be captured from the original analog signal. The greater the
number of bits, the greater the dynamic range that can be captured by the
digital version of the analog signal. With the 16 bits used on CD audio, each
sample can be divided into 65,536 levels (2 to power 16), which translates to
a dynamic range of 96dB. In other words, 16 bits can capture a range of sound
from pure silence to 96 dB in loudness. 

As a rule of thumb, the highest frequency that can be preserved when 
converting from analog to digital is less than half the sampling rate. 
Thus, the 44.1kHz sample rate used by CD audio can capture frequencies up to
about 20kHz. 

At a high enough sampling rate and bit depth, it is difficult for the average
person to distinguish between the original and digitized version of a sound.

    Redbook Audio is a 44.1kHz/16-bit format used on song tracks on standard
    CDs playable in a stereo system CD player. Redbook Audio format must be 
    converted to MP3, Windows Media (WMA) or WAV format in order to reside
    in the music library on your PC hard drive. Redbook Audio cannot be stored
    on a PC hard drive. Instead, digital audio files are stored on your PC
    hard drive as 'files' with the extensions MP3, WMA or WAV. The term 
    'redbook' came from the color of the binder in which the specification 
    was first published. 

The Redbook Audio standard that defines CD audio, includes two audio tracks 
to provide stereo, a sampling rate of 44.1 kHz and a 16-bit depth. While 
this preserves much of the original sound quality, it also requires a great 
deal of storage space and a system capable of handling a high audio 
'bit rate '.


                           AUDIO FORMATS:
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                         Audio Compression
                         -----------------
Compression techniques can significantly reduce the bit rates of digital
audio. 

    Windows Media is a group of technologies created by Microsoft to 
    distribute media content. See Microsoft's website 
    www.microsoft.com/windowsmedia) for more information. 

The most popular compression methods, MP3 and Windows Media (WMA), use a 
technique called perceptual encoding. This compression format is based on 
the principle that some of the original sound cannot be perceived by the 
human ear and therefore can be removed without being perceived by the 
listener. 

For example, if a loud sound at a particular frequency dominates a weaker 
sound at a different frequency, the weaker sound can be removed without 
significantly affecting the overall quality. Also, sounds at certain 
frequencies are less significant to the overall sound and can be encoded 
less precisely, thereby conserving more file space. 
 

                         WAV Format
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The WAV format is an uncompressed copy of a CD audio track that preserves
CD-quality Redbook audio.  Although this is the purest form of audio, it also
takes up a significant amount of hard drive space (a typical three-minute song
consumes more than 30MB of hard disk space).  Playback of uncompressed WAV
audio will be most important for audiophiles who can discern the loss of
dynamic range and other limitations of compressed audio formats.


           MP3 and Windows Media Compressed Audio Formats
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MP3 and Windows Media (WMA) formats are 'compressed' versions of the WAV
format and consume as little as 10% of the hard drive space of an equivalent
WAV file—typically 1 MB for a minute of music. The drawback is that some of
the music quality is lost due to the compression process. With less 
compression, the file is larger but the difference in audio quality is less
detectable. There is no quality setting for WAV format, since it must be 
fixed at 44.1kHz/16bits.


           Disk Space for Recording Digital Audio Files
           --------------------------------------------
The disk space required for storing digital audio depends on the audio bit
rate, as described above. Stereo, CD-quality, WAV format digital audio
generates a bit rate of 1.4 Mbps, which translates to 10.5 megabytes of hard
drive space per minute (60 sec x 1.4Mbps / 8 bits per byte). Thus, a typical
four-minute song recorded in WAV format will consume about 42 MB of hard drive
space.

In contrast, the WMA format at the highest quality setting reduces the bit
rate to 192kbps. Thus, the same one-minute file will consume only 1.4MB 
(60 sec x 192kbps/ 8 bits per byte). For a four minute song, this translates
to 5.7MB, or about 1/10th the size of an equivalent WAV file. 

Thus, an hour of music recorded as an uncompressed stereo WAV files will
consume about 650 MB (the storage capacity of a CD), while the same hour of
music recorded in the WMA format at 192kbps will consume about 84MB. To most
listeners, the difference in audio quality is imperceptible. 

WMA Format maximum bite rates Update:  
  If you select the Windows Media Lossless format in rip setting, only 
  in Windows XP you can increase the audio rip bit rate for music CDs 
  that ranges from 470 to 940 Kbps. However, this format is not supported 
  by the AudioTron.
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