* WHAT ARE THEY? 

Digital audio is made of lots of tiny 'snapshots' that happen many times a
second. The 'size' of the individual 'snapshot' is expressed as the number 
of bits (0's and 1's, or 'off' and 'on' states) stored in a single digital
'word', while the number of times each second that a 'snapshot' is taken is 
expressed as the 'sampling rate'.  

So, you can look at digital audio as a sort of movie. The individual snapshots
are 'frames' that go by fast enough to make the images appear to move. The
rate at which the frames go by in a movie is expressed as frames per second 
(fps). 

The bits are pieces of each word, which is called a sample. In a CD player,
there are 44,100 16-bit samples of the audio per second, and there are two
streams for stereophonic playback. This makes for a 'sampling rate' that is
called 'stereo 16-bit 44.1 kHz' (k = 1000, Hz = cycles per second).  

To clarify the analogy :

'Bit' or Digital 'Word'   =  Snapshot (analogous to a frame in a movie)
'Bit Depth' or 'Word Length'  =  Size of the snapshot 
'Sampling Rate'  =  Frames Per Second (the speed at which the film travels
   through the projector)


* WHY ARE THERE SO MANY DIFFERENT FORMATS?

As in all things electronic, there are different standards for different
formats developed by different companies. The most commonly used format for
stereo digital audio in the home is the CD Audio format (Red Book audio). 
CD Audio is formatted as stereo 16-bit 44.1 kHz sampling rate. Here is a list
of common digital audio formats with their bit depths and sampling rates:

Format                    Bit Depth       Sampling Rate
------                    ---------       -------------
CD Audio (stereo)         16 bits         44.1 kHz
DAT (stereo)              16 bits         48 kHz (most can also use 44.1 kHz)
DAT (extended play)       16 bits         32 kHz
MiniDisc (compressed)     16 bits         48 kHz
MiniDisc (extended play)  16 bits         32 kHz
ADAT XT (black face)      16 bits         48 kHz
ADAT XT20                 20 bits         48 kHz
DVD Audio (projected)     24 bits         96 kHz
Pro MO disk               24 bits         96 kHz (192 kHz, 88.2 kHz also used)

In general, the greater the bit depth the higher the resolution and the lower
the amount of noise in the audio signal. Also, the higher the sampling rate
the greater the frequency response of the digital recorder.

* HOW DOES THE SAMPLING RATE AFFECT THE QUALITY OF THE RECORDING?

At one-half the frequency of the sampling rate there is a sub-harmonic
generated that needs to be filtered out so that it will not be heard by 
the listener after digital to analog conversion. 
This is called the Nyquist Frequency. 

The sampling rate of 44.1 kHz for CD Audio was chosen because it is a little 
more than twice the acknowledged upper limit of human hearing, 20 kHz 
(20,000 cycles per second). The Nyquist frequency oscillation occurs at 
22.05 kHz when a sampling rate of 44.1 kHz is employed. 

A steep filter is employed in typical CD players and DAT recorders that 
allows frequencies up to 20 kHz to pass through while almost completely 
filtering out the 22.05 kHz oscillations. This way the full 20 Hz - 20 kHz 
audio band can be reproduced, while the Nyquist frequency subharmonic is 
rendered inaudible.


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