MadSci Network: Engineering

Re: why do telephones, am & fm radio, cd and dat use different sampling rates

Date: Mon Feb 4 01:46:47 2002
Posted By: Martin Smith, Grad student, Engineering, B.E., M.EngSc., Uni of Qld / now employed by an airline
Area of science: Engineering
ID: 1010295610.Eg

Telephones, and commercial AM and FM radios are all analogue devices and do 
not use sample methods.

CDs DATs .WAV computer files etc are digital and use sampling to record 
analogue signals.

When an analogue signal (such as speech) is digitised the signal is sampled 
at a given frequency.  That is the "value" of the signal is determined at 
set intervals.  This introduces a possible error when the analogue signal 
is reconstructed from the sampled data.

Basically the higher the frequency of sampling the less error there is in 
reconstruction.  Or alternatively you can think that the higher the 
sampling rate, the higher the signal frequency that can be successfully 
reconstructed.  You need to sample at least twice as fast as the highest 
signal frequency there is.  So if you wanted to reconstruct a sine wave at 
10kHz, you would need to sample at least at 20Khz.

Remember a complex signal can be though of as an infinite series of sine 
waves at different and increasingly higher frequencies (fourier transform 
analysis).  So the faster you sample the "higher" of the simple sine wave 
frequencies that make up the complex signal can be reconstructed.

So very basically CDs DATs .WAV computer files, MP3 etc all have a defined 
sampling rate.  That rate determines the highest sine wave frequency that 
can be reconstructed faithfully, which also determines the quality of the 
recording.  However higher sampling means you need more storage.  It comes 
down to a trade off.  The fact that the human ear can only hear frequencies 
below 20KHz also plays a role in determining what sampling rate to use.

Martin Smith

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