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fundamental - simple question - on audio samples

Started by Srini March 24, 2006
what exactly is the physical significance of the samples in a recorded
sound stream like say in a wav file. i realize it is a result of AD
conversion of a signal but beyond that i am drawing a blank. is this
the amplitude of the sound wave? can someone point me to any reference.
my own searches have led to lot of conceptual understanding but not
specific.

thanks

srini

Srini wrote:

> what exactly is the physical significance of the samples in a recorded > sound stream like say in a wav file. i realize it is a result of AD > conversion of a signal but beyond that i am drawing a blank. is this > the amplitude of the sound wave? can someone point me to any reference. > my own searches have led to lot of conceptual understanding but not > specific. > > thanks > > srini >
If it's uncompressed samples then you can think of the numbers in the wav file as some constant times the voltage present on the ADC input at the sampling instant. In reality it's more complicated than that because most sound acquisition is done through sigma-delta converters which contain considerable on-board filtering, but for the sake of your question you can just pretend that it's simple. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/
Srini wrote:
> what exactly is the physical significance of the samples in a recorded > sound stream like say in a wav file. i realize it is a result of AD > conversion of a signal but beyond that i am drawing a blank. is this > the amplitude of the sound wave? can someone point me to any reference. > my own searches have led to lot of conceptual understanding but not > specific.
Think of it as a numeric representation of the voltage generated by a microphone. The voltage is recorded as numbers at regular intervals, all the same. The intervals must be short enough so that there will be at least two samples of every cycle of the highest expected frequency. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Thanks a whole bunch Tim and Jerry.

If I may follow up a bit - simplistically, to perform the reverse ie
the play back, clearly there has to be a "standard" mapping between the
physical quantity - say the voltage and the digitized samples. or
perhaps each DAC will output different absolute volts but scaled by the
sample value.

regards

srini

Srini wrote:
> If I may follow up a bit - simplistically, to perform the reverse ie > the play back, clearly there has to be a "standard" mapping between the > physical quantity - say the voltage and the digitized samples. or > perhaps each DAC will output different absolute volts but scaled by the > sample value.
For something like an mp3 or wav file there is no standard mapping between digitized samples and absolute voltage or sound pressure. That's why there's a volume control; and you may have to fiddle with the volume control depending on the microphone/speaker efficiency and/or recording level. The sound technicial can even play with the recording level within one recording, or the AGC may be doing it automatically. IMHO. YMMV. -- rhn A.T nicholson d.0.t C-o-M
You are right that each DAC may output a different absolute voltage, but in all 
cases, the voltage will be proportional to the sample value (or as you put it, 
scaled by the sample value).

And to answer your original question, yes the sample (ideally) represents the 
amplitude of the sound wave.  But as others have pointed out, it is quite a few 
steps removed from that.  First the sound wave is converted to voltage by a 
transducer of some type, typically a microphone.  Then that microphone voltage 
is probably amplified and filtered in various ways before it hits the ADC.  But 
the ideal is that the sample value represents the amplitude.  (This assumes you 
are talking about recording a real acoustic signal, as opposed to a synthesized 
voltage, e.g. from an electronic instrument.)

-- 
Jon Harris
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"Srini" <RSVasan1007@gmail.com> wrote in message 
news:1143233270.673844.256720@g10g2000cwb.googlegroups.com...
> Thanks a whole bunch Tim and Jerry. > > If I may follow up a bit - simplistically, to perform the reverse ie > the play back, clearly there has to be a "standard" mapping between the > physical quantity - say the voltage and the digitized samples. or > perhaps each DAC will output different absolute volts but scaled by the > sample value. > > regards > > srini >