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Thread: Audio DAC Accuracy [Tech]
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face BY : Don Hyde email (remove spam text)



The relationship between DAC bits and SNR (signal-to-noise ratio) is
actually pretty simple.  SNR is normally expressed in dB, doubling the
voltage of a signal increases its level by 6 dB (approximately).  The SNR of
a signal produced by a DAC is the ratio of the maximum signal output to the
step size.  Adding a bit to a DAC doubles the ratio of full-scale output to
minimum step size, therefor it increases the SNR by 6 dB.  An 8-bit DAC has
an SNR of 48 dB, which sounds pretty good (telephones use 8 bits at 8K
samples/second), but not as good as your stereo.  On a good day, FM radio
can give you 70 or 80 dB SNR (equivalent to around 12 bits at 30K samples or
so), which is not as good as a CD which, with 16 bits can give you 96 dB
SNR, which is better than almost anyone's ears.

120 dBA (absolute sound pressure) is often considered the threshold of pain
(I think mine is lower, but I guess it's pretty subjective).  A quiet room
might get as low as 40 dBA, so you probably have never experienced more than
about 80 dB SNR in real life, which is the reasoning behind the choice of 16
bits for CD's -- more than you get in real life, with some to spare.

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See also: www.piclist.com/techref/io/audio.htm?key=audio
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