[linux-audio-user] (no subject)

[Steve Harris]

On Mon, Apr 12, 2004 at 06:21:33AM -0500, Martin McCormick wrote:
> 	Now, here is something I am a bit unsure of, myself.  I know,
> from actual observations that a straight PCM output from your basic
> A/D converter if read as unsigned numbers moves in steps from 0 to all
> 1's on.  I honestly have not tried to interpret those data as signed
> numbers because it wasn't convenient at the time.  If one wants to
> have a valid representation of what the wave form is doing for
> graphical or calculation purposes, then the mid-point would have to be
> what one would call 0 level with -1 being one below and 1 being 1
> above, etc.  I am certainly not arguing with anyone, but am a wee bit
> confused as to the correct way to represent the numbers.

Audio AD converters are asymmetrical - abs(minimum_voltage) is higher than
maximum_voltage. Audio AD converters sometimes have unsigned modes, but
they seem to have different behaviour. I checked up recently to make sure
JACK was doing the right thing(TM) when converting between floats and
ints. This stuff is explained in detail in the AD converter spec sheets
for the AD converters uned in the m-audio delta cards.
 
> 	This is probably off-topic, but any graphical software that shows
> you your music wave forms or does DSP functions in an arithmetic
> manner that is based upon an AC model has to behave as if the
> mid-point value was 0.

The midpoint value in 2's complement is -0.5bits, which does not represent
0volts.

- Steve