[PD] Playing and recording simultaneously

[Pedro Lopes]

As far as I know: sampling ticks-sync from A/D or D/A is a responsibility of
the soundcard (or at least all the architecture behind it: a/d + soundcard
driver). Pd just accesses that to fetch the values or to write to it, It
wouldn't make any sense that it would change the sync* - but maybe someone
with deeper knowledge of pd-arch can assure that.

*I'm thinking from a point of view of audio applications that I've
prototyped in the past from scratch (C), there's no access besides reading
or writing data to the audio buffers provided by the OS, so apart from the
latency that the audio app adds (apart form all the other latency that
Mathieu already pointed out) it shouldn't interfere with the sampling sync,
that's responsibility of the sound card[1]


[1] Some cards offer you their own sample-sync possibilities such as some
MOTU hardware... (
http://www.motu.com/techsupport/technotes/phase-accurate-versus-sample-accurate-sync)
although I do not recommend MOTU =P

On Sun, Jul 4, 2010 at 9:26 PM, Henrique <jhgoulart at gmail.com> wrote:

> Thanks for your observation, Mathieu.
>
> However, I think the total delay itself is not a problem, because it will
> appear in my acoustic response estimate. That is, if I compute the
> convolution of the estimated impulse response with the excitation signal,
> the result should have the same delay (assuming the convolutive model is
> valid). So, both the digital delay and the analog filtering that occur are
> also part of my estimated acoustic response.
>
> But, in order to the IR estimation be correct, there must be a
> synchronization between the clock frequencies of the: (1) D/A converter at
> the output generator and the (2) A/D converter on the input recorder. If the
> "sampling ticks" of these clocks are not (at least approximately)
> synchronized, then a significant error is expected on the estimate.
>
> --
> Henrique
>
>
>
> On Sun, Jul 4, 2010 at 5:12 PM, Mathieu Bouchard <matju at artengine.ca>wrote:
>
>> On Sun, 4 Jul 2010, Henrique wrote:
>>
>>  However, I have a question about how Pure Data works when I
>>> simultaneously play a waveform and record the output using a microfone (that
>>> is required for the estimate). Is the clock frequency of the D/A converter
>>> which produces the output syncrhonized with the clock frequency of the A/D
>>> converter wich produces the recorded signal? That is a very important
>>> question when trying to measure an acoustic response, and I couldn't find
>>> the answer at puredata.info website.
>>>
>>
>> The total delay is the sum of the logical delay as written in the audio
>> settings dialogue of Pd, plus the digital in-delay and out-delay of the
>> soundcard and/or driver, plus any analogue delay introduced by the soundcard
>> due to filtering, plus the microphone's delay, plus the speaker's delay,
>> plus the room's delay (relative to position and orientation of both speaker
>> and microphone).
>>
>> In the end, any digital delay can be counted by easy addition, whereas the
>> analogue delays are frequency-dependent and thus have to be counted as
>> filters. So, to measure a room's response, you'd first just subtract the
>> digital delay, but after that, for the analogue effects, you'd need to
>> deconvolve instead (but I suppose that you already know that).
>>
>> It may be tricky to know the digital delay beforehand... but if you put
>> the microphone and speaker really next (in)to each other, then just look in
>> your recording for the point when the response begins, then it might be
>> quite close to a digital delay, IF your impulsion contains enough
>> high-frequencies. But I don't know how close it is, as I haven't tried it.
>>
>> The total digital delay is soundcard-dependent, driver-dependent, and
>> OS-dependent, on top of being dependent on a setting in pd.
>>
>>  _ _ __ ___ _____ ________ _____________ _____________________ ...
>> | Mathieu Bouchard, Montréal, Québec. téléphone: +1.514.383.3801
>
>
>
> _______________________________________________
> Pd-list at iem.at mailing list
> UNSUBSCRIBE and account-management ->
> http://lists.puredata.info/listinfo/pd-list
>
>


-- 
Pedro Lopes
contacto: jazz at radiozero.pt
website: http://web.ist.utl.pt/Pedro.Lopes
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puredata.info/pipermail/pd-list/attachments/20100704/98ca8ef6/attachment.htm>