On 14/03/2014 14:27, Olli Niemitalo wrote:
(I tried to send this earlier with images but forgot that attachments
are not allowed.)
I recently found out that not all codecs / sound cards use symmetrical
sinc-type filters in their ADC's / DAC's. This is the output-->input
impulse response of my M-Audio Fast Track Ultra sound card:
http://yehar.com/Fast%20Track%20Ultra%2048%20kHz%20output-input%20ir.jpg
It looks more like a minimum-phase lowpass filter. The marker at
sample #29 indicates what PortAudio thinks the latency is: 29 samples.
But in this case latency, if expressed in terms of group delay,
actually depends on the frequency and is only as much as PortAudio
says for frequencies that are beyond human hearing:
..
But portaudio only states the software i/o buffer latency, it knows
nothing directly of internal codec latencies. You would need to subtract
the (two-way?) buffer latency portaudio reports, and then measure or
compute how much of the remainder is down to the dac filtering. I am not
entirely sure how accurate that portaudio estimate is anyway - it's an
area of portaudio that may not be entirely bug-free. It has been
discussed on the portaudio list fairly recently IIRC.
Richard Dobson
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