[pulseaudio-discuss] Latency problem with long latencies

[Pierre-Louis Bossart]

On 3/20/16 2:24 PM, Georg Chini wrote:
> On 20.03.2016 19:13, Alexander E. Patrakov wrote:
>> 20.03.2016 22:41, Georg Chini пишет:
>>> Hello,
>>>
>>> I am still working on module-loopback and hit a problem that I cannot
>>> explain.
>>> When running a HDA card with long latency (333ms) I see that the
>>> resulting
>>> latency is not stable but varies around 300 usec. What is worse, the
>>> changes
>>> in latency are not even reported, module-loopback sees a completely
>>> stable
>>> value ( +/-10 usec).
>>> My setup: I have a bitscope dual channel USB oscilloscope attached to
>>> another
>>> machine. This device also has a function generator that I set to 1kHz
>>> rectangle.
>>> The signal is input to the HDA card and fed to the output of the same
>>> card via
>>> module-loopback. Input and output signal are measured using the
>>> oscilloscope.
>>> The movement of the edge of the output signal now shows the "latency
>>> jitter"
>>> which can be made visible by using the overlay mode of the oscilloscope.
>>>
>>> The result for HDA (module-loopback running with 500ms adjust time
>>> and 1s
>>> latency) is shown at http://georg.chini.tk/hda_333ms_3.png. Some of the
>>> original
>>> input signal is somehow fed into the output signal, don't know where
>>> this happens,
>>> so please ignore it as it has no impact on the result.
>>
>> No idea either, just some blind pokes...
>>
>> 1. Are you using the trivial resampler? Does anything change if you
>> change the resampler type?
>> 2. Do you get the same trace with a "weird" latency (not something
>> that can be derived from one second by multiplication or division by
>> small factors)? E.g. something like 318 ms.
>>
>> To me, it looks like something is sometimes creating an
>> off-by-one-sample error when copying the data. Which is exactly the
>> job of the trivial resampler, that's why the question.

why is a resampler needed when doing a loopback on the same card?

>>
> Hi Alexander,
>
> no, I am working with speex-float-5. I just re-tested zitaj-bridge, and
> I was wrong,
> it shows the same behavior. I am also seeing double lines, so it is
> probably deeper
> down in the alsa driver.
> I also tested your 318 ms - same result.
> To rule out artifacts produced by the oscilloscope I fed the input
> signal directly to the
> second channel - this looks fine.
> The choice of the resampler does no longer have any impact on the
> performance
> of module-loopback. For small latencies (70ms - 100ms) I can keep the
> latency jitter
> now well below 100 usec for HDA -> USB on my system, this corresponds to a
> difference of 35 mm in hearing distance. When the USB device is run in
> batch mode
> the result is even better but less robust to changes in system load.
> zitaj-bridge is still somewhat better for HDA -> USB and USB -> HDA (~50
> usec)
> but always fails after a few minutes complaining about excessive timing
> errors
> even for HDA -> HDA on the same card.
>
>
>
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