[pulseaudio-discuss] Latency problem with long latencies

[Pierre-Louis Bossart]

On 3/28/16 9:30 AM, Georg Chini wrote:
> On 28.03.2016 16:27, Pierre-Louis Bossart wrote:
>> 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?
>
> The resampler is there for the more general case when the two cards
> are different. I just have no system with two HDA cards, so I have to
> do my tests HDA -> HDA on the same card.

Understood, the point was whether you can remove this resampler to look 
for the root cause.