[pulseaudio-discuss] [PATCH 07/13] loopback: Refactor latency initialization

[Georg Chini]

On 22.11.2015 14:26, Alexander E. Patrakov wrote:
> 22.11.2015 17:21, Georg Chini wrote:
>> The other big problem is that you cannot determine the number
>> of cycles you will need to correct the initial latency error because
>> this error is unknown before the first adjustment cycle.
>
> You can circumvent this problem by sending zeros instead of the actual 
> data until you correct the initial latency error well enough. And, 
> because we are sending zeros, nobody cares if there are big frequency 
> steps. So one cycle is always enough to correct the initial latency 
> error once it is known, and then we can unmute the sound.

I do not understand what you are proposing. Are you saying we should 
skip samples
until we are near the requested latency? So send no audio for at least 
one adjust time?
That would mean that you have to reduce the adjust time drastically.
BTW, I did some experiments with shorter adjust times and could not see 
a significant
improvement over the 1s interval.

>
>> When you calculate that safety margin you also have to consider
>> that the controller might overshoot, so you temporarily could
>> get less latency than you requested.
>
> This is definitely impossible with the controller in PATCH 04/13 
> modified so that min_cycles is always 1. Indeed, by design, such 
> controller corrects exactly 100% of the latency error in one step, 
> without paying attention that it might be noticeable. And with 
> min_cycles > 1, it corrects less than 100% of the error, so cannot 
> make the situation any worse than it is. I.e. here overshoot would 
> mean correcting less than 0% or more than 100% of the error, and it 
> just can't happen.
>

In theory yes, but practice you are wrong here. It does overshoot 
sometimes, especially
with long adjust times because we are taking measurements that have some 
error.
Consider that the measured latency is 2 ms too long (this can easily 
happen in the first cycles),
then you will correct 2 ms too much. With short latencies, even half a 
millisecond might
be significant.

> OTOH, the PI-controller that I am currently working on can indeed 
> overshoot. I think a good idea would be to detect situations where the 
> latency error is excessive or would be excessive during the next step, 
> and correct this situation with your controller with min_cycles = 1, 
> instead of my controller. That's, without paying attention to artifacts.
>
> I will post this controller (and, as requested, perform measurements 
> with the trivial resampler) in the near future.
>
Thanks, please also test the patch I sent on Friday.