[pulseaudio-discuss] [PATCH v6 14/25] source.c, sink.c: Implement pa_{source, sink}_get_raw_latency_within_thread() calls

[Georg Chini]

On 15.08.2016 23:40, Tanu Kaskinen wrote:
> On Sun, 2016-06-05 at 21:05 +0200, Georg Chini wrote:
>> In certain situations, the reported sink or source latency may become
>> negative.
> What are those certain situations?

I'm not really sure, I believe the reports get negative when the initial
start time of the calculation is wrong. So the reported source or sink
latencies have a permanent offset which can not be avoided because
all reported times/latencies have errors.

>
>> This does not indicate that the latency is indeed negative but can be considered
>> merely an offset error. The current get_latency() calls truncate negative latencies
>> because they do not make sense from a physical point of view.
>> In the context of module loopback, negative source or sink latencies are acceptable
>> because the overall latency never becomes negative. Truncating negative values leads
>> to discontinuities in the latency reports, therefore this patch implements functions
>> in source.c and sink.c which can return the raw value without truncation.
> What's the practical problem with the discontinuities? A negative
> latency report is always incorrect, so changing it to 0 can only make
> the latency report closer to truth. The explanation isn't sufficient
> for me to understand why that is such a big problem that it warrants
> adding this stuff to the core.

Actually you recommended to add it to the core.
 From a practical perspective your argument is not important. You only 
consider
an isolated source or sink latency, The end-to-end latency never gets 
negative
and should be continuous.

In effect, the calculated end-to-end latency might really be slightly 
too small,
because of an incorrect offset, but this offset applies to ALL reported 
latencies,
thereby lowering the reported end-to-end latency not only when one of the
components gets negative.
Truncating the latency when it gets negative would mean sometimes 
ignoring the
offset and sometimes not which not does not look like a good idea.

Let me give you an example of what I am seeing:

       source      sink    buffer   total
1.        0.5         10      50.5      61
2.       -0.4         12      49.4      61
3.        0.2           9      51.8      61

If you truncated the negative latency in the second measurement, you would
end up with 61.4 ms, which may be nearer to the "real" end-to-end 
latency, but
will trigger an unwanted and unneeded correction due to an isolated jump 
in the
time series.

> If this stuff makes sense, there should be a comment somewhere
> explaining what the "raw latency" means and why we care about it.
>