[pulseaudio-discuss] [PATCH 12/13] loopback: Validate the rate parameter

[Alexander E. Patrakov]

18.12.2015 11:47, Georg Chini wrote:
> On 18.12.2015 06:49, Tanu Kaskinen wrote:
>> On Wed, 2015-02-25 at 19:43 +0100, Georg Chini wrote:
>>> ---
>>>   src/modules/module-loopback.c | 5 +++++
>>>   src/pulse/sample.c            | 5 ++++-
>>>   2 files changed, 9 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/src/modules/module-loopback.c
>>> b/src/modules/module-loopback.c
>>> index d22afb5..7474ef2 100644
>>> --- a/src/modules/module-loopback.c
>>> +++ b/src/modules/module-loopback.c
>>> @@ -1039,6 +1039,11 @@ int pa__init(pa_module *m) {
>>>           goto fail;
>>>       }
>>> +    if (ss.rate < 4000 || ss.rate > PA_RATE_MAX) {
>>> +        pa_log("Invalid rate specification, valid range is 4000 Hz
>>> to %i Hz", PA_RATE_MAX);
>>> +        goto fail;
>>> +    }
>>> +
>> I pushed this change...
>>
>>>       if (pa_modargs_get_value(ma, "format", NULL))
>>>           format_set = true;
>>> diff --git a/src/pulse/sample.c b/src/pulse/sample.c
>>> index 1331c72..69ac83f 100644
>>> --- a/src/pulse/sample.c
>>> +++ b/src/pulse/sample.c
>>> @@ -106,7 +106,10 @@ int pa_sample_format_valid(unsigned format) {
>>>   }
>>>   int pa_sample_rate_valid(uint32_t rate) {
>>> -    return rate > 0 && rate <= PA_RATE_MAX;
>>> +    /* The extra 1% is due to module-loopback: it temporarily sets
>>> +     * a higher-than-nominal rate to get rid of excessive buffer
>>> +     * latency */
>>> +    return rate > 0 && rate <= PA_RATE_MAX * 101 / 100;
>> ...but I left this one out. The two changes fix two separate issues, so
>> they should be in separate patches.
>>
>> Making pa_sample_rate_valid() accept values above PA_RATE_MAX isn't
>> very nice, but I can see how it's better than the current behaviour,
>> which probably can cause the daemon to crash. Alexander mentioned in
>> IRC that he'd prefer doing the adaptive resampling inside module-
>> loopback rather than using pa_sink_input_set_rate(), and this change
>> demonstrates one good reason for that. I'm not asking you to implement
>> that change, but it certainly would be nice.
>>
>> --
>> Tanu
> Yes, doing it inside the module would really be a good idea. The
> main issue my controller is fighting against are the systematic
> errors introduced by changing the rate of the sink input. When
> you change the rate, the next latency report has an extremely
> high error.
> Can you point me to some code that implements resampling
> so that I have an example of what to do?

Look at the ffmpeg resampler inside PulseAudio tree, it's the simplest 
possible one. But I'd say that it's pointless to look at the code 
without understanding the math behind all of that, so please look at 
these links, too:

https://en.wikipedia.org/wiki/Sinc_filter
https://ccrma.stanford.edu/~jos/resample/resample.pdf

-- 
Alexander E. Patrakov