[FFmpeg-devel] [PATCH] ffplay: use 48 kHz for ALSA output

[Marton Balint]

On Thu, 21 Apr 2016, Reimar Döffinger wrote:

> On 20.04.2016, at 23:59, Marton Balint <cus at passwd.hu> wrote:
>
>> Signed-off-by: Marton Balint <cus at passwd.hu>
>> ---
>> ffplay.c | 5 +++++
>> 1 file changed, 5 insertions(+)
>> 
>> diff --git a/ffplay.c b/ffplay.c
>> index 804bcbc..89a34d2 100644
>> --- a/ffplay.c
>> +++ b/ffplay.c
>> @@ -2578,12 +2578,17 @@ static int audio_open(void *opaque, int64_t wanted_channel_layout, int wanted_nb
>>     static const int next_nb_channels[] = {0, 0, 1, 6, 2, 6, 4, 6};
>>     static const int next_sample_rates[] = {0, 44100, 48000, 96000, 192000};
>>     int next_sample_rate_idx = FF_ARRAY_ELEMS(next_sample_rates) - 1;
>> +    char driver_name[32];
>>
>>     env = SDL_getenv("SDL_AUDIO_CHANNELS");
>>     if (env) {
>>         wanted_nb_channels = atoi(env);
>>         wanted_channel_layout = av_get_default_channel_layout(wanted_nb_channels);
>>     }
>> +    /* By using the default dmix sample rate we should be able to avoid the
>> +     * ALSA resampler because using it causes small buffers and underruns. */
>
> That only makes sense if the alsa device used actually uses dmix. Plus I 
> am generally sceptical about such a hackish workaround (in particular 
> making assumptions on internals like default sample rate) for what 
> sounds like an ALSA issue (assuming we can't/shouldn't just request a 
> larger buffer or query the "native" sample rate).

You're right, this is a hack, and probably alsa is buggy, or SDL could 
create the player differently. I tried to reproduce the issue 
with pure alsa, unfotunately without success.

In the meantime I also found another workaround, setting the 
SDL_AUDIO_ALSA_SET_BUFFER_SIZE environment variable, as this causes the 
initialized alsa buffer to have 4 periods instead of 2, which also makes 
the issue disappear. Setting this (if the variable is not already defined) 
is an acceptable workaround?

I see little chance of me digging into the deep of ALSA, I have no better 
idea which benefits everybody other than setting the 
SDL_AUDIO_ALSA_SET_BUFFER_SIZE environment variable.

Regards,
Marton