[vlc-devel] [PATCH 5/5] wasapi: add an option to handle exclusive mode

[Rémi Denis-Courmont]

I don't see how it reduces latency, no. Audio latency is largely bound by the packet/period size, not by exclusive mode.

Le 25 octobre 2019 10:17:21 GMT+03:00, Thomas Guillem <thomas at gllm.fr> a écrit :
>
>
>On Thu, Oct 24, 2019, at 20:55, Rémi Denis-Courmont wrote:
>> Le torstaina 24. lokakuuta 2019, 15.37.13 EEST Thomas Guillem a écrit
>:
>> > cf. WASAPI_EXCLUSIVE_LONGTEXT
>> > ---
>> >  modules/audio_output/wasapi.c | 174
>+++++++++++++++++++++++++++++++++-
>> >  1 file changed, 170 insertions(+), 4 deletions(-)
>> > 
>> > diff --git a/modules/audio_output/wasapi.c
>b/modules/audio_output/wasapi.c
>> > index daa946f8afc..91defb250fb 100644
>> > --- a/modules/audio_output/wasapi.c
>> > +++ b/modules/audio_output/wasapi.c
>> > @@ -588,6 +588,154 @@ static void Stop(aout_stream_t *s)
>> >      free(sys);
>> >  }
>> > 
>> > +/*
>> > + * This function will try to find the closest PCM format that is
>accepted
>> > by + * the sound card. Exclusive here means a direct access to the
>sound
>> > card. The + * format arguments and the return code of this function
>behave
>> > exactly like + * IAudioClient_IsFormatSupported().
>> > + */
>> > +static HRESULT GetExclusivePCMFormat(IAudioClient *c, const
>WAVEFORMATEX
>> > *pwf, +                                     WAVEFORMATEX
>**ppwf_closest) +{
>> > +    HRESULT hr;
>> > +    const AUDCLNT_SHAREMODE exclusive =
>AUDCLNT_SHAREMODE_EXCLUSIVE;
>> > +
>> > +    *ppwf_closest = NULL;
>> > +
>> > +    /* First try the input format */
>> > +    hr = IAudioClient_IsFormatSupported(c, exclusive, pwf, NULL);
>> > +
>> > +    if (hr != AUDCLNT_E_UNSUPPORTED_FORMAT)
>> > +    {
>> > +        assert(hr != S_FALSE); /* S_FALSE reserved for shared mode
>*/
>> > +        return hr;
>> > +    }
>> > +
>> > +    /* This format come from vlc_ToWave() */
>> > +    assert(pwf->wFormatTag == WAVE_FORMAT_EXTENSIBLE);
>> > +    const WAVEFORMATEXTENSIBLE *pwfe = (void *) pwf;
>> > +    assert(IsEqualIID(&pwfe->SubFormat,
>&KSDATAFORMAT_SUBTYPE_IEEE_FLOAT)
>> > +        || IsEqualIID(&pwfe->SubFormat,
>&KSDATAFORMAT_SUBTYPE_PCM));
>> > +
>> > +    /* Allocate the output closest format */
>> > +    WAVEFORMATEXTENSIBLE *pwfe_closest =
>> > +        CoTaskMemAlloc(sizeof(WAVEFORMATEXTENSIBLE));
>> > +    if (!pwfe_closest)
>> > +        return E_FAIL;
>> > +    WAVEFORMATEX *pwf_closest = &pwfe_closest->Format;
>> > +
>> > +    /* Setup the fallback arrays. There are 3 properties to check:
>the
>> > format, +     * the samplerate and the channels. There are maximum
>of 4
>> > formats to +     * check, 3 samplerates, and 2 channels
>configuration. So,
>> > that is a +     * maximum of 4x3x2=24 checks */
>> > +
>> > +    /* The format fallback order is dependent of the input format.
>We don't
>> > +     * want to use a high quality format when it's not needed but
>we
>> > prefer to +     * use a high quality format instead of a lower one
>*/
>> > +    static const uint16_t bits_pcm8_fallback[] =  {  8, 16, 24, 32
>};
>> > +    static const uint16_t bits_pcm16_fallback[] = { 16, 24, 32, 8 
>};
>> > +    static const uint16_t bits_pcm24_fallback[] = { 24, 32, 16, 8 
>};
>> > +    static const uint16_t bits_pcm32_fallback[] = { 32, 24, 16, 8 
>};
>> > +
>> > +    static const size_t bits_fallback_size =
>> > ARRAY_SIZE(bits_pcm8_fallback); +
>> > +    const uint16_t *bits_fallback;
>> > +    switch (pwf->wBitsPerSample)
>> > +    {
>> > +        case 64: /* fall through */
>> > +        case 32: bits_fallback = bits_pcm32_fallback; break;
>> > +        case 24: bits_fallback = bits_pcm24_fallback; break;
>> > +        case 16: bits_fallback = bits_pcm16_fallback; break;
>> > +        case 8:  bits_fallback = bits_pcm8_fallback;  break;
>> > +        default: vlc_assert_unreachable();
>> > +    }
>> > +
>> > +    /* Check the input samplerate, then 48kHz and 44.1khz */
>> > +    const uint32_t samplerate_fallback[] = {
>> > +        pwf->nSamplesPerSec,
>> > +        pwf->nSamplesPerSec == 48000 ? 0 : 48000,
>> > +        pwf->nSamplesPerSec == 44100 ? 0 : 44100,
>> > +    };
>> > +    const size_t samplerate_fallback_size =
>> > ARRAY_SIZE(samplerate_fallback); +
>> > +    /* Check the input number of channels, then stereo */
>> > +    const uint16_t channels_fallback[] = {
>> > +        pwf->nChannels,
>> > +        pwf->nChannels == 2 ? 0 : 2,
>> > +    };
>> > +    const size_t channels_fallback_size =
>ARRAY_SIZE(channels_fallback);
>> 
>> Really the only point in exclusive mode is bit exact. If you need to
>convert, 
>> there is really no point, especially if the sample rate and/or the
>channel 
>> mappings differ. This is waaaaay needlessly complicated.
>
>An other point of exclusive is to reduce the audio latency.
>
>> 
>> -- 
>> レミ・デニ-クールモン
>> http://www.remlab.net/
>> 
>> 
>> 
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