Re: PCM hw mixing with volume

[Manuel Jander <manuel.jander@mat.utfsm.cl>]

Hi Ove,

On Wed, 2004-03-10 at 10:16, Ove Kaaven wrote:
> > Please start writing what the API should provide.
> 
> Well, the requirements that raised this thread should be fairly clear.
> For example,
> 
> ALTERNATIVE 1
> 
> snd_pcm_set_volume(snd_pcm_t* pcm, int volume)
> 
> and
> 
> snd_pcm_set_pan(snd_pcm_t* pcm, int pan)
> 
> using whatever value range makes the most sense, and perhaps some query
> on whether these controls are available from the pcm info. It's
> acceptable to have to explicitly ask for these controls using hw_params.
> The plug plugin could insert the route/volume plugin if these controls
> are asked for.

I don't like this. We would end with tons of different functions.

> ALTERNATIVE 2
> 
> snd_pcm_set_volume(snd_pcm_t* pcm, int vol_left, int vol_right)
> 
> but I don't expect this to be useful, since there are probably some
> oddball devices out there that aren't able to control left and right
> volume independently like this.

Same as Alternative 1...

> ALTERNATIVE 3
> 
> Let each PCM channel have its own mixer control, like the EMU10K1
> currently do. That is, use snd_pcm_info_get_subdevice() to get the
> index, then look up a volume control with a well-known name and the
> given index using the snd_ctl API to control the volume of that PCM
> stream. The name and semantics of that mixer control would have to be
> standardized and be the same on all devices that are capable of this
> feature.

This seems to me the best approach. But the control name space must be
choosen well.

> Whatever way is chosen, it's simple enough to work with.
> 
> But if we get into the realm of spatial sound, it can get trickier, of
> course. I might suggest to create a new snd_pcm_fx_params_t or
> something, which can be loaded into a PCM stream to change the volume,
> pan, or other effect. The available effects should depend on which was
> requested in hw_params. Specifically, the following effects would be
> desirable:
> 
> * Volume
> * Pan (not available if 3D sound is requested)
> * Frequency (can be used for software-calculated doppler shift, or for
> "wavetable synthesis"-like purposes)
> * 3D position (relative to listener)
> * Velocity (for hardware-calculated doppler shift)
> * Cone inside/outside angles, direction, outside volume

The above parameters can be handled by ALSA directly right now (not
implemented yet of course). Indeed i'm implementing such controls for
the aureal driver right now.

> * Min/max distance (wherever the sound is heard)

This parameters require resource management, and ALSA does not yet
provide that. I'm not sure if that can be done inside ALSA.

> There are also settings that can be considered "global" for all sounds
> in the game, but I'm not sure how they could really be considered such
> using the PCM API, so it's acceptable to have to go through every PCM
> stream and update the parameters of each one, if that's easier to
> implement. If not, there must be a way to attach each 3D-aware PCM
> stream to some kind of master structure which can hold these parameters.
> This master can be a designated PCM stream with special hw_params.
> Global settings include:
> 
> * 3D position, orientation (forward and upward directions), and velocity
> of listener, to allow streams to use absolute, not relative,
> coordinates, if they want to
> * Distance, rolloff, and doppler factors (I don't remember the exact
> semantics of these, but can look it up)

> These will be required anyway for full HW-accelerated OpenAL support, of
> course.

Yeah, I want full OpenAL too. Lets do it !

> The API should also be designed to allow for further parametrized
> effects to be applied to a particular stream, for example reverb
> effects. For what it's worth, DirectSound has currently defined Gargle,
> Chorus, Flanger, Echo, Distortion, Compressor, and a couple of Reverb
> types...

Best Regards

Manuel Jander



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