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Re: [fluid-dev] API design: fluid_synth_process()

From: Tom M.
Subject: Re: [fluid-dev] API design: fluid_synth_process()
Date: Wed, 02 May 2018 16:40:27 +0200

> fluid_synth_process() behaves like fluid_synth_nwrite_float()

Except for the fact that it doesnt handle fx channels.

> Consequently nout and out array must be set to sufficient size (nout >= 2 x 
> synth->audio_channels).

Not necessarily. I think fluid_synth_process() should also be usable for simple 
stereo mixing, i.e. nout == 2. In fact I believe that no matter how many output 
buffers the user calls it with, fluid_synth_process should always render all 
playing voices to those buffers (provided that nout >= 2).

The channel layout indeed can be used by fluidsynths internal rendering engine. 
I'd suggest to use / extend the current calling convention. As you already said 
"out" currently contains an array of planar buffers for normal, dry, stereo 
audio (alternating left and right). Like:

out[0]  = left_buffer_channel_1
out[1]  = right_buffer_channel_1
out[2]  = left_buffer_channel_2
out[3]  = right_buffer_channel_2
out[ i*2 + 0 ]  = left_buffer_channel_i
out[ i*2 + 1 ]  = right_buffer_channel_i

where 0 <= i < fluid_synth_count_audio_channels()

Just following to those "normal" audio buffers we could place the effects 
buffers like:

out [ fluid_synth_count_audio_channels() + 0 ] = left_buffer_fxchannel_1 
(currently hardcoded to reverb)
out [ fluid_synth_count_audio_channels() + 1 ] = right_buffer_fxchannel_1 
(currently reverb)
out [ fluid_synth_count_audio_channels() + 2 ] = left_buffer_fxchannel_2 
(currently chorus)
out [ fluid_synth_count_audio_channels() + 3 ] = right_buffer_fxchannel_2 
(currently chorus)

out [ fluid_synth_count_audio_channels() + k*2 + 0 ] = left_buffer_fxchannel_k 
(if this will ever be added)
out [ fluid_synth_count_audio_channels() + k*2 + 1 ] = right_buffer_fxchannel_k

where 0 <= k < fluid_synth_count_effects_channels()

If surround audio would ever be implemented, channel layout could look like:

out[ i*5 + 0 ]  = left_front_buffer_channel_i
out[ i*5 + 1 ]  = right_front_buffer_channel_i
out[ i*5 + 2 ]  = center_front_buffer_channel_i
out[ i*5 + 3 ]  = left_rear_buffer_channel_i
out[ i*5 + 4 ]  = right_rear_buffer_channel_i

We could directly use this channel layout for rvoice_mixer internally, provided 
that the requested number of audio frames to synthesize is a multiple of 
fluid_synth_get_internal_bufsize() so we dont have to hold any temporary audio 

Disadvantage: Not very user friendly. There is no clear separation between dry 
and effects channels anymore, like done in fluid_synth_nwrite_float(). This 
will lead to ambiguous situations: if the user calls fluid_synth_process() with 
4 output buffers, is he requesting the first two normal stereo audio channels, 
or one stereo channel and one effects channel? We could (ab)use the "in" 
parameter for effect buffers to avoid this ambiguity, unless someone sees a 
better use-case for the "in" param?


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