From eedb7d32009215288a777c32caaa02e81cc7aa9c Mon Sep 17 00:00:00 2001 From: David Reid Date: Mon, 27 Jan 2020 20:17:01 +1000 Subject: [PATCH] Add some documentation for channel conversion. --- research/ma_data_converter.h | 55 ++++++++++++++++++++++++++++++++++++ 1 file changed, 55 insertions(+) diff --git a/research/ma_data_converter.h b/research/ma_data_converter.h index e8f29281..536a31c5 100644 --- a/research/ma_data_converter.h +++ b/research/ma_data_converter.h @@ -5,6 +5,61 @@ #include "ma_resampler.h" +/************************************************************************************************************************************************************** + +Channel Conversion +================== +Channel conversion is used for channel rearrangement and conversion from one channel count to another. The `ma_channel_converter` API is used for channel +conversion. Below is an example of initializing a simple channel converter which converts from mono to stereo. + + ```c + ma_channel_converter_config config = ma_channel_converter_config_init(ma_format, 1, NULL, 2, NULL, ma_channel_mix_mode_default, NULL); + result = ma_channel_converter_init(&config, &converter); + if (result != MA_SUCCESS) { + // Error. + } + ``` + +To process perform the conversion simply call `ma_channel_converter_process_pcm_frames()` like so: + + ```c + ma_result result = ma_channel_converter_process_pcm_frames(&converter, pFramesOut, pFramesIn, frameCount); + if (result != MA_SUCCESS) { + // Error. + } + ``` + +It is up to the caller to ensure the output buffer is large enough to accomodate the new PCM frames. + +The only formats supported are ma_format_s16 and ma_format_f32. If you need another format you need to convert your data manually which you can do with +ma_pcm_convert(), etc. + +Input and output PCM frames are always interleaved. Deinterleaved layouts are not supported. + + +Channel Mapping +--------------- +In addition to converting from one channel count to another, like the example above, The channel converter can also be used to rearrange channels. When +initializing the channel converter, you can optionally pass in channel maps for both the input and output frames. If the channel counts are the same, and each +channel map contains the same channel positions with the exception that they're in a different order, a simple shuffling of the channels with be performed. If, +however, there is not a 1:1 mapping of channel positions, or the channel counts differ, the input channels will be mixed based on a mixing +mode which is specified when initializing the `ma_channel_converter_config` object. + +When converting from mono to multi-channel, the mono channel is simply copied to each output channel. When going the other way around, the audio of each output +channel is simply averaged and copied to the mono channel. + +In more complicated cases blending is used. The `ma_channel_mix_mode_simple` mode will drop excess channels and silence extra channels. For example, converting +from 4 to 2 channels, the 3rd and 4th channels will be dropped, whereas converting from 2 to 4 channels will put silence into the 3rd and 4th channels. + +The `ma_channel_mix_mode_rectangle` mode uses spacial locality based on a rectangle to compute a simple distribution between input and output. Imagine sitting +in the middle of a room, with speakers on the walls representing channel positions. The MA_CHANNEL_FRONT_LEFT position can be thought of as being in the corner +of the front and left walls. + +Finally, the `ma_channel_mix_mode_custom_weights` mode can be used to use custom user-defined weights. Custom weights can be passed in as the last parameter of +`ma_channel_converter_config_init()`. + +**************************************************************************************************************************************************************/ + typedef struct { ma_format format;