The libasound library supports devices with up to eight voices.
Configuration of the libasound library is based on the maximum number of voices supported in hardware. If the numbers of source and destination voices are different, then snd_pcm_plugin_params() instantiates a voice converter.
The default voice conversion behavior is as follows:
From | To | Conversion |
---|---|---|
Mono | Stereo | Replicate channel 1 (left) to channel 2 (right) |
Stereo | Mono | Average channels 1 and 2 |
Mono | 4-channel | Replicate channel 1 to all other channels |
Stereo | 4-channel | Replicate channel 1 (front left) to channel 3 (rear left), and channel 2 (front right) to channel 4 (rear right) |
You can use the voice conversion API to configure the conversion behavior and place any source channel in any destination channel slot:
The actual conversion is controlled by the snd_pcm_voice_conversion_t structure, which is defined as follows:
typedef struct snd_pcm_voice_conversion { uint32_t app_voices; uint32_t hw_voices; uint32_t matrix[32]; } snd_pcm_voice_conversion_t
The matrix member forms a 32-by-32-bit array that specifies how to convert the voices. The array is ranked with rows representing application voices, voice 0 first; the columns represent hardware voices, with the low voice being LSB-aligned and increasing right to left.
For example, consider a mono application stream directed to a 4-voice hardware device. A bit array of:
matrix[0] = 0x1; // 00000001
causes the sound to be output on only the first hardware channel. A bit array of:
matrix[0] = 0x9; // 00001001
causes the sound to appear on the first and last hardware channel.
Another example would be a stereo application stream to a 6-channel (5.1) output device. A bit array of:
matrix[0] = 0x1; // 00000001 matrix[1] = 0x2; // 00000010
causes the sound to appear on only the front two channels, while:
matrix[0] = 0x5; // 00000101 matrix[1] = 0x2; // 00000010
causes the stream signal to appear on the first four channels—likely the front and rear pairs, but not on the center or low-frequency effects (LFE) channels. The bitmap used to describe the hardware (i.e., the columns) depends on the hardware, and you need to be mindful of the actual hardware you'll be running on to properly map the channels. For example:
If you call snd_pcm_plugin_get_voice_conversion() before the voice conversion plugin has been instantiated, the function fails and returns -ENOENT. In QNX Neutrino 6.6 or later, snd_pcm_plugin_set_voice_conversion() instantiates the plugin if it doesn't already exist.