Pulse Code Modulation audio mixer

The Pulse Code Modulation (PCM) audio mixer lets you mix a PCM stream into another PCM stream. It can for example be useful for mixing a tone and an audio stream together. This library is useful for developing applications that offer audio features, for example using the nRF5340 Audio DK.

The library offers the following types of mixing support:

  • Stereo stream into another stereo stream

  • Combinations of mono to mono

  • Mono to stereo: channel left or right or left+right

Configuration

To enable the library, set the CONFIG_PCM_MIX Kconfig option to y in the project configuration file prj.conf.

API documentation

Header file: include/pcm_mix.h
Source file: lib/pcm_mix/pcm_mix.c
group pcm_mix

Pulse Code Modulation audio mixer library.

Enums

enum pcm_mix_mode

Values:

enumerator B_STEREO_INTO_A_STEREO
enumerator B_MONO_INTO_A_MONO
enumerator B_MONO_INTO_A_STEREO_LR
enumerator B_MONO_INTO_A_STEREO_L
enumerator B_MONO_INTO_A_STEREO_R

Functions

int pcm_mix(void *const pcm_a, size_t size_a, void const *const pcm_b, size_t size_b, enum pcm_mix_mode mix_mode)

Mixes two buffers of PCM data.

Note

Uses simple addition with hard clip protection. Input can be mono or stereo as long as the inputs match. By selecting the mix mode, mono can also be mixed into a stereo buffer. Hard coded for the signed 16-bit PCM.

Parameters

  • pcm_a – [in/out] Pointer to the PCM data buffer A.

  • size_a – [in] Size of the PCM data buffer A (in bytes).

  • pcm_b – [in] Pointer to the PCM data buffer B.

  • size_b – [in] Size of the PCM data buffer B (in bytes).

  • mix_mode – [in] Mixing mode according to pcm_mix_mode.

Return values

  • 0 – Success. Result stored in pcm_a.

  • -EINVAL – pcm_a is NULL or size_a = 0.

  • -EPERM – Either size_b < size_a (for stereo to stereo, mono to mono) or size_a/2 < size_b (for mono to stereo mix).

  • -ESRCH – Invalid mixing mode.