Multiplexer/Demultiplexer

Introduction

The multiplexer/demultiplexer component copies its input audio channels into output audio channels according to a specific routing matrix. Multiplexer has multiple input audio streams and a single audio output stream. Demultiplexer has a single input stream and multiple output streams. In the SOF codebase, multiplexer and demultiplexer are implemented in a single component as the operations and configurations overlap heavily.

../../_images/muxdemux.png

Figure 19 Multiplexer has exactly 1 output stream and demultiplexer has exactly 1 input stream.

Configuration

The component configuration defines how audio channels are copied from input to output stream. As the ASoC/SOF audio stream can have up to 8 audio channels, a stream-to-stream specific 8 x 8 routing matrix defines the channel mapping from input to output. Because every stream is fully configurable, we have a matrix for all multiplexer input streams or all demultiplexer output streams. The 8 x 8 binary matrix takes up to 64 bits and is controlled with eight unsigned char values.

../../_images/mux.png

Figure 20 Example of multiplexer configuration matrices with 2 input streams. In this artificial mux example, Input stream 1’s channel 1 is copied to both output channels; Input stream 2’s single channel is copied to output channel 1.

../../_images/demux.png

Figure 21 Example of demultiplexer configuration matrices with 2 output streams. In this artificial demux example, Input stream’s channel 1 is copied to all output streams channels (1 channel to 4 identical channels in 2 output streams).

Using a routing matrix means also that this component can leave out channels or mix multiple channels into one.

Topology

Previous figures show that the routing matrix is difficult to parametrize in order to be easily understandable. As it is sent to firmware with 64 bits, it is quite tedious to easily see the binary routings from hexadecimal or integer values. SOF topology m4 macros have helpers to “visualize” the matrix for easier configuration.

The following example from pipe-volume-demux-playback.m4 shows how to define 2 routing matrices and a demux component:

# pipeline_id, channels, matrix_rows
define(matrix1, `ROUTE_MATRIX(PIPELINE_ID, 2,
                             `BITS_TO_BYTE(1, 0, 0 ,0 ,0 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 1, 0 ,0 ,0 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 1 ,0 ,0 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,1 ,0 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,0 ,1 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,0 ,0 ,1 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,0 ,0 ,0 ,1 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,0 ,0 ,0 ,0 ,1)')')

# pipeline_id, channels, matrix_rows
define(matrix2, `ROUTE_MATRIX(5, 2,
                             `BITS_TO_BYTE(1, 0, 0 ,0 ,0 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 1, 0 ,0 ,0 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 1 ,0 ,0 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,1 ,0 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,0 ,1 ,0 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,0 ,0 ,1 ,0 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,0 ,0 ,0 ,1 ,0)',
                             `BITS_TO_BYTE(0, 0, 0 ,0 ,0 ,0 ,0 ,1)')')

# frame_format, num_channels, num_streams, route_matrix
MUXDEMUX_CONFIG(demux_priv, 2, 2, 2, LIST(`     ', `matrix1,', `matrix2'))

# demux Bytes control with max value of 255
C_CONTROLBYTES(DEMUX, PIPELINE_ID,
        CONTROLBYTES_OPS(bytes, 258 binds the mixer control to bytes get/put handlers, 258, 258),
        CONTROLBYTES_EXTOPS(258 binds the mixer control to bytes get/put handlers, 258, 258),
        , , ,
        CONTROLBYTES_MAX(, 304),
        ,
        demux_priv)

# Mux 0 has 2 sink and source periods.
W_MUXDEMUX(0, 1, PIPELINE_FORMAT, 2, 2, LIST(`         ', "DEMUX"))

In the above example you can see that the routing matrices have only “diagonal” 1’s, which means that input stream’s channels are copied to corresponding output streams channels.

ALSA control

Multiplexer configuration is loaded in the kernel/firmware boot as part of the ALSA binary control in topology, but can be also controlled through ALSA controls.

The complex binary control blob can be created with a generic python tool:

python sof_gen_blob.py -a 3 14 0 -t 18 -m 3H I 1B 8B 3B I 1B 8B 3B -v "2 2 2" "1" "2" "1 2 4 8 16 32 64 128" "0 0 0" "5" "1" "1 1 4 8 16 32 64 128" "0 0 0"

It produces the following output:

sof m4 and ALSA conf format:
`       bytes "0x53,0x4f,0x46,0x00,0x12,0x00,0x00,0x00,0x3c,'
`       0x00,0x00,0x00,0x00,0xe0,0x00,0x03,0x00,'
`       0x00,0x00,0x00,0x02,0x00,0x02,0x00,0x02,'
`       0x00,0x00,0x00,0x01,0x00,0x00,0x00,0x02,'
`       0x01,0x02,0x04,0x08,0x10,0x20,0x40,0x80,'
`       0x00,0x00,0x00,0x05,0x00,0x00,0x00,0x01,'
`       0x01,0x01,0x04,0x08,0x10,0x20,0x40,0x80,'
`       0x00,0x00,0x00,'

sof ctl tool format:
(4607827, 18, 60, 50388992, 0, 2, 2, 2, 1, 2, 1, 2, 4, 8, 16, 32, 64, 128, 0, 0, 0, 5, 1, 1, 1, 4, 8, 16, 32, 64, 128, 0, 0, 0)

The sof-ctl tool can be then used to set the parameters through ALSA control:

sof-ctl -Dhw:0 -n 22 -s demux_coeffs.txt