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# Digital mixer

The Mixer thread(s) take outgoing audio from the Decouple thread and incoming audio from the Audio Hub thread. It then applies the volume to each channel and passes incoming audio on to Decouple and outgoing audio to Audio Hub. The volume update is achieved using the built-in 32bit to 64bit signed multiply-accumulate function (`macs`). The mixer is implemented in the file `mixer.xc`.

The mixer takes (up to) two threads and can perform eight mixes with up to 18 inputs at sample rates up to 96kHz and two mixes with up to 18 inputs at higher sample rates. The component automatically reverts to generating two mixes when running at the higher rate.

The mixer can take inputs from either:

- The USB outputs from the host—these samples come from the Decouple thread.

- The inputs from the audio interfaces on the device—these samples come from the Audio Hub thread and includes samples from digital input streams.

Since the sum of these inputs may be more than the 18 possible mix inputs to each mixer, there is a mapping from all the possible inputs to the mixer inputs.

After the mix occurs, the final outputs are created. There are two possible output destinations for each mix.

- The USB inputs to the host—these samples are sent to the Decouple thread.

- The outputs to the audio interface on the device—these samples are sent to the Audio Hub thread

For each possible output from the device, a mapping exists to inform the mixer what its source is. The possible sources are the output from the USB host, the inputs from the Audio Hub thread or the outputs from the mixes.

Essentially the mixer/router can be configured such that any device input can be used as an input to any mix or routed directly to any device output. Additionally, any device output can be derived from any mixer output or any device input.

As mentioned in Audio Request: Volume control, the mixer can also handle processing of volume controls. If the mixer is configured to handle volume but the number of mixes is set to zero (such that the thread is solely doing volume setting) then the component will use only one thread. This is sometimes a useful configuration for large channel count devices since it offloads volume processing from the buffering sub-system.

A sequence diagram showing the communication between Audio Hub, Decouple and mixer threads is shown in Fig. 5. `mixer1` thread exchanges data with Decouple and Audio Hub along with any volume control operations and performs the mixing operations for the even output channel numbers. The mixing for the odd channels is offloaded to the `mixer2` thread.

> Fig. 5 Mixer communication sequence diagram

The mixer can also be configured in passthrough mode (`MAX_MIX_COUNT` = 0), as shown in Fig. 6. In this mode, the `mixer2` thread is not present and the `mixer1` exchanges data with Audio Hub and Decouple along with any volume control operations without doing any actual mixing.

> Fig. 6 Mixer in passthrough mode

## Control

The mixer tasks can receive the control commands from the host via USB Control Requests to Endpoint 0. The Endpoint 0 thread relays these to the Mixer threads(s) via a channel (`c_mix_ctl`). These commands are described in Table 34.

Table 34 Mixer control commands
Command | Description
--- | ---
`SET_SAMPLES_TO_HOST_MAP` | Sets the source of one of the audio streams going to the host.
`SET_SAMPLES_TO_DEVICE_MAP` | Sets the source of one of the audio streams going to the audio driver.
`SET_MIX_MULT` | Sets the multiplier for one of the inputs to a mixer.
`SET_MIX_MAP` | Sets the source of one of the inputs to a mixer.
`SET_MIX_IN_VOL` | If volume adjustment is being done in the mixer, this command sets the volume multiplier of one of the USB audio inputs.
`SET_MIX_OUT_VOL` | If volume adjustment is being done in the mixer, this command sets the volume multiplier of one of the USB audio outputs.

## Host control

The mixer can be controlled from a host PC by sending requests to Endpoint 0. XMOS provides a simple command line based sample application demonstrating how the mixer can be controlled. This is intended as an example of how you might add mixer control to your own control application. It is not intended to be exposed to end users.

For details, consult the README file in the host_usb_mixer_control directory. A list of arguments can also be seen with:

    $ ./xmos_mixer --help

The main requirements of this control utility are to

- Set the mapping of input channels into the mixer

- Set the coefficients for each mixer output for each input

- Set the mapping for physical outputs which can either come directly from the inputs or via the mixer.

Note

The flexibility within this configuration space is such that there are often multiple ways of producing the desired result. Product developers may only want to expose a subset of this functionality to their end users.

Whilst using the XMOS host control example application, consider the example of setting the mixer to perform a loop-back from analogue inputs 1 & 2 to analogue outputs 1 & 2.

Note

The command outputs shown are examples; the actual output will depend on the mixer configuration.

The following will show the index for each device output along with which channel is currently mapped to it. In this example the analogue outputs 1 & 2 are 0 & 1 respectively:

    $ ./xmos_mixer --display-aud-channel-map

      Audio Output Channel Map
      ------------------------

    0 (DEVICE OUT - Analogue 1) source is  0 (DAW OUT - Analogue 1)
    1 (DEVICE OUT - Analogue 2) source is  1 (DAW OUT - Analogue 2)
    2 (DEVICE OUT - SPDIF 1) source is  2 (DAW OUT - SPDIF 1)
    3 (DEVICE OUT - SPDIF 2) source is  3 (DAW OUT - SPDIF 2)
    $ _

The DAW Output Map can be seen with:

    $ ./xmos_mixer --display-daw-channel-map

      DAW Output To Host Channel Map
      ------------------------

    0 (DEVICE IN - Analogue 1) source is  4 (DEVICE IN - Analogue 1)
    1 (DEVICE IN - Analogue 2) source is  5 (DEVICE IN - Analogue 2)
    $ _

Note

In both cases, by default, these bypass the mixer.

The following command will list the channels which can be mapped to the device outputs from the Audio Output Channel Map. Note that, in this example, analogue inputs 1 & 2 are source 4 & 5 and Mix 1 & 2 are source 6 & 7:

    $ ./xmos_mixer --display-aud-channel-map-sources

      Audio Output Channel Map Source List
      ------------------------------------

    0 (DAW OUT - Analogue 1)
    1 (DAW OUT - Analogue 2)
    2 (DAW OUT - SPDIF 1)
    3 (DAW OUT - SPDIF 2)
    4 (DEVICE IN - Analogue 1)
    5 (DEVICE IN - Analogue 2)
    6 (MIX - Mix 1)
    7 (MIX - Mix 2)
    $ _

Using the indices from the previous commands, we will now re-map the first two mixer channels (Mix 1 & Mix 2) to device outputs 1 & 2:

    $ ./xmos_mixer --set-aud-channel-map 0 6
    $ ./xmos_mixer --set-aud-channel-map 1 7
    $ _

The effect of this can be confirmed by re-checking the map:

    $ ./xmos_mixer --display-aud-channel-map

      Audio Output Channel Map
      ------------------------

    0 (DEVICE OUT - Analogue 1) source is  6 (MIX - Mix 1)
    1 (DEVICE OUT - Analogue 2) source is  7 (MIX - Mix 2)
    2 (DEVICE OUT - SPDIF 1) source is  2 (DAW OUT - SPDIF 1)
    3 (DEVICE OUT - SPDIF 2) source is  3 (DAW OUT - SPDIF 2)
    $ _

Analogue outputs 1 & 2 are now derived from the mixer, rather than directly from USB. However, since the mixer is mapped, by default, to just pass the USB channels through to the outputs no functional change will be observed.

The mixer nodes need to be individually set. The nodes in `mixer_id` 0 can be displayed with the following command:

    $ ./xmos_mixer --display-mixer-nodes 0

      Mixer Values (0)
      ----------------

                           Mixer outputs
                                    1              2
      DAW - Analogue 1       0:[0000.000]   1:[  -inf  ]
      DAW - Analogue 2       2:[  -inf  ]   3:[0000.000]
      DAW - SPDIF 1          4:[  -inf  ]   5:[  -inf  ]
      DAW - SPDIF 2          6:[  -inf  ]   7:[  -inf  ]
      AUD - Analogue 1       8:[  -inf  ]   9:[  -inf  ]
      AUD - Analogue 2      10:[  -inf  ]  11:[  -inf  ]
    $ _

Note

The USB audio reference design has only one unit so the `mixer_id` argument should always be 0.

With mixer outputs 1 & 2 mapped to device outputs analogue 1 & 2; to get the audio from the analogue inputs to device outputs mixer_id 0 node 8 and node 11 need to be set to 0db:

    $ ./xmos_mixer --set-value 0 8 0
    $ ./xmos_mixer --set-value 0 11 0
    $ _

At the same time, the original mixer outputs can be muted:

    $ ./xmos_mixer --set-value 0 0 -inf
    $ ./xmos_mixer --set-value 0 3 -inf
    $ _

Now audio inputs on analogue 1 and 2 should be heard on outputs 1 and 2 respectively.

As mentioned above, the flexibility of the mixer is such that there will be multiple ways to create a particular mix. Another option to create the same routing would be to change the mixer sources such that mixer outputs 1 and 2 come from the analogue inputs 1 and 2.

To demonstrate this, the changes documented above should be undone (resetting the device will yield the same result):

    $ ./xmos_mixer --set-value 0 8 -inf
    $ ./xmos_mixer --set-value 0 11 -inf
    $ ./xmos_mixer --set-value 0 0 0
    $ ./xmos_mixer --set-value 0 3 0
    $ _

The mixer should now have the default values. The sources for mixer 0 output 1 and 2 can now be changed using indices from the Audio Output Channel Map Source list:

    $ ./xmos_mixer --set-mixer-source 0 0 4

       Set mixer(0) input 0 to device input 4 (AUD - Analogue 1)
    $ ./xmos_mixer --set-mixer-source 0 1 5

       Set mixer(0) input 1 to device input 5 (AUD - Analogue 2)
    $ _

Re-running the following command will show that the first column now has “AUD - Analogue 1 and 2” rather than “DAW (Digital Audio Workstation i.e. the host) - Analogue 1 and 2” confirming the new mapping. Again, by playing audio into analogue inputs 1/2 this can be heard looped through to analogue outputs 1/2:

    $ ./xmos_mixer --display-mixer-nodes 0