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# Audio stream formats

The design currently supports up to three different stream formats for playback, selectable at run time. This is implemented using standard Alternative Settings to the Audio Streaming interfaces.

An Audio Streaming interface can have Alternate Settings that can be used to change certain characteristics of the interface and underlying endpoint. A typical use of Alternate Settings is to provide a way to change the subframe size and/or number of channels on an active Audio Streaming interface. Whenever an Audio Streaming interface requires an isochronous data endpoint, it must at least provide the default Alternate Setting (Alternate Setting 0) with zero bandwidth requirements (no isochronous data endpoint defined) and an additional Alternate Setting that contains the actual isochronous data endpoint. This zero bandwidth alternative setting 0 is always implemented by the design.

For further information refer to 3.16.2 of USB Audio Device Class Definition for Audio Devices, release 2.0

Customisable parameters for the Alternate Settings provided by the design are as follows.:

- Audio sample resolution

- Audio sample subslot size

- Audio data format

Note

Currently only a single format is supported for the recording stream

By default the design exposes two sets of Alternative Settings for the playback Audio Streaming interface, one for 16-bit and another for 24-bit playback. When DSD is enabled an additional (32-bit) alternative is exposed.

## Audio subslot

An audio subslot holds a single audio sample. See USB Device Class Definition for Audio Data Formats for full details. This is represented by bSubslotSize in the devices descriptor set.

An audio subslot always contains an integer number of bytes. The specification limits the possible audio subslot size to 1, 2, 3 or 4 bytes per audio subslot.

Since the xcore is a 32-bit machine the value 4 is typically used for bSubSlot - this means that packing/unpacking samples to/from packets is computationally trivial. Other values can, however, be used and the values 4, 3 and 2 are supported.

Values other than 4 may be used for the following reasons:

- Bus-bandwidth needs to be efficiently utilised. For example maximising channel-count/sample-rates in full-speed operation.

- To support restrictions with certain hosts. For example, historically many Android based hosts support only 16 bit samples in a 2-byte subslot.

bSubSlot size is set using the following defines:

- When running in high-speed:

  - HS_STREAM_FORMAT_OUTPUT_1_SUBSLOT_BYTES

  - HS_STREAM_FORMAT_OUTPUT_2_SUBSLOT_BYTES

  - HS_STREAM_FORMAT_OUTPUT_3_SUBSLOT_BYTES

- When running in full-speed:

  - FS_STREAM_FORMAT_OUTPUT_1_SUBSLOT_BYTES

  - FS_STREAM_FORMAT_OUTPUT_2_SUBSLOT_BYTES

  - FS_STREAM_FORMAT_OUTPUT_3_SUBSLOT_BYTES

## Audio sample resolution

An audio sample is represented using a number of bits (bBitResolution) less than or equal to the number of total bits available in the audio subslot i.e. bBitResolution <= bSubslotSize * 8). The design supports values 16, 24 and 32.

bBitResolution is set using the following defines:

- When operating at high-speed:

  - HS_STREAM_FORMAT_OUTPUT_1_RESOLUTION_BITS

  - HS_STREAM_FORMAT_OUTPUT_2_RESOLUTION_BITS

  - HS_STREAM_FORMAT_OUTPUT_3_RESOLUTION_BITS

- When operating at full-speed:

  - FS_STREAM_FORMAT_OUTPUT_1_RESOLUTION_BITS

  - FS_STREAM_FORMAT_OUTPUT_2_RESOLUTION_BITS

  - FS_STREAM_FORMAT_OUTPUT_3_RESOLUTION_BITS

## Audio format

The design supports two audio formats, PCM and, when “Native” DSD is enabled, Direct Stream Digital (DSD). A DSD capable DAC is required for the latter.

The USB Audio Raw Data format is used to indicate DSD data (2.3.1.7.5 of USB Device Class Definition for Audio Data Formats). This use of a RAW/DSD format in an alternative setting is termed by XMOS as Native DSD

The following defines affect both full-speed and high-speed operation:

- STREAM_FORMAT_OUTPUT_1_DATAFORMAT

- STREAM_FORMAT_OUTPUT_2_DATAFORMAT

- STREAM_FORMAT_OUTPUT_3_DATAFORMAT

The following options are supported:

- UAC_FORMAT_TYPEI_RAW_DATA

- UAC_FORMAT_TYPEI_PCM

Note

Currently DSD is only supported on the output/playback stream

Note

4 byte slot size with a 32 bit resolution is required for RAW/DSD format

Native DSD requires driver support and is available in the Thesycon Windows driver via ASIO.