XMOS xCORE-200 Multi-channel Audio board Design Manual page 49

USB Audio Design Guide 49/110
Standard DSD has a sample size of 1 bit and a sample rate of 2.8224MHz - this is
64x the speed of CD. This equates to the same data-rate as a 16 bit PCM stream at
176.4kHz.
In order to clearly identify when this PCM stream contains DSD and when it contains
PCM some header bits are added to the sample. A 24-bit PCM stream is therefore
used, with the most significant byte being used for a DSD marker (alternating 0x05
and 0xFA values).
When enabled, if USB audio design detects a un-interrupted run of these samples
(above a defined threshold) it switches to DSD mode, using the lower 16-bits as
DSD sample data. When this check for DSD headers fails the design falls back to
PCM mode. DoP detection and switching is done completely in the Audio/I2S core
(audio.xc). All other code handles the audio samples as PCM.
The design supports higher DSD/DoP rates (i.e. DSD128) by simply raising the
underlying PCM sample rate e.g. from 176.4kHz to 352.8kHz. The marker byte
scheme remains exactly the same regardless of rate.
DoP requires bit-perfect transmission - therefore any audio/volume processing will
break the stream.
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