forked from PAWPAW-Mirror/lib_xua
101 lines
2.9 KiB
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101 lines
2.9 KiB
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S/PDIF Receive
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==============
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XMOS devices can support S/PDIF receive up to 192kHz - see ``lib_spdif`` for full specifications.
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The S/PDIF receiver module uses a clock-block and a buffered one-bit port.
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The clock-block is divided of a 100 MHz reference clock. The one bit port is buffered to 4-bits.
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The receiver code uses this clock to over sample the input data.
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The receiver outputs audio samples over a *streaming channel end* where data can be input using the
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built-in input operator. ``lib_spdif`` also provides API functions that wrap up this communication.
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The S/PDIF receive function never returns. The 32-bit value from the channel input comprises:
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.. list-table:: S/PDIF RX Word Structure
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:header-rows: 1
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:widths: 10 32
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* - Bits
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-
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* - 0:3
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- A tag (see below)
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* - 4:28
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- PCM encoded sample value
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* - 29:31
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- User bits (parity, etc)
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The tag has one of three values:
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.. list-table:: S/PDIF RX Tags
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:header-rows: 1
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:widths: 10 32
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* - Tag
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- Meaning
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* - FRAME\_X
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- Sample on channel 0 (Left for stereo)
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* - FRAME\_Y
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- Sample on another channel (Right if for stereo)
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* - FRAME\_Z
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- Sample on channel 0 (Left), and the first sample of a frame; can be used if the user bits need to be reconstructed.
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See S/PDIF, IEC 60958-3:2006, specification for further details on format, user bits etc.
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Usage and Integration
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---------------------
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Since S/PDIF is a digital steam the devices master clock must be synchronised to it. This is typically
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done with an external device. See `Clock Recovery` (:ref:`usb_audio_sec_clock_recovery`).
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.. note::
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Due to the requirement for this clock recovery S/PDIF receive can only be used in Asynchronous
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mode.
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The S/PDIF receive function communicates with the Clock Gen core, which in turn passes audio data to the
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Audio Hub core. The Clock Gen core also handles locking to the S/PDIF clock source
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(see :ref:`usb_audio_sec_clock_recovery`).
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Ideally the parity of each word/sample received should be checked. This is done using the built in
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``crc32`` function (see ``xs1.h``):
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.. literalinclude:: lib_xua/src/core/clocking/clockgen.xc
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:start-after: //:badParity
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:end-before: //:
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If bad parity is detected the word/sample is ignored, otherwise the tag is inspected for channel
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(i.e. left or right) and the sample stored.
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The following code snippet illustrates how the output of the S/PDIF receive component could is used::
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while(1)
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{
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c_spdif_rx :> data;
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if(badParity(data)
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continue;
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tag = data & 0xF;
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/* Extract 24bit audio sample */
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sample = (data << 4) & 0xFFFFFF00;
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switch(tag)
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{
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case FRAME_X:
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case FRAME_X:
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// Store left
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break;
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case FRAME_Z:
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// Store right
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break;
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}
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}
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The Clock Gen core stores samples in a small FIFO before they are communicated to the Audio Hub core.
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