S/PDIF transmit

lib_xua supports the development of devices with S/PDIF transmit through the use of lib_spdif. The XMOS S/SPDIF transmitter component runs in a single thread and supports sample-rates upto 192 kHz.

The S/PDIF transmitter thread takes PCM audio samples via a channel and outputs them in S/PDIF format to a port. A lookup table is used to encode the audio data into the required format.

It receives samples from the Audio I/O thread two at a time (for left and right). For each sample, it performs a lookup on each byte, generating 16 bits of encoded data which it outputs to a port.

S/PDIF sends data in frames, each containing 192 samples of the left and right channels.

Audio samples are encapsulated into S/PDIF words (adding preamble, parity, channel status and validity bits) and transmitted in biphase-mark encoding (BMC) with respect to an external master clock.

Table 35 S/PDIF capabilities

Sample frequencies	44.1, 48, 88.2, 96, 176.4, 192 kHz
Master clock ratios	128x, 256x, 512x
Library	lib_spdif

Clocking

Fig. 10 D-Type jitter reduction

The S/PDIF signal is output at a rate dictated by the external master clock. The master clock must be 1x 2x or 4x the BMC bit rate (that is 128x 256x or 512x audio sample rate, respectively). For example, the minimum master clock frequency for 192kHz is therefore 24.576 MHz.

This resamples the master clock to its clock domain (oscillator), which introduces jitter of 2.5-5 ns on the S/PDIF signal. A typical jitter-reduction scheme is an external D-type flip-flop clocked from the master clock (as shown in Fig. 9).

Usage

The interface to the S/PDIF transmitter thread is via a normal channel using streaming builtin functions (outuint, inuint). Data format should be 24-bit left-aligned in a 32-bit word: 0x12345600

The following protocol is used on the channel:

Table 36 S/PDIF Component Protocol

outct	New sample rate command
outuint	Sample frequency (Hz)
outuint	Master clock frequency (Hz)
outuint	Left sample
outuint	Right sample
outuint	Left sample
outuint	Right sample
...
...

This communication is wrapped up in the API functions provided by lib_spdif.

Output stream structure

The stream is composed of words with the structure shown in Table 37. The channel status bits are 0x0nc07A4, where c=1 for left channel, c=2 for right channel and n indicates sampling frequency as shown in Table 38.

Table 37 S/PDIF stream structure

Bits
0:3	Preamble	Correct B M W order, starting at sample 0
4:27	Audio sample	Top 24 bits of given word
28	Validity bit	Always 0
29	Subcode data (user bits)	Unused, set to 0
30	Channel status	See below
31	Parity	Correct parity across bits 4:30

Table 38 Channel status bits

Frequency (kHz)	n
44.1	0x0
48	0x2
88.2	0x8
96	0xA
176.4	0xC
192	0xE