SPRUIE9D May 2017 – May 2024 DRA74P , DRA75P , DRA76P , DRA77P
An S/PDIF frame is composed of two subframes (see Figure 26-117). For linear coded audio applications, the rate of frame transmission normally corresponds exactly to the source sampling frequency fs. The S/PDIF format clock rate is therefore 128 × fs (128 = 32 cells per subframe × 2 clocks per cell × 2 subframes per sample). For example, for an S/PDIF stream at a 192-kHz sampling frequency, the serial clock is 128 × 192 kHz = 24.58 MHZ.
In 2-channel operation mode, the samples taken from both channels are transmitted by time multiplexing in consecutive subframes. Both subframes contain valid data (cell 28 validity bits for A- and B- channels, both set to '0'). The first subframe (left or A channel in stereophonic operation and primary channel in monophonic operation) normally starts with preamble M. However, the preamble of the first subframe changes to preamble B once every 192 frames to identify the start of the block structure used to organize the channel status information. The second subframe (right or B channel in stereophonic operation and secondary channel in monophonic operation) always starts with preamble W.
In single-channel operation mode in a professional application, the frame format is the same as in the 2-channel mode. Data is carried in the first subframe and may be duplicated in the second subframe. If the second subframe is not carrying duplicate data, cell 28 (validity bit) is set to logical 1.