Every audio sample transmitted in a subframe consists of 32 S/PDIF time intervals (or cells), numbered 0 to 31. Figure 24-116 shows a subframe.
- Time intervals 0–3 carry one of the three permitted preambles to signify the type of audio sample in the current subframe. The preamble is not encoded in BMC format, and therefore the preamble code can contain more than two consecutive 0 or 1 logical states in a row. See Table 24-327.
- Time intervals 4–27 carry the audio sample word in linear 2s-complement representation. The MSB is carried by time interval 27. When a 24-bit coding range is used, the LSB is in time interval 4. When a 20-bit coding range is used, time intervals 8-27 carry the audio sample word with the LSB in time interval 8. Time intervals 4–7 may be used for other applications and are designated auxiliary sample bits.
- If the source provides fewer bits than the interface allows (20 or 24), the unused LSBs are set to logical 0. For a nonlinear PCM audio application or a data application, the main data field can carry any other information.
- Time interval 28 carries the validity bit (V) associated with the main data field in the subframe.
- Time interval 29 carries the user data channel (U) associated with the main data field in the subframe.
- Time interval 30 carries the channel status information (C) associated with the main data field in the subframe. The channel status indicates if the data in the subframe is digital audio or some other type of data.
- Time interval 31 carries a parity bit (P) such that time intervals 4–31 carry an even number of 1s and an even number of 0s (even parity). As listed in Table 24-327, the preambles (time intervals 0–3) are also defined with even parity.
As listed in Table 24-327, the McASP DIT generates only one polarity of preambles, and it assumes the previous logical state is 0. This is because the McASP assures an even-polarity encoding scheme when transmitting in DIT mode. If an underrun condition occurs, the DIT resynchronizes to the correct logic level on the AXRn pin before continuing with the next transmission.
Table 24-327 Preamble CodesPreamble Code(1) | Previous Logical State | Logical States on pin AXRn(2) | Description |
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B (or Z) | 0 | 1110 1000 | Start of a block and subframe 1 |
M (or X) | 0 | 1110 0010 | Subframe 1 |
W (or Y) | 0 | 1110 0100 | Subframe 2 |
(1) Historically, preamble codes are referred to as B, M, and W. For use in professional applications, preambles are referred to as Z, X, and Y, respectively.
(2) The preamble is not BMC-encoded. Each logical state is synchronized to the serial clock. These eight logical states make up time slots (cells) 0 to 3 in the S/PDIF stream.