0
3 4
7 8
27 28
31
Sync
preamble
Auxiliary
LSB
MSB V U C P
Audio sample word
Validity flag
User data
Channel status
Parity bit
Functional Description
22.3.3.2.2 Subframe Format
Every audio sample transmitted in a subframe consists of 32 S/PDIF time intervals (or cells), numbered
from 0 to 31.
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
.
•
Time intervals 4-27 carry the audio sample word in linear 2s-complement representation. The most-
significant bit (MSB) is carried by time interval 27. When a 24-bit coding range is used, the least-
significant bit (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 (either 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 may 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 shown in
, the preambles (time intervals 0-3) are
also defined with even parity.
Figure 22-12. S/PDIF Subframe Format
Table 22-5. Preamble Codes
Preamble Code
(1)
Previous Logical State
Logical States on pin AXRn
(2)
Description
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, W. For use in professional applications, preambles are referred to as Z, X,
Y, respectively.
(2)
The preamble is not BMC encoded. Each logical state is synchronized to the serial clock. These 8 logical states make up time
slots (cells) 0 to 3 in the S/PDIF stream.
As shown in
, the McASP DIT only generates one polarity of preambles and it assumes the
previous logical state to be 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.
3780
Multichannel Audio Serial Port (McASP)
SPRUH73H – October 2011 – Revised April 2013
Copyright © 2011–2013, Texas Instruments Incorporated