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24.0.18.2 S/PDIF Coding Format
The McASP transmitter supports the S/PDIF format with 3.3V biphase-mark encoded output. The S/PDIF
format is supported by the digital audio interface transmit (DIT) transfer mode of the McASP. This section
briefly discusses the S/PDIF coding format.
24.0.18.2.1 Biphase-Mark Code (BMC)
In S/PDIF format, the digital signal is coded using the biphase-mark code (BMC). The clock, frame, and
data are embedded in only one signal—the data pin AXR[n]. In the BMC system, each data bit is encoded
into two logical states (00, 01, 10, or 11) at the pin. These two logical states form a cell. The duration of
the cell, which equals to the duration of the data bit, is called a time interval. A logical 1 is represented by
two transitions of the signal within a time interval, which corresponds to a cell with logical states 01 or 10.
A logical 0 is represented by one transition within a time interval, which corresponds to a cell with logical
states 00 or 11. In addition, the logical level at the start of a cell is inverted from the level at the end of the
previous cell.
Figure 24-9
As shown in
Figure
clock is always programmed to 128 × f
how this clock rate is derived based on the S/PDIF format). The device receiving in S/PDIF format can
recover the clock and frame information from the BMC signal.
Internal
to McASP
(unencoded)
At pin
(at pin AXR[n])
Data (Unencoded)
1086
Multichannel Audio Serial Port (McASP)
and
Table 24-1
show how data is encoded to the BMC format.
24-9, the frequency of the clock is twice the unencoded data bit rate. In addition, the
, where f
s
Figure 24-9. Biphase-Mark Code (BMC)
Clock
128 x Fs
Time interval
Data
1
0
Biphase
mark signal
1 0 1 1 0 1
Table 24-1. Biphase-Mark Encoder
Previous State at Pin
AXR[n]
0
0
1
1
Copyright © 2013–2016, Texas Instruments Incorporated
is the sample rate (see
s
1
1
0
0
1
0
1
0 0
1
1
0 1 0 0 1 0 1 0
BMC-Encoded Cell Output at AXR[n]
0
1
0
1
SPRUH82C – April 2013 – Revised September 2016
www.ti.com
Section 24.0.18.2.3
for details on
0
1
1
0
1
1
Cell
11
00
10
01
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