Time interval
1 0 1 1 0 0 1 0 1 1 0
1 0 1 1 0 1 10 0 0 11 0 1 0 0 1 0 1 0 11
Cell
Clock
128 x Fs
Data
(unencoded)
Biphase
mark signal
(at pin AXRn)
Internal
to McASP
At pin
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Functional Description
22.3.3.2 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.
22.3.3.2.1 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 AXRn. 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 22-11 and Table 22-4 show how data is encoded to the BMC format.
As shown in Figure 22-11, the frequency of the clock is twice the unencoded data bit rate. In addition, the
clock is always programmed to 128 × f
s
, where f
s
is the sample rate (see Section 22.3.3.2.3 for details on
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.
Figure 22-11. Biphase-Mark Code (BMC)
Table 22-4. Biphase-Mark Encoder
Previous State at Pin
Data (Unencoded) AXRn BMC-Encoded Cell Output at AXRn
0 0 11
0 1 00
1 0 10
1 1 01
3779
SPRUH73H–October 2011–Revised April 2013 Multichannel Audio Serial Port (McASP)
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