Chapter 22 Multichannel Audio Serial Port (McASP).pdf
Chapter 22 Multichannel Audio Serial Port (McASP).pdf
Chapter 22 Multichannel Audio Serial Port (McASP).pdf
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Functional Description www.ti.com<br />
<strong>22</strong>.3.3.2 S/PDIF Coding Format<br />
The <strong>McASP</strong> transmitter supports the S/PDIF format with 3.3V biphase-mark encoded output. The S/PDIF<br />
format is supported by the digital audio interface transmit (DIT) transfer mode of the <strong>McASP</strong>. This section<br />
briefly discusses the S/PDIF coding format.<br />
<strong>22</strong>.3.3.2.1 Biphase-Mark Code (BMC)<br />
In S/PDIF format, the digital signal is coded using the biphase-mark code (BMC). The clock, frame, and<br />
data are embedded in only one signal—the data pin AXRn. In the BMC system, each data bit is encoded<br />
into two logical states (00, 01, 10, or 11) at the pin. These two logical states form a cell. The duration of<br />
the cell, which equals to the duration of the data bit, is called a time interval. A logical 1 is represented by<br />
two transitions of the signal within a time interval, which corresponds to a cell with logical states 01 or 10.<br />
A logical 0 is represented by one transition within a time interval, which corresponds to a cell with logical<br />
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<br />
previous cell. Figure <strong>22</strong>-11 and Table <strong>22</strong>-4 show how data is encoded to the BMC format.<br />
As shown in Figure <strong>22</strong>-11, the frequency of the clock is twice the unencoded data bit rate. In addition, the<br />
clock is always programmed to 128 × f s, where f s is the sample rate (see Section <strong>22</strong>.3.3.2.3 for details on<br />
how this clock rate is derived based on the S/PDIF format). The device receiving in S/PDIF format can<br />
recover the clock and frame information from the BMC signal.<br />
Internal<br />
to <strong>McASP</strong><br />
At pin<br />
Clock<br />
128 x Fs<br />
Data<br />
(unencoded)<br />
Biphase<br />
mark signal<br />
(at pin AXRn)<br />
Figure <strong>22</strong>-11. Biphase-Mark Code (BMC)<br />
Time interval<br />
1 0 1 1 0 0 1 0 1 1 0<br />
1 0 1 1 0 1 0 1 0 0 1 1 0 1 0 0 1 0 1 0 1 1<br />
Table <strong>22</strong>-4. Biphase-Mark Encoder<br />
Previous State at Pin<br />
Data (Unencoded) AXRn BMC-Encoded Cell Output at AXRn<br />
0 0 11<br />
0 1 00<br />
1 0 10<br />
1 1 01<br />
3936 <strong>Multichannel</strong> <strong>Audio</strong> <strong>Serial</strong> <strong>Port</strong> (<strong>McASP</strong>) SPRUH73E–October 2011–Revised May 2012<br />
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