Cirrus Logic AN22 - Manual (Page 2)

AN22
2 AN22REV2
auxiliary data fields. If it is 20 bits or less, the aux-
iliary field can be used for other applications such
as voice. The parity bit generates even parity and
can detect an odd number of transmission errors in
the sub-frame. The validity bit, when low, indicates
the audio sample is fit for conversion to analog.
The user and channel status bits are sent once per
sample and, when accumulated over a number of
samples, define a block of data. The user bit chan-
nel is undefined and available to the user for any
purpose. The channel status bit conveys, over an
entire block, important information about the audio
data and transmission link. Each of the two audio
channels has its own channel status data with a
block structure that repeats every 192 samples.
As shown in Figure 2, two consecutive sub-frames
are defined as a frame, containing channels A and
B, and 192 frames define a block. The preambles
that identify the start of a sub-frame are different
for each of the two channels with another unique
one identifying the beginning of a channel status
block.
Modulation and Preambles
The data is transmitted with biphase-mark encod-
ing to minimize the DC component and to allow
clock recovery from the data. As illustrated in
Figure 3, the 1’s in the data have transitions in the
center, and the 0’s do not, after biphase-mark en-
coding. Also, the biphase-mark data switches po-
larity at every data bit boundary. Since the value of
the data bit is determined by whether there is a tran-
sition in the center of the bit, the actual polarity of
the signal is irrelevant.
Each sub-frame starts with a preamble. This allows
a receiver to lock on to the data within one
sub-frame. There are three defined preambles: one
Aux Data
0 3 4 7
LSB
8
Audio Data Preamble
28 29 30 31
V U C P MSB
27 bit
Sub-frame
Validity
User Data
Channel Status Data
Parity Bit
Figure 1. Sub-frame Format
Channel A Z Channel B Y Channel A X Channel B Y Channel A X Channel B Y X
Sub-frame Sub-frame
Start of Channel Status Block
Frame 0 Frame 191 Frame 1
Preambles
Figure 2. Frame/Block Format
1 0 1 1 0 0 1 1 0 1 0 1
Clock
(2 times bit rate)
Data
Biphase-Mark
Data
1 0 0 0 1 1
Figure 3. Biphase-Mark Encoding