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PrecedingState: 0 1
Preamble:
ChannelCoding: ChannelCoding: Description:
X
111 000 10 000 111 01 Channel Su1 bframe
Y
111 010 00 000 101 11 Channel Su2 bframe
Z
111 100 00 000 011 11 Channel1Subframe andBlockStart
Cl cko
(2xSourceBi atetR )
Source Data
Coding
(NR )Z
AES3Channel
Coding
(BiphaseMark)
Insert Preamble
Code Below
PrecedingState,fr ofthepreviousFrameomtheParitybit .
PreambleZ(BlockStart)
PreambleCoding
0
0
1
1
DIGITAL INTERFACE TRANSMITTER (DIT) OPERATION
SRC4382
SBFS030C – JANUARY 2006 – REVISED SEPTEMBER 2007
The binary non-return to zero (NRZ) formatted audio and status source data for bits 4 through 31 of each
subframe are encoded utilizing a Biphase Mark format for transmission. This format allows for clock recovery at
the receiver end, as well as making the interface insensitive to the polarity of the balanced cable connections.
The preambles at the start of each subframe are encoded to intentionally violate the Biphase Mark formatting,
making their detection by the receiver reliable, as well as avoiding the possibility of audio and status data
imitating the preambles. Figure 64 shows the Biphase Mark and preamble encoding.
Although the AES3 standard originally defined transmission for sampling rates up to 48kHz, the interface is
capable of handling higher sampling rates, given that attention is paid to cable length and impedance matching.
Equalization at the receiver may also be required, depending on the cable and matching factors. It is also
possible to transmit and decode more than two channels of audio data utilizing the AES3 or related consumer
interfaces. Special encoding and/or compression algorithms are utilized to support multiple channels, including
the Dolby
®
AC-3, DTS, MPEG-1/2, and other data reduced audio formats.
Figure 64. Biphase Mark Encoding
The DIT encodes a given two-channel or data-reduced audio input stream into an AES3-encoded output stream.
In addition to the encoding function, the DIT includes differential line driver and CMOS buffered output functions.
The line driver is suitable for driving balanced or unbalanced line interfaces, while the CMOS buffered output is
designed to drive external logic or line drivers, as well as optical transmitter modules. Figure 65 illustrates the
functional block diagram for the DIT.
The input of the DIT receives the audio data for Channels 1 and 2 from one of four possible sources: Port A, Port
B, the DIR, or the SRC. By default, Port A is selected as the source. The DIT also requires a master clock
source, which may be provided by either the MCLK input (pin 25) or RXCKO (the DIR recovered master clock
output). A master clock divider is utilized to select the frame rate for the AES3-encoded output data. The
TXDIV[1:0] bits in control register 0x07 are utilized to select divide by 128, 256, 384, or 512 operation.
Channel status and user data for Channels 1 and 2 are input to the AES3 encoder via the corresponding
Transmitter Access (TA) data buffers. The TA data buffers are in turn loaded from the User Access (UA) buffers,
which are programmed via the SPI or I
2
C host interface, or loaded from the DIR Receiver Access (RA) data
buffers. The source of the channel status and user data is selected utilizing the TXCUS[1:0] bits in control
register 0x09. When the DIR is selected as the input source, the channel status and user data output from the
DIT is delayed by one block in relation to the audio data.
The validity (V) bit may be programmed using one of two sources. The VALSEL bit in control register 0x09 is
utilized to select the validity data source for the DIT block. The default source is the VALID bit in control register
0x07, which is written via the SPI or I
2
C host interface. The validity bit may also be transferred from the AES3
decoder output of the DIR, where the V bit for the DIT subframes tracks the decoded DIR value frame by frame.
The parity (P) bit will always be generated by the AES3 encoder internal parity generator logic, such that bits 4
through 31 of the AES3-encoded subframe are even parity.
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