ADSP-21368 SHARC Processor Hardware Reference 9-3
S/PDIF Transmitter/Receiver
A frame is uniquely composed of two subframes. The first subframe nor-
mally starts with preamble X. However, the preamble changes to preamble
Z once every 192 frames. This defines the block of frames structure used
to organize the channel status information. The second subframe always
starts with preamble Y.
Subframe Format
Each frame consists of two subframes. Figure 9-2 shows an illustration of
a subframe, which consists of 32 time slots numbered 0 to 31. A subframe
is 64 unit intervals in length. The first four time slots of each subframe
carry the preamble information. The preamble marks the subframe start
and identifies the subframe type. The next 24 time slots carry the audio
sample data, which is transmitted in a 24-bit word with the least signifi-
cant bit (LSB) first. When a 20-bit coding range is sufficient, time slots 8
to 27 carry the audio sample word with the LSB in time slot 8. Time slots
4 to 7 may be used for other applications. Under these circumstances, the
bits in time slots 4 to 7 are designated auxiliary sample bits. If the source
provides fewer bits than the interface allows (either 20 or 24), the unused
LSBs are set to logic 0.
This functionality is important when using the SPDIF receiver in com-
mon applications where there are multiple types of data to handle. If there
are PCM audio data streams as well as encoded data streams, for example a
CD audio stream and a DVD audio stream with encoded data, there is a
danger of incorrectly passing the encoded data directly to the DAC. This
results in the ‘playing’ of encoded data as audio, causing loud odd noises
to be played. The non-audio flag provides an easy method to mark the this
type of data.
After the audio sample word, there are four final time slots which carry:
1. Validity bit (time slot 28). The validity bit is logic 0 if the audio
sample word is suitable for conversion to an analog audio signal,
and logic 1 if it is not. This bit is set if the
CHST_BUF_ENABLE bit
To Next Page
Loading...
Need help?
General
