7-2
Audio Decoder
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
descriptions of these bits.
7.2.2
Starting,
Stopping and
Controlling the
Rate of the
Decoder
The external controller starts the decoder, typically in response to the
synchronization requirements of the presentation unit. The decoder can
also be stopped under software control. In applications, it is often neces-
sary to adjust the audio playback to accommodate drift between the
encoder and decoder clocks, or to accommodate missing or erroneous
audio frames. The L64005 allows a section of an audio frame to be
repeated, or to be skipped on demand. This slows down or speeds up
the effective play rate and allows the decoder to come back into synchro-
nization. The ADRS[1:0] bits in Group 6, Register 54 control the play
When the decoder is paused, the output PCM value holds the same data
as the last sample before the pause occurred. No samples are dropped
when normal play resumes from pausing the decoder. Pausing the
decoder is different from stopping the decoder. If the decoder is stopped,
it resynchronizes to the input stream when restored, and loses samples.
Note that if dropping or repeating the frame segments is unacceptable,
the user can adjust the sample rate up or down with the NCO. However,
with an oversampling DAC, be careful to keep BCLK phase constant—
that is, the ratio of
m/n must remain an integer.
In those applications where the device is congured in the parallel mode
and decoding PES or program streams, you can set the ASBA (Group
6, Register 54 bit 5) and the audio parser will assume that data is byte
aligned. This will help avoid the situation where audio start codes are
‘emulated’, meaning where a bit pattern that looks like a start code
occurs somewhere else in the stream. This can cause the PES parser to
discard large portions of data until a real PES start code is found and
byte alignment and synchronization is re-established. This feature cannot
be used in serial elementary mode because sync words are not byte
aligned in this mode.
The host selectable audio parameters may be safely programmed and
the decoder started after the ASPV (Audio Synchronization and Param-
eters Valid) bit is set. This bit (Group 6, Register 54, bit 6) indicates that