Decoding Flow Control
11-9
11.3.1 Audio Decoder Play Mode
The Audio Decoder Play Mode bits in Register 355 (
page 4-105) pause
the decoder, set it to normal play, and increase or decrease the play
speed by skipping or repeating samples. The decimation or interpolation
factors are one out of every 16 samples for the MPEG and Dolby Digital
Decoders and one out of every 8 samples for the Linear PCM Decoder.
The mode selections are:
0b00 - Pause. The decoder is paused and the last pair of PCM and
S/P DIF samples are repeated, effectively muting the output, until the
play mode is changed. The Audio ES Channel Buffer keeps lling
with new data. The decoder continues to decode and write to the
output buffer until the output buffer overows and stops the audio
read pointer of the Audio ES Channel Buffer. Since there is no
guarantee that the channel read pointer will stop at the end of a
frame, decoder resynchronization is required when the mode is
changed again.
A prolonged audio pause will cause the Audio ES Channel Buffer to
overow and the system parser to deassert the channel request
signals, stopping video as well as audio.
0b01 - Normal Play. The decoder is playing at normal speed. When
the PCM output buffer or S/P DIF output buffer empties and the next
output signal is requested by the audio DAC or S/P DIF decoder, the
last pair of output samples is repeated.
0b10 - Fast Play. The Dolby Digital and MPEG Decoder skip one out
of every 16 pairs of samples and play at 16/15 normal speed. The
Linear PCM Decoder performs fast play at 8/7 normal speed. When
the Linear PCM bitstream is at 48 kHz, every eighth pair of PCM
samples is skipped. When it is at 96 kHz, every fteenth and
sixteenth pair of PCM samples and every eighth pair of S/P DIF
samples are skipped.
0b11 - Slow Play. The Dolby Digital and MPEG Decoders repeat
every sixteenth pair of samples and play at 16/17 normal speed. The
Linear PCM Decoder runs at 8/9 normal speed in Slow Play Mode.
When the Linear PCM bitstream is at 48 kHz, every eighth pair of
PCM samples is played twice. When Fs is 96 kHz, every fteenth
and sixteenth pair of PCM samples and every eighth of S/P DIF
samples are played twice.