Audio Decoder Overview
10-3
When the host starts the selected audio decoder, audio frames/packets
are retrieved from the Audio ES Channel Buffer in SDRAM and decoded
and formatted. The three decoders parse most of the parameters from
the bitstream and store them in registers in the Host Interface. The host
reads these registers and writes decoder commands to other registers to
modify the audio.
The MPEG Decoder reproduces 16-bit audio samples from the bitstream
with 24-bit internal processing precision. The packetized Linear PCM
samples can be 16, 20, or 24 bits in length. The host can override the
bitstream sample resolution for all of the decoders by setting the
Overwrite Quantization bit in Register 366 (
page 4-89
) and programming
the Host Quantization bits in the same register for 16, 20, or 24-bit
samples. The decoders truncate or extend the samples accordingly.
Table 10.1
Audio Decoder Modes
Mode
Bits [2:0]
DAC Output
S/P DIF (IEC958) Output
0b000
MPEG decoder
MPEG decoder output PCM samples con-
verted to IEC958 format
0b001
Reserved
0b010
MPEG Decoder
MPEG Formatter
0b011
Reserved
0b100
Linear PCM Decoder
Linear PCM Decoder
NOTE: If the sample frequency in the Linear
PCM bitstream is 96 kHz, then the IEC958
output is derived from an on-chip filter that
converts from 96-kHz to 48-kHz sample
frequency.
0b101
Linear PCM Decoder output decimated
through on-chip filter to convert from
96-kHz to 48-kHz sample rate. This mode
should be selected if the output desired is
through a DAC that supports a 48-kHz
sample frequency only.
Same as DAC, converted to IEC958 format.
0b110
CD bypass
S/P DIF bypass
0b111
PCM FIFO
PCM FIFO