L64005 MPEG-2 Audio/Video Decoder Technical Manual
1-25
Final Rev F
Copyright 1996 by LSI Logic Corporation. All rights reserved.
1.6.5.2 Audio Output
Audio output is over a three-wire serial PCM stream. The signals are:
BCLK, a serial bit clock; ASDATA, a serial PCM data output; and LRCLK,
the left/right channel indication. In addition, the emphasis output is
optionally available.
1.6.5.3 Audio Decode Rate Control
The L64005 decodes audio data at a rate proportional to the audio sam-
ple frequency. The sample frequency is either derived internally from the
27-MHz SYSCLK reference, or externally from the oversampling clock
reference input, ACLK. When using the internal reference clock, BCLK is
derived from the 27-MHz reference using an numerically controlled oscil-
lator (NCO) which acts as a N/M clock divider circuit. Note that in this
case, care must be taken to avoid non-integer results in order maintain
a phase constant BCLK frequency. If not, the phase is jittered about the
desired frequency and although this will work for in-expensive broadband
DAC’s, it’s generally not acceptable for the more commonly used over-
sampling DAC’s. If an external reference is selected, care must be taken
to ensure that the reference is locked to the actual audio sample rate in
the encoder. This can be done with a VCO or VCXO with a control volt-
age derived from the audio presentation time stamp, or otherwise locked
to the encoder. Some oversampling DAC’s have very good on board
sample frequency generation circuitry. Failing to adhere to these precau-
tions will result in eventual underow or overow in the audio channel
buffer unless other buffer management precautions are taken.
1.6.5.4 Support for Low Sampling Frequencies
The L64005 also supports Layer 1 and Layer 2 decoding at sample rates
of 24 kHz, 22.05 kHz and 16 kHz as dened in ISO/IEC 13818-3, an
extension to the MPEG audio denition. The new denitions for the sam-
pling frequency eld, bitrate index eld, and bit allocation are selected
when the frame header ID bit is zero.
1.6.5.5 Signicant Differences from the L64002
See “Notice for L64002 Users,” in the Preface for more detail.