
7-8
Memory Interface
The second item that contributes to additional channel buffering
requirements is related to A/V synchronization. In most DVD systems,
the A/V sync can be maintained very precisely and generally does not
add to the channel buffer requirements. However, in a transport system,
the A/V sync error can accumulate and require additional buffer space.
The space needed is calculated in a manner similar to the real-time
decode calculation. The additional bits are determined by the maximum
error time provided by the A/V synchronization mechanism. For example,
if the maximum A/V sync error is 10 ms, then 10 ms x 15 Mb/s = 150,000
additional bits (18,750 bytes) are required. Additional space is similarly
needed for audio data.
The third item requiring additional channel buffering is caused by the use
of a slave mode pixel interface to the NTSC/PAL encoder. In this system
conguration, the decoder is locked to the external VSYNC and cannot
start decoding at a channel start until the next VSYNC arrives. This
results in a decode start delay of up to one eld time or 20 ms in a PAL
system. The additional space required is then 20 ms x 15 Mb/s =
300,000 bits (37,500 bytes). Although audio decoding starts immediately,
the audio must be delayed 20 ms to maintain A/V synchronization. A
well-controlled DVD system can avoid this problem by starting the disc
play at VSYNC time so it does not need the additional buffering.
In most cases, the size of the SPU channel must be larger than one
SPU unit since conditions can occur where one SPU unit is being
decoded while another is being received. This generally requires twice
the channel buffer size because the L64021 does not have instantaneous
SPU decode.
It is the host’s responsibility to program the start and end SDRAM
address for all the channel buffers, the video frame stores, and the OSD