L64105 LSI Logic Corporation, L64105 Datasheet - Page 235

• Manufacturer Part Number: L64105
• Description: Mpeg-2 Audio/video Decoder
• Manufacturer: LSI Logic Corporation

Table 7.2
The area consumed by the channel buffering is defined in
ISO/IEC 13818 . There are a number of items that affect the size of the
channel buffering needed in a system.
The MPEG model is based upon an ideal decoder which can
instantaneously decode an image at the decode time. Real
implementations may take up to one frame time to decode an image.
This results in bits backing up in the channel buffer for one frame time
until the picture is decoded and removed from the channel. This
phenomenon is known as real-time decode and it requires additional
space in the video channel buffer. The additional space can be
calculated as the frame time x bit rate. A PAL system at 15 Mbps with a
40-ms frame time requires an additional 600,000 bits (75 Kbytes) in the
video channel buffer.
A similar calculation can be done for the audio frame time and bit rate.
For the audio real-time decode overflow, if the A/V sync provides
accuracy to within one frame time, the maximum size of the audio real-
time decode overflow is upper bounded by the size of the audio channel.
This restriction exists because, in one frame time of audio decode, the
system cannot input more than one channel buffer size of audio bits.
The second item that contributes to additional channel buffering
requirements is related to A/V synchronization. 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 Mbps = 150,000
additional bits (18,750 bytes) are required. Additional space is similarly
needed for audio data.
Memory Channel Buffer Allocation
Item
System Header channel
3 Video Frame stores
OSD storage area
Example NTSC SDRAM Allocation (Cont.)
Size (bytes)
512
1,555,200
optional
7-7