L64105 LSI Logic Corporation, L64105 Datasheet - Page 398

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

The DCT coefficient in the upper left location (0, 0) of the block
represents the zero horizontal and zero vertical frequencies and is known
as the DC coefficient. The DC coefficient is proportional to the average
pixel value of the 8 x 8 block, and additional compression is provided
through predictive coding because the difference in the average value of
neighboring 8 x 8 blocks tends to be relatively small.
The other coefficients represent one or more nonzero horizontal or
nonzero vertical spatial frequencies and are called AC coefficients. The
quantization level of the coefficients corresponding to the higher spatial
frequencies favors the creation of an AC coefficient of zero by choosing
a quantization step size such that the human visual system is unlikely to
perceive the loss of the particular spatial frequency, unless the coefficient
value lies above the particular quantization level. The statistical encoding
of the expected runs of consecutive zero-valued coefficients of higher-
order coefficients accounts for some coding gain.
To cluster nonzero coefficients early in the series and to encode as many
zero coefficients as possible following the last nonzero coefficient in the
ordering, the coefficient sequence is specified to be a zigzag ordering .
Zigzag ordering concentrates the highest spatial frequencies at the end
of the series. The MPEG-2 standard includes additional block scanning
orders.
After block scanning has been performed, the encoder performs
run-length coding on the AC coefficients. This process reduces each
8 x 8 block of DCT coefficients to a number of events represented by a
nonzero coefficient and the number of preceding zero coefficients.
Because many coefficients are likely to be zero after quantization,
run-length coding increases the overall compression ratio.
The encoder then performs variable-length coding (VLC) on the resulting
data. VLC is a reversible procedure for coding that assigns shorter
codewords to frequent events and longer codewords to less frequent
events, thereby reducing the number of bits necessary to represent a
data set without losing any information. Huffman encoding is a
particularly well known form of VLC.
The final compressed video data is now ready for transmission to either
a local storage device from which a video decoder may later retrieve and
decompress the data, or to a remote video decoder via cable, or direct
satellite broadcast, for example.
A-4
Video/Audio Compression and Decompression Concepts