6
Recommendation H.261
(03/93)
The filter is switched on/off for all six blocks in a macroblock according to the macroblock type (see 4.2.3, MTYPE).
3.2.4
Transformer
Transmitted blocks are first processed by a separable two-dimensional discrete cosine transform of size 8 by 8. The output
from the inverse transform ranges from –256 to
+255 after clipping to be represented with 9 bits. The transfer function of the
inverse transform is given by:
f x y
C u C v F u v
x
u
y
v
u
v
(
) =
( ) ( ) (
) cos[ (
)
] cos[ (
)
]
,
,
/
/
1
4
0
7
0
7
2
1
16
2
1
16
=
=
∑ ∑
+
+
π
π
with
u, v, x, y
= 0, 1, 2, . . ., 7
where
x,y
= spatial coordinates in the pel domain,
u,v
= coordinates in the transform domain,
C u
( )
1 / 2
=
for u
= 0; otherwise 1,
C v
( )
1/ 2
=
for v
= 0; otherwise 1.
NOTE – Within the block being transformed, x
= 0 and y = 0 refer to the pel nearest the left and top edges of the picture,
respectively.
The arithmetic procedures for computing the transforms are not defined, but the inverse one should meet the error tolerance
specified in Annex A.
3.2.5
Quantization
The number of quantizers is 1 for the INTRA dc coefficient and 31 for all other coefficients. Within a macroblock the same
quantizer is used for all coefficients except the INTRA dc one. The decision levels are not defined. The INTRA dc
coefficient is nominally the transform value linearly quantized with a stepsize of 8 and no dead-zone. Each of the other 31
quantizers is also nominally linear but with a central dead-zone around zero and with a step size of an even value in the range
2 to 62.
The reconstruction levels are as defined in 4.2.4.
NOTE – For the smaller quantization step sizes, the full dynamic range of the transform coefficients cannot be represented.
3.2.6
Clipping of reconstructed picture
To prevent quantization distortion of transform coefficient amplitudes causing arithmetic overflow in the encoder and
decoder loops, clipping functions are inserted. The clipping function is applied to the reconstructed picture which is formed
by summing the prediction and the prediction error as modified by the coding process. This clipper operates on resulting pel
values less than 0 or greater than 255, changing them to 0 and 255, respectively.
3.3
Coding control
Several parameters may be varied to control the rate of generation of coded video data. These include processing prior to the
source coder, the quantizer, block significance criterion and temporal sub-sampling. The proportions of such measures in the
overall control strategy are not subject to recommendation.
When invoked, temporal sub-sampling is performed by discarding complete pictures.
3.4
Forced updating
This function is achieved by forcing the use of the INTRA mode of the coding algorithm. The update pattern is not defined.
For control of accumulation of inverse transform mismatch error a macroblock should be forcibly updated at least once per
every 132 times it is transmitted.
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