H.261 Datasheet, PDF (6/31 Pages) List of Unclassifed Manufacturers – LINE TRANSMISSION OF NON-TELEPHONE SIGNALS

2.7 Error handling
The transmitted bit-stream contains a BCH code (Bose, Chaudhuri and Hocquengham) (511,493) forward error correction
code. Use of this by the decoder is optional.
2.8 Multipoint operation
Features necessary to support switched multipoint operation are included.
3
Source coder
3.1 Source format
The source coder operates on non-interlaced pictures occurring 30 000/1001 (approximately 29.97) times per second. The
tolerance on picture frequency is ± 50 ppm.
Pictures are coded as luminance and two colour difference components (Y, C_B and C_R). These components and the codes
representing their sampled values are as defined in CCIR Recommendation 601.
Black = 16
White = 235
Zero colour difference = 128
Peak colour difference = 16 and 240.
These values are nominal ones and the coding algorithm functions with input values of 1 through to 254.
Two picture scanning formats are specified.
In the first format (CIF), the luminance sampling structure is 352 pels per line, 288 lines per picture in an orthogonal
arrangement. Sampling of each of the two colour difference components is at 176 pels per line, 144 lines per picture,
orthogonal. Colour difference samples are sited such that their block boundaries coincide with luminance block boundaries as
shown in Figure 2. The picture area covered by these numbers of pels and lines has an aspect ratio of 4:3 and corresponds to
the active portion of the local standard video input.
NOTE – The number of pels per line is compatible with sampling the active portions of the luminance and colour difference
signals from 525- or 625-line sources at 6.75 and 3.375 MHz, respectively. These frequencies have a simple relationship to those in CCIR
Recommendation 601.
The second format, quarter-CIF (QCIF), has half the number of pels and half the number of lines stated above. All codecs
must be able to operate using QCIF. Some codecs can also operate with CIF.
Means shall be provided to restrict the maximum picture rate of encoders by having at least 0, 1, 2 or 3 non-transmitted
pictures between transmitted ones. Selection of this minimum number and CIF or QCIF shall be by external means (for
example via Recommendation H.221).
3.2 Video source coding algorithm
The source coder is shown in generalized form in Figure 3. The main elements are prediction, block transformation and
quantization.
The prediction error (INTER mode) or the input picture (INTRA mode) is subdivided into 8 pel by 8 line blocks which are
segmented as transmitted or non-transmitted. Further, four luminance blocks and the two spatially corresponding colour
difference blocks are combined to form a macroblock as shown in Figure 10.
Recommendation H.261 (03/93)
3