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

For transmission using H.261, the still image is sub-sampled 2:1 horizontally and vertically into four sub-images in
the currently transmitted video format. Figure D.1 shows the sub-sampling pattern on the still image. The samples labelled 0,
1, 2 and 3 form the four sub-images 0, 1, 2 and 3, respectively.
0
3
0
3
0
3
0
3
1
2
1
2
1
2
1
2
0
3
0
3
0
3
0
3
1
2
1
2
1
2
1
2
0
3
0
3
0
3
0
3
1
2
1
2
1
2
1
2
FIGURE D.1/H.261
Sub-sampling pattern
D.3 Picture layer multiplex
When HI_RES is “0”, the two lower bits of the temporal reference (TR) identify one of the four sub-images 0, 1, 2 or 3. The
three higher bits of the TR shall be set to “0”.
The encoder transmits a still image by setting HI_RES to “0” and transmitting the four sub-images 0, 1, 2 and 3 in sequential
order. It is allowed to transmit more than one frame for each sub-image, but should not go back once it starts transmitting the
next sub-image. The encoder is allowed to resume motion video at any time by setting HI_RES back to “1”.
NOTE – The reference memory for the current frame is always the previous frame, regardless of whether a frame is motion
video or still image.
D.4 Multipoint considerations
A still image transmitted within the video bit-stream can be broadcast on a multipoint connection by broadcasting the video.
The MCV (multipoint command visualization-forcing) and Cancel-MCV commands defined in Recommendation H.230
provide for this capability. A terminal could force an MCU to broadcast its video by sending MCV, and then return to the
previous mode of operation by sending Cancel-MCV. MCUs are required to implement these commands, but they are
optional for terminals.
D.5 Other considerations
– All the video coding modes are allowed (intra-frame, inter-frame, motion compensation, etc.);
– the multiplex arrangement below the picture layer remains the same (group of blocks, macroblocks, etc.);
– the maximum number of bits allowed per frame (sub-image) should not be exceeded (256 Kbits for CIF and 64
Kbits for QCIF);
– forward error correction is not affected.
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Recommendation H.261 (03/93)