H.261 Datasheet, PDF(25/31 Page) List of Unclassifed Manufacturers – LINE TRANSMISSION OF NON-TELEPHONE SIGNALS

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H.261 Datasheet, PDF (25/31 Pages) List of Unclassifed Manufacturers – LINE TRANSMISSION OF NON-TELEPHONE SIGNALS

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Tran smissio n o rder

Da ta

493

C od ed dat a

(S 1 S2 S3 S4 S5 S6 S 7 S8 ) = ( 00 011 01 1)

T 150 246 0-90/d0 8

Par ity

FIGURE 13/2H.261...[D08] = 9.5

F ill (all â1â)

49 2

FIGURE 13/H.261

Error correcting frame

5.3 Video coding delay

This item is included in this Recommendation because the video encoder and video decoder delays need to be known to allow

audio compensation delays to be fixed when H.261 is used to form part of a conversational service. This will allow lip

synchronization to be maintained. Annex C recommends a method by which the delay figures are established. Other delay

measurement methods may be used but they must be designed in a way to produce similar results to the method given in

Annex C.

5.4 Forward error correction for coded video signal

5.4.1 Error correcting code

The transmitted bitstream contains a BCH (511,493) forward error correction code. Use of this by the decoder is optional.

5.4.2

Generator polynomial

g(x) = (x9 + x4 + 1) (x9 + x6 + x4 + x3 + 1)

Example: For the input data of â01111 . . . 11â (493 bits) the resulting correction parity bits are

â011011010100011011â (18 bits).

5.4.3 Error correction framing

To allow the video data and error correction parity information to be identified by a decoder an error correction framing

pattern is included. This consists of a multiframe of eight frames, each frame comprising 1 bit framing, 1 bit fill indicator

(Fi), 492 bits of coded data (or fill all 1s) and 18 bits parity. The frame alignment pattern is:

(S1S2S3S4S5S6S7S8) = (00011011).

See Figure 13 for the frame arrangement. The parity is calculated against the 493-bits including fill indicator (Fi).

The fill indicator (Fi) can be set to zero by an encoder. In this case only 492 consecutive fill bits (fill all 1s) plus parity are

sent and no coded data is transmitted. This may be used to meet the requirement in 5.2 to provide video data on every valid

clock cycle.

Recommendation H.261 (03/93)

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