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Recommendation H.261
(03/93)
T 1502 430-90/ d01
C od in g cont ro l
S our ce
co der
So ur ce
d ecod er
V id eo mu ltiplex
cod er
Vid eo mu ltiplex
de co de r
T ra nsm ission
bu ffe r
Re ceivin g
buf fe r
Tr a nsm ission
co de r
C od ed
bit s tre am
Vid eo
sign al
a) Vid eo cod er
b) Vid eo d ecod er
Exte rn al con tro l
R ece iving
de cod er
FIGURE 1/H.261
Outline block diagram of the video codec
FIGURE 1/H.261...[D01] = 9 CM
2.1
Video input and output
To permit a single Recommendation to cover use in and between regions using 625- and 525-line television standards, the
source coder operates on pictures based on a common intermediate format (CIF). The standards of the input and output
television signals, which may, for example, be composite or component, analogue or digital and the methods of performing
any necessary conversion to and from the source coding format are not subject to Recommendation.
2.2
Digital output and input
The video coder provides a self-contained digital bit stream which may be combined with other multi-facility signals (for
example as defined in Recommendation H.221). The video decoder performs the reverse process.
2.3
Sampling frequency
Pictures are sampled at an integer multiple of the video line rate. This sampling clock and the digital network clock are
asynchronous.
2.4
Source coding algorithm
A hybrid of inter-picture prediction to utilize temporal redundancy and transform coding of the remaining signal to reduce
spatial redundancy is adopted. The decoder has motion compensation capability, allowing optional incorporation of this
technique in the coder.
2.5
Bit rate
This Recommendation is primarily intended for use at video bit rates between approximately 40 kbit/s and 2 Mbit/s.
2.6
Symmetry of transmission
The codec may be used for bidirectional or unidirectional visual communication.
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