One of the initial aims in the design of a new low bit rate video codec was to replace H.261 with a more efficient one, that took the name of H.263. Hence the functionalities of H.261, but with some improvements, need to be included in this codec; they appear in Annex C of the H.263 specification that can be activated or disabled as desired [25-C]. Since this annex was introduced prior to all the other annexes, which later on were called optionalities, we introduce this annex prior to all the other options.
In H.263 the decoder can be instructed to alter its normal decoding mode and provide some extra display functions. Instructions for the alterations may be issued by an external device such as Recommendation H.245, which is a control protocol for multimedia communications [9]. Some of the commands and the actions are as follows.
This signal causes the decoder to freeze its displayed picture until a freeze release signal is received or a time-out period of at least six seconds has expired. A frozen picture is much better perceived by the viewer than, say, a broken picture due to channel errors, or if the encoder cannot deliver the compressed bit stream on time for a continuous display.
This command causes the encoder to encode its next picture in intra mode, with coding parameters to avoid buffer overflow. This mode in conjunction with the back channel reduces the probability of error propagation into the subsequent pictures. This mode improves the resilience of the codec to channel errors.
Freeze picture release is a signal from the encoder, which has responded to a fast update request, and allows a decoder to exit from its freeze mode and display decoded pictures in the normal manner. This signal is transmitted by the PTYPE in the picture header of the first picture coded in response to the fast update request.
In a multipoint connection, a multipoint control unit (MCU) can assemble two to four video bit streams into one video bit stream, so that at the receiver up to four different video signals can be displayed simultaneously. In H.261, this can be done on a quad screen by only editing the GOB header, but in H.263 it is more complex due to a different GOB structure, overlap motion estimation, multiple motion vectors etc. Therefore, in H.263 a special continuous presence multipoint mode is provided in which four independent video bit streams are transmitted in the four logical channels of a single H.263 video bit stream.