 described in the following sections. All three visible portion of the 625 system and the picture repetition rate utilise an 8 by 8 DCT. being exactly half the field rate of the 60Hz format. Control PCM video in Variable Quantiser length Buffer coders Compressed data out Inverse quantiser Inverse DCT Frame Intra Inter store(s) Motion vector Vectors search Figure 57.28 Generic diagram of motion compensated hybrid DPCM/DCT coders 57/28 Video transmission At bit rates below about 1Mbit/s it may be subjectively preferable Weighting matrices are applied to the transform coefficients and to reduce the coded picture rate and introduce motion jerkiness these are downloaded to the decoder, enabling the encoder to opti- rather than accept the full amount of spatial quantisation noise. mise them to the source material. B-pictures are not used as the basis The motion compensation range is +15 pels and +15 lines. for constructing any others and it has been found possible to quan- Weighting of transform coefficients is not incorporated. tise them more coarsely than the others. This can yield a small saving in bits. 57.4.11 ISO/MPEG Draft International Standard 11172 57.4.12 CCIR Recommendation 723 The mandate of the ISO Working Group is the coding of video and This Recommendation covers coding of television signals for associated audio for storage applications. The first item in the broadcast contribution quality at bit rates of the order of 30Mbit/s workplan is for bit rates up to about 1.5Mbit/s and is addressed by to 45 Mbit/s (CCIR, 1990c). The algorithm handles CCIR 601 4:2:2 the Draft International Standard 11172 (ISO, 1992). The algorithm input video without any internal subsampling. has much in common with CCITT Recommendation H.261 but also In addition to intra coding there are an inter-frame mode and an some significant differences. A very wide range of picture formats inter-field mode. The inter-frame one uses the field of the previous is possible to suit television and computer applications. The full frame with the