8.6 Video broadcasting

Currently more than ninety-five percent of MPEG-2 coded video is for broadcasting applications, carried via terrestrial, satellite and cable TV networks to homes. In Europe, the standard ITU-R 601 video is encoded in the range of 3–8 Mbit/s, depending on the scene content of the video. The lower end of the bit rate is for head-and-shoulders type video such as the video clip of a news reader, and the higher bit rates are required for the critical scenes such as sports programs, similar to the snap shot shown in Figure 8.10. Normally, scenes with grass and tree leaves, which have detailed texture and random motion due to wind, if they appear alongside a plain scene, like a lake or stream, are the most difficult to code. Random motion of the detailed area makes motion estimation useless, and for a limited bit rate budget, increase in quantiser step size will cause blocking artefacts in the plain areas. For HDTV video, the required bit rate is of the order of 20 Mbit/s.

In both terrestrial and satellite TV, for better channel utilisation, several TV programs may be multiplexed and then digitally modulated on to a carrier. At the destination, the receiver, known as the set top box, separates the channels, decodes each program and feeds the individual analogue signals to the television set for display. Although the same multiplexing technique can be used, because digital modulation techniques for terrestrial and satellite are different, unfortunately the same set top box cannot be used for both.

For multiplexing of TV programs, the individual bit streams are first decoded and are then reencoded to a new target bit rate. For optimum multiplexing, the target bit rate for each TV channel is made dependent on the content (statistics) of each program, and hence it is called statistical multiplexing. Here, more complex video might be assigned higher bit rates and since video complexity may vary over time, then for optimum statistical multiplexing we need to monitor the video complexity continuously.

One way of calculating the complexity of a scene in a video program is to define the scene complexity as the sum of the complexity indices of its I, P and B-pictures in a group of pictures (GOP) [16]. For each picture type, the complexity index is the product of its average quantiser step size to its bit rate in that frame. For example, the scene complexity index (SCI) of a video with a GOP structure of N = 12 and M = 3, which has one I, three P and eight B-pictures is:

(8.9)

where I, P and B are the target bit rates for the I, P and B pictures, and Q_I, Q_P and Q_B are their respective average quantiser step sizes. After calculating SCI for each TV program, the total bit rate is divided between the TV channels in proportion to their SCI. Values of the SCI can be continuously calculated on a frame by frame basis (within a window of a GOP), to provide optimum statistical multiplexing.

One of the main attractions of digital satellite TV is the benefit of broadcasting many TV programmes from a single transponder. In the analogue era, one satellite transponder with a bandwidth of 36 MHz could accommodate only one frequency modulated (FM) TV programme, whereas currently about 6–8 digital TV programmes can be multiplexed into 27 Msymbol/s and are accommodated in the same transponder. There are even stations that squeeze about 10–15 digital TV programmes into a transponder, albeit at a slightly lower video quality. In addition to this increase of the number of TV channels, the required transmitted power for digital can be of the order of 10–20 per cent of analogue, or for the same power, the satellite dishes can be made much smaller (45–60 cm diameter dishes compared to 80 cm used in analogue). Digital terrestrial TV also benefits from the low power transmitters.

In digital terrestrial TV, normally one programme is digitally modulated into an 8 MHz (European) UHF channel [17]. The bit stream prior to channel modulation is orthogonal frequency division multiplexed (OFDM) into 1705 carriers (2000 carriers is also an option) and the channel modulation is a 64-QAM. At a higher modulation rate (e.g. 256-QAM) it is even possible to accommodate an 18–24 Mbit/s bit stream into the same 8 MHz UHF channel, thus being able to multiplex 2-3 digital programmes (or even higher for poorer quality) into one existing analogue UHF terrestrial channel.

Since at the baseband a 4–8 Mbit/s MPEG-2 video is OFDM modulated into almost 2000 carriers, then each bit of the video signals is transmitted at a rate of 2–4 kbit/s. Such a low data rate (large interval) is very robust against interference, similar to a high frequency burst of noise, and can be cleaned up easily. Thus OFDM is particularly attractive for ghost-free TV broadcasting in big cities, where multiple reflections from tall buildings can create interference (a common problem with analogue TV). Moreover, it is possible to cover the whole broadcast TV network (nationwide programmes not regional programmes) with a single frequency, since interference is not a problem. This will release a lot of wireless bandwidth for other communication services. Finally, similar to satellite, the transmitter power can be reduced by a factor of ten.

The price paid for all these benefits of digital TV is the sensitivity of the digital TV to channel errors. During heavy rain or snow, pictures become blocky or in the more severe cases, a complete loss of picture (picture freeze). This is the main disadvantage of digital TV since, in analogue TV, weaker reception may cause snowy pictures, which is better than picture break up or freeze in digital TV. To alleviate this problem, layered video coding with unequal error protection to various layers may be used. Or one may use the more intelligent technique of distributing the transmitter power among the layers, such that the picture quality gradually degrades, closer to quality degradation in analogue TV.