Dishing up TV

Dishing up TVIn a paper to be given at Cable & Satellite 98 (May 18-20) in London, Chris Carter reviews the design of digital satellite receivers The digital satellite receiver mass market started in the summer of 1994 with the launch of the DirecTV/USSB satellite service in the US. Subscribers now number around three million for DirecTV. Growth was quick because, unlike in Europe, in the US there were no existing small-dish analogue services, so the market went directly to digital. A broadcast TV resolution digital video sequence needs over 200Mbits/s to transmit without compression. Such a data rate would need many times the bandwidth of the original analogue signal to distribute, so compression is essential for digital video to have any benefit for broadcasting. The chosen compression stanadard was MPEG-2 which uses both spatial and temporal compression to reduce this data rate to a more manageable 4-8Mbits/s with only a minimal loss of quality. Satellite transponder bandwidths are typically in the range of 26-54MHz leading to useful bit rates in the range of 20 to 60Mbits/s. So, with compression, around 4-10 digital channels can be transmitted on a single transponder (compared with only one with analogue transmission). This is of great benefit to a service provider as they can generate more revenue from the extra channels, whilst cutting the cost of transmitting each one. Also lucrative new services, only possible with digital, such as near-video-on-demand (NVOD) can be offered. NVOD works by transmitting the same movie on multiple channels simultaneously but with the start time staggered by, say, 20 minutes on each channel. For example, for a 2 hour film broadcast on six channels at 20 minute intervals, the viewer is on average no more than 10 minutes from the start of the next broadcast. For pay-TV services, such as NVOD, the transport stream is most likely scrambled to prevent un-authorised viewing. In these cases some hardware i.e. a descrambler, and some software is needed to make the data usable. Authorisation is normally done by interaction with a smart card, containing data identifying the user and allowing the subscriber to be charged. This can be done by using pre-paid cards, or the set-top box itself can contact a subscriber management centre for authority, using a low speed modem (e.g. 1200 bits/s) for communication and the smartcard to identify the viewer. All this together with the infrastructure required at the transmission end and in the subscriber management centre is known as the conditional access (CA) system. In order for transmission equipment manufacturers, receiver manufacturers and service providers to install systems that work together a considerable amount of work has gone into developing standards for the whole system, not just the compression part. Most operators around the world now conform to the European Digital Video Broadcast (DVB) family of specifications which cover most such requirements. The DVB standards specify a mechanism for transmitting CA data and the descrambler hardware, but they do not specify how these are to be used together. This is at the discretion of the CA provider. Well known examples of companies providing such systems include Irdeto, News Digital Systems and France Telecom. There may be several independent video, audio and data streams contained within the one transport stream. To allow for these to be identified and separated, the different streams relating to different channels are put into ‘packets’ with each being labelled with a header (containing the Packet IDentifier or PID) to identify to which channel the packet belongs. In the case of a TV channel, it is then the task of a transport demultiplexer to separate out the video and audio packets (selected by their PID) for the channel that the viewer is wanting to watch. Once the video and audio packets are demultiplexed, they then need to be decompressed and displayed. For this, MPEG-2 audio and video decoders are required. The decoded video must also be re-encoded to either PAL or NTSC for display on a standard television set. To ensure that the video and audio are synchronised, software on the main processor needs to check the time stamps that are sent with each packet and make sure that the outputs are correctly aligned. To allow the set-top box to ‘know’ which PIDs relate to which program, service information (SI) data is also transmitted as part of the transport stream. Such SI data also includes information on which transponder contains which channel, so the software in the box can control the tuner to pick-out the required transponder. Chris Carter is with SGS-Thomson Microelectronics

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