The digital TV reception
Digital TV reception is normally provided by an integrated receiver decoder (IRD) commonly known as a set-top box (STB). The STB may be either a standard (SD) or high definition (HD) as well as satellite or terrestrial. Digital terrestrial television (DTTV) may also be incorporated within the TV receiver in what is known as integrated digital television (iDTV). In this chapter, we will look at the component parts of a SD and HD STBs for both the terrestrial and satellite transmission media.
The digital receiver/decoder of set-top-box
The set-top-box, STB is a self-contained tuner/decoder device in that it tunes to the required channel, extracts and decodes the selected pro- gramme data, checks the access rights of the user and produces picture, sound and other services as instructed.
Digital signals are fed to the STB in the same way as for analogue TV broadcasting. Thus, in the case of satellite transmission, an outdoor dish and low noise block (LNB) are necessary to receive the signals and convert them to a suitable intermediate frequency (IF). For terrestrial television, the input to the STB is obtained directly from a terrestrial aerial. And for cable too, the signal is obtained directly from the network. The output from the SD digital decoder is in the form of analogue video and audio signals, that is fed to an analogue TV receiver either directly via a SCART component video or s-video connections or as a UHF-modulated signal using the aerial socket at the back of the receiver (Figure 18.1). In the case of HD, digitised HD video and sound are available on a single HDMI connection or components video and L/R sound outlets. Video and audio from HD decoders are also available on the normal SCART or s-video ports, but this time in SD video only.
System overview
Figure 18.2 shows the basic components of a digital receiver/decoder STB. The channel decoder, also known as the front end, is specific to the broad- casting media: satellite, terrestrial or cable. It extracts the required MPEG transport stream from the RF or IF (in the case of satellite reception) signal. It consists of a tuner, ADC, an appropriate demodulator and a forward error correction (FEC) processor. The transport stream containing
standard 204-byte packets belonging to one or more television pro- grammes is fed into the transport demultiplexer. Before processing the transport stream, the demultiplexer sends the bitstream to the conditional access module (CAM), which controls the user access rights to the selected programme or service. The CAM interrogates a smart card to find out if the user has an active subscription to the selected programme. If access is granted or the programme is free view, the transport stream is routed back to the transport demultiplexer. The demultiplexer selects the transport packets belonging to the chosen programme and reassembles them to reconstruct the packetised elementary streams (PESs) of the programme. A fast static RAM (SRAM) or synchronous DRAM (SDRAM) memory is used to store selected audio, video and service packets on their way to the MPEG decoder.
The MPEG decoder carries out the video and audio decoding of the audio and video PESs. It converts the video PES data stream back into its original components: Y (luminance) and CR and CB (chrominance). The picture is reconstructed from the I, P and B frames. This reconstruction requires simultaneous storage of these frames, hence the need for a large video memory in the form of a DRAM or a faster SDRAM buffer. The three components are made available for direct connection to a TV receiver or they may be sent to an embedded PAL encoder to convert the digital video into an analogue PAL composite video, which is fed directly to a television set via a SCART connector or into a UHF modulator for a UHF output.
The audio PES packets are decoded using the same rules as adopted in the encoding stage to produce left and right analogue audio signals. The same DRAM chip is used for audio buffering which also provides an approximately 1-s delay to ensure audio and video synchronisation. This delay is necessary given that the processing of the video signals takes a longer time to complete than the processing of audio signals. The left and right analogue audio signals from the audio decoder may be fed to a summing amplifier to produce mono sound for the UHF modulator. A separate stereo output (L and R) is also provided, which may be sent to the SCART connector.
The system requires a high level of programming and control. This is performed by a 16-bit or a 32-bit microprocessor (not shown). Control is exercised using normal control signals such as read/write; reset and inter- rupt request (IRQ) as well as the I2C serial control bus. A non-volatile flash memory chip is used to store start-up and other software programs. Upgrading of the software may be carried out off-air by first loading the new software into the DRAM chips and then transferring it to the non- volatile flash memory.