RECEIVER OVERVIEW
It is plain from the description above that the DTV signal is a complex one which requires a great deal of sorting out at the receiving end. The DTV receiver is to all intents and purposes a powerful computer, with memory, software and high-speed data ports. The simplest possible representation of a DTV receiver is given in Fig. 12.15. It consists of two main blocks: a channel decoder and an MPEG decoder. The input signal enters the tuner, which works in the same way as in an analogue receiver – a local oscillator beats against the carrier to produce an i.f. signal. The analogue products are A−D converted to produce digital signals as datastreams representing the phase angles of the carriers. In the following stage, the demodulator, the carrier-phase data is decoded into datastreams corresponding to
those sent by the transmitter, but with propagation and other errors still present. They are identified and where possible corrected by the FEC (Forward Error Correction) stage. If connection/repair is not possible the corrupt packet is flagged by the FEC decoder section. The signal emerging from the channel decoder section is, as near as possible, an exact replica of that sent – Fig. 12.10. The channel decoder section of the receiver is dependent on the application: cable, satellite or terrestrial, which use different modulation systems and carrier frequencies. The MPEG decoder is common, in its working principle, to all three.
In the decoder section the MPEG datastream first encounters the transport demultiplexer, which separates out the different channels in the transport stream, based on the PID data in each packet, routeing those identified as pay or subscription ones through the conditional access block. Others are passed direct to the data decoders, one each for video and audio. The video decoder section expands the data and builds it back into a complete signal, using 2 Mb or more of storage DRAM. The output from the video decoder consists of three consecutive bytes, one each for Y, Cr and Cb components. In the PAL encoder the signal is converted back to a form compatible with all existing equipment, though RGB is more commonly used for TV coupling, and gives better on-screen results. The audio decoder, meanwhile, decodes and expands the serial audio data- stream, presenting it in simple PCM mode to the DAC block, which recreates the original stereo audio signal for passage to the VCR or TV.
The two control centres have different roles. The microcontroller shown here in the channel decoder section decodes user commands; operates the front-panel display; monitors supply rails; and controls and co-ordinates the operation of the channel decoder section. The main microprocessor, bottom right, is primarily concerned with the control of MPEG functions: it governs the transport stream demultiplexing; the video and audio decoding processes; the conditional access communication; and it provides an interface for the RS232 port.