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Digital TV
A Technical FAQ
Rev 4.0.  June 14th 2000

by Chris Muriel
chris.muriel@analog.com

What is Digital Satellite Television
Part 4) Simple Block Diagram of digital receiver.

 


Some observations on the afforementioned diagram:

DAC = Digital-to-analogue converter
ADC = Analogue-to-digital converter

The video encoder typically contains 4 or more DACs which run at video rates & quality. This infers 8 bit video DAC's (not cheap). 3 are needed for RGB; another is needed for composite video out (PAL or SECAM). Some use 10 bit DAC's & the difference *may* be visible by viewing sharp transitions like black to white -hint : have a look at the On Screen Display if you want to try to spot this effect. I have not included the conditional access module for simplicity.
See section 5 for more detail on this (including a diagram).

Complex IC's are needed for many of these blocks.

A qpsk demodulator/ADC/Viterbi decoder can easily cost around $7-$13 in manufacturers volumes! The mpeg transport demultiplexer & decoders cost even more!I haven't included the CPU & memory (usually around 1-3Meg. is needed & some of this may be fast,expensive SRAM). Perhaps you can now see why the digital receivers cost a lot more than the analogue ones!!! It's worth noting that on Astra,a Network Information Table (NIT) is transmitted every 10 seconds on every DVB/mpeg transponder. The information sent includes the FEC,S/R,frequencies etc.


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