MÔJ KOŠÍK
Môj košík je prázdny

TV, TV-SAT

Trochu teórie

Distribúcia digitálnych signálov v systémoch káblovej televízie
Obmedzenie počtu analógových programov, ktoré je možné v sieti káblovej televízie distribuovať (okolo 60) je hlavným dôvodom pre rastúci záujem o digitálnu technológiu.
QAM modulácia.
QAM modulácia je optimálna pre káblové siete. Používa sa v hyper pásme (S kanály) a UHF pásme. Kvôli potrebe kompatibility s kanálovým rastrom a prenosovými požiadavkami sa používajú 8 MHz kanály (MPEG-2 kompresie, QAM modulácie).
Pri použití MPEG-2 kompresie je možné v rámci jedného fyzického kanálu distribuovať až 8 digitálnych televíznych programov (podobne ako u satelitného televízneho vysielania), zatiaľ čo každý analógový televízny program, používajúci AM moduláciu, obsadzuje celý kanál.

Samozrejme tiež existuje možnosť kombinovania ako digitálnych, tak analógových programov v rámci jednej siete. Digitálne programy obvykle zaberajú rozsah od S17 do S38.
QPSK modulácia.
Digitálne satelitne televízne vysielanie používajúce QPSK moduláciu a MPEG-2 kompresiou tiež umožňuje prenos až 8 programov z jedného transpondéra. Kvôli vlastnostiam QPSK modulácie (Quadrature Phase Shift Keying) je potrebný 40 Mbps priechod a 36 MHz pásmo.
QPSK - QAM transmodulácia.
Digitálne satelitne televízne vysielanie je často označované ako DVB-S (Digital Video Broadcasting - Satellite), digitálne káblové televízne vysielanie - DVB-C (Digital Video Broadcasting - Cable), digitálne pozemne televízne vysielanie - DVB-T (Digital Video Broadcasting - Terrestrial).
Rozdiel v šírke fyzických kanálov medzi DVB-S a DVB-C prenosmi neumožňuje prosté frekvencie presunúť sa zo satelitného IF do pásma káblovej televízie.
It means that, when we want to distribute in cable network programs from one transponder, there is necessity of changing the original modulation into QAM, allowing to reach higher transmission speeds in narrower band than that required for QPSK. More sophisticated QAM modulation requires higher S/N ratio that will guarantee BER level not larger than 9*10-6.

The table below shows the required signal levels, C/N ratios, and the BER values for proper reception of programs with QAM modulation. For comparison there is given data for other types of modulation.
  TelevisionRadio
  AM64QAMFMQPSKFM
  analog terr.TV
DVB-Canalog sat TV
DVB-Sanalog radio
Signal level
dBuV57-8045-7044-7745-7040-70
C/NdB>43>28>15>11>38
BER -9*10-6-9*10-6-
The idea of QPSK-QAM transmodulation
The advantage of this solution is full transparency for data stream - there are not needed additional devices for processing free to air and encrypted TV channels.
Transmodulators.
The devices are offered by many manufacturers, among others Alcad, Hirchmann, Kathrein, and Fagor.
Alcad produces TQ-501 transmodulators - each panel supports conversion from one transponder (in contrast with QAM-2000 head station). The advantage of the devices is output range of 47 to 862 MHz (S1-CH69).
A very interesting station, taking into consideration its price and quality is CSE 2000 of Hirschmann, using CHD 2000Q modules that can utilize the whole S1-69CH range.
It is also offered more advanced solution - CSE 3200, prepared for CDQ 3935 and CDQ 3940 transmodulators..
UFO-Compact station from Kathrein is prepared for UFO 385 panels working in S17-S37 range.
In large cable TV networks there are often used transmodulators of Fagora (Spain) - SDT 6300, SDT 7300, SDT 6400, and 7400.
This short review of transmodulators shows that there is no problem with building required head station. But the subscribers also need additional equipment - special cable receivers, usually called (digital) set-top boxes (STB).
Digital cable receivers.
In the case of distribution of digital signals in cable TV network, every user has to have DVB-C receiver. For example, there are available CSR 3402 QAM receivers from Hirschmann, Mediamaster C from Nokia, etc.
Practical solution.
The diagram shown below shows a typical configuration of cable TV network distributing digital programs. It is important that distribution of digital signals does not require to change the cabling or amplifiers. Generally, every modern cable TV network providing analog channels can be easily adapted to distribution of digital broadcasts. Theoretically, for 60 analog channels, we can get a digital equivalent of 480 programs.
Diagram of typical installation using QPSK-QAM transmodulation
The prospects of digital solutions in cable TV networks.
So far, the popularity of distribution of digital programs in cable networks in Poland is low. The system is used only by Aster company.
We reckon that the growth of interest is hampered by competitive offers of satellite platforms that usually provide their services with almost free satellite receivers.
Another disadvantage of cable offers is still limited number of available digital programs (usually only from several transponders), whereas satellite platforms give access to 50 or more transponders and 500 or so programs.