The limitation of the number of analog programs that can be distributed in cable TV network (about 60) is the main reason for growing interest in digital technology.
QAM modulation.
QAM modulation is optimal for cable networks. It is used in hyper-band (S channels) and UHF band. Due to the need of compatibility with channel raster and transmission requirements, there are used 8 MHz channels (MPEG-2 compression, QAM modulation).
Employing MPEG-2 compression, it is possible to distribute within one physical channel up to 8 digital TV programs (similarly to sat TV), while any analog TV program, using AM modulation, occupied the entire channel.
Of course, there is possibility of combining in one network both digital and analog programs. The digital ones usually take the range from S17 to S38.
QPSK modulation.
Digital satellite TV using QPSK modulation and MPEG-2 compression also allows to transmit up to 8 programs from one transponder. Due to the features of QPSK modulation (Quadrature Phase Shift Keying), there is needed 40 Mbps throughput and 36 MHz band.
QPSK - QAM transmodulation.
Digital satellite TV is often denoted as DVB-S (Digital Video Broadcasting - Satellite), digital cable TV - DVB-C (Digital Video Broadcasting - Cable), digital terrestrial TV - DVB-T (Digital Video Broadcasting - Terrestrial).
The difference in the width of physical channels between DVB-S and DVB-C transmissions does not allow simple frequency shift from the first satellite IF into cable TV range.
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.
    Television Radio
    analog terr.TV
DVB-C analog sat TV
DVB-S analog radio
Signal level
dBuV 57-80 45-70 44-77 45-70 40-70
C/N dB >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.
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.