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Internet CCTV GUIDE - Modulators - junction of CCTV and SMATV/MATV system
Modulators are used for conversion and distribution of audio and video signals from different sources (CCTV cameras, SAT receivers etc.) via the existing antenna installation. The RF signal coming from the modulator can be received by any TV receiver connected to the installation. CCTV applications allow to monitor several locations in one or several TV channels. Of course, the images from CCTV cameras can be recorded on VCRs or DVRs.
There are 2 types of modulators used in the TV/CCTV technology:
  • DSB - AM (dual side band, amplitude modulation)
  • VSB - AM (vestigial side band, amplitude modulation
They differ in the spectrum, which means the width of the frequency band. VSB-AM modulation is used by terrestrial TV transmitters and higher-class modulators, for example those in the headend stations of cable TV. VSB-AM modulator can work without an extra space between channels (e.g. on 50, 51, 52, 53, 54, 55, etc. channels), which allows the maximum channel capacity. However, it costs several times more than DSB-AM modulator.
DSB-AM modulation is used in simple modulators built in sat receivers, VCRs etc. DSB-AM modulators, due to broader spectrum, require one-channel spacing (they can work e.g. on 50, 52, 54, 56 ... channels). It should be remembered that the free channel has to be the one below the working channel.
The spectrum of DSB - AM and VSB - AM signals
Generally, there are two groups of modulators on the market: simple and inexpensive modulators for home users and professional and semi-professional devices.
Examples from the first group are Signal Digital Classic R95176 (sound subcarrier: 6.5 MHz), R95175 (sound subcarrier 5.5 MHz - PAL-G) and Economy R95178. The output channel can be set within whole UHF band (ch. 21-69). They are equipped with AC/DC adapter and RF cable. Typical output level: 70 dBuV. An interesting and cheap RF modulator is MixPol R95164 that uses PLL synthesis and can work on channels 1-69 and S1-S38.
The group of semi-professional modulators includes, among others, MT-21P R871721 (Terra).
MT-30A R871731 MT-30B R871732 MT-30C R871733 (Terra) belong to the professional group of modulators and use VSB-AM modulation.
Most modulators work on 21-69 channels (sometimes on channels 6-12 and S9-S16), usually in B/G standard, although there is no problem with accessing D/K versions. The input connectors are usually in chinch or mini-DIN form. Typical output level is 80 dBuV and there is almost always possibility of adjusting this parameter.
If there is a need to add a few modulators to RF installation, and for the reason of too many channels already distributed it is not possible (e.g. only one free channel is available), the images from several CCTV cameras have to be put into a divider and then the output signal should modulate the one free channel.
Without such a problem, very useful in practice are 2, 4, 6 - channel modulators (R95165, R951664,R95167), which allow distribution of images from several cameras using one AC/DC adapter.
6-channel TV modulator: Mixpol MDP-6 (ch. 1-69)
MIXPOL modulator - 6 inputs, channels S1-S38, K1-12, 21-69
Demodulator R87201 is used to change high frequency TV signal to baseband audio-video signal. This device is sold together with remote control which is used for configuration i.e. programming and storing selection of input channels. Demodulator can be used together with additional modulator for creating simple and cheap CCTV surveillance system - when we have no possibility to lay down additional cables and want to transmit signals from several CCTV cameras through one cable.
Audio-Video Demodulator ICC-1
Caution. In the above scheme, the images from individual cameras are available on different channels - only the image from selected camera (after setting adequate channel on the demodulator) can be displayed on the monitor. If we want to have, for further processing, all channels available at he same time, we need to use the same quantity of demodulators as modulators.
Broadband TV Combiner - ST 1-60Flush Outlet: Satel GAR-BG-DK (end-of-line)2-Way TV/FM Splitter R-2 SignalAudio-Video Demodulator ICC-1Audio-Video Demodulator ICC-1Video Card - Geovision GV-250(2-input, with software)75-ohm terminating resistor for outlets of Satel
Example of using modulators and demodulators for transmission of signals through antenna installation - from surveillance cameras to digital recording card
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