Audio - Video Modulators
Modulators are used for distribution of audio/video signals from different sources (cameras, SAT receivers, DVD players etc.) via the existing antenna installations. The signal coming from a modulator can be received by any TV receiver. Modulators come in 2 versions: with or without built-in power unit. If they don't have their own power supply, an external device will be needed. There are 2 types of modulators used in the TV technology:
- DSB - AM (dual side band, Amplitude Modulation)
- VSB - AM (vestigial side band, Amplitude Modulation)
They differ in the used spectrum, which means the width of the frequency band. VSB-AM modulation is used in the terrestrial TV applications and in the higher-class modulators, for example those in the headend stations of cable TV. Such modulator, due to VSB modulation, can work without an extra space between channels (e.g. 50, 51, 52, 53, 54, 55, etc), which allows a greater channel capacity.
In case of application of the VSB-AM modulators, it is crucial to even out the channel output level. It is ideal if all the channels are at the same level. The recommended max difference should not exceed 3dB, some receivers will tolerate differences greater than that, in the range of 8-10 dB.
Generally, the max difference should not exceed the following:
- 3dB (for adjacent channels),
- 6dB (at any point in 60 MHz range),
- 12dB (in 30 - 1000 MHz range).
DSB-AM modulation is used in very simple modulators working in SAT tuners, VCR's, TV games, collective antennas. Modulators using the DSB-AM modulation, due to a wider bandwidth used by each channel, have to be set every second channel with one channel space between them (e.g. channels 50, 52, 54, 56, 58, etc).
The experience shows that in such a case the level differences may be greater and it is necessary to reduce the capacity of the network. It is recommended that the difference should not be greater than 6dB. If there is no choice and such differences in levels are unavoidable, each receiver in the network should be tested for quality of reception.
The spectrum of DSB - AM and VSB - AM signals
Generally, there are 2 groups of modulators on the market: The least complicated and inexpensive modulators for home users and professional and semi-professional modulators.
An example of the first group may be MOD-1 (Inel), K-3 (Kayto), and ME-1 (Sat-Service) modulators. They usually work in channels 28-43 (sometimes 21-69), B/G standard with 5.5 MHz audio or, less frequently, D/K standard with 6.5 MHz audio. Most of them generate a test signal in the form of 2 vertical lines which is used for tunning in the channel in televisions. They use chinch or SCART connectors and come with AC/DC adapter (12V/100mA output). Their typical output level is 70 dBuV. They are only made in one version - dual band.
Professional modulators include, among many others, MS-301 (Alcad), MS-401 (Alcad), MS-501 (Alcad), MT-09 (Terra), MT-09P (Terra), MT-10 (Terra). They are the modulators aspiring to this group, although some of them have a lower purity of spectrum or don't have certain useful functions. MT-11, MT-11P, MT-12P, MT-14 (Terra), MAW-800 and MAW-810 (Telkom-Telmor) are the best examples.
Most of them work in channels 21-69 (sometimes 6-12 and S9-S16), usually in the B/G standard (often with possibility of selecting B/G or D/K standard). The input connectors usually are chinch (RCA) type, or mini-DIN, not the SCART. Typical output level is - 80 dBuV, and it is almost always adjustable. Some of them can work on the channels 1-5, 6-12, 21-69, S1-S18, S19-S38, which makes the entire range TV RF spectrum. Only in this group there are available single (vestigial) band modulators.
Example of application of modulators within antenna installation in a detached house. More about such installation here
Example of application of modulators in a simple monitoring system. Both modulators and cameras are powered via coaxial cable. More about monitoring here
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