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What is 22kHz and What is DiSEqC ?

In order to squeeze more channels into a given frequency band width, channels are transmitted with vertical OR horizontal polarisation. (It doesn't matter what the word means, just accept it).

The conventional LNB known as the "Marconi (polarisation) Switching LNB" responds to the supply voltage to change the polarisation. If the supply voltage going up the dish cable is less than 15 volts, the LNB will "see" only vertically polarised transmissions. If the voltage is more than 15 volts, it will "see" only horizontally polarised transmissions.

The older type of LNB had a separate polariser to do this job and was able to use the voltage to select High Band or Low Band operation (by changing the internal oscillator frequency).

So the modern LNB has a problem if it has to switch between two frequency bands. The problem was overcome with the introduction of the "Universal" LNB.

The "Universal" LNB switches polarisation with voltage but it also switches its internal oscillator for "High Band" when it "hears" a 22kHz tone. Specificallly, the oscillator changes from 9.75 GHz to 10.6 GHz.

Recently manufactured receivers incorporate a 22kHz tone generator which is menu-selectable on a per-channel basis. (One exception is the Ferguson SRD6 which has a tone generator but no menu selection. The tone comes on automatically if you program a channel for a higher frequency).

Older receivers require an external tone generator box in order to make the "Universal" LNB select High Band. The external box can be controlled by a manually operated switch or by some feature of the receiver which will put a voltage onto its control wire to switch it on.

An alternative use for the 22kHz tone is to control an external switching box whichfeeds signals from one of a pair of LNBs into the receiver. For example, you could have two separate LNBs on a dish connected to a switching box which is itself connected to the receiver by a single coaxial cable. When the box "hears" a 22kHz tone it swaps to the other LNB.

Digital Satellite Equipment Control (DiSEqC) is an extension of this idea. It relies on a switching box which detects the 22kHz tone pulsing rapidly on and off. In this way, a specially designed receiver can control numerous LNBs through a DiSEqC switching box.

For a more comprehensive discussion of DiSEqC, read "What Satellite TV" magazine March 1997 issue and "TELEVISION servicing" magazine, November 1997 issue.


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Copyright ©1997 SatCure
Version 1.1 updated on 3/5/99
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