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Old 8th Jan 2017, 3:37 pm   #1
Hammonds
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Default What's this Labgear device (Cable Equaliser)

Hi Folks

Before I dispose of this item, please could anyone tell me what it is and whether it has any value before it's skipped.

Many thanks

Dave
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Old 8th Jan 2017, 3:42 pm   #2
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Default Re: What's this Labgear device?

No idea of its value but it appears to be a line conditioning module to equalise the channel strength of individual channels for a communal aerial system.

Frank
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Old 8th Jan 2017, 3:56 pm   #3
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Default Re: What's this Labgear device?

Used in terraced houses and blocks of flats etc that were sharing ariels. Each channel could be fine tuned. Would imagine with everything being digital now that these are no longer required.

Only way to know it's worth to someone is to pop it on an auction site.

Cheers
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Old 8th Jan 2017, 4:08 pm   #4
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Default Re: What's this Labgear device?

It's set up for channels 21, 24, 37, 31 and 37, which from memory were the analogue TV channels broadcast from Sandy Heath, and possibly other transmitters. The 'Channel 5' signal on channel 37 was much weaker than the others, so a device like this would have been used to equalise the levels of all of them on a distribution system as others have said.

Now that analogue TV is no longer broadcast on those channels, it's not likely to be very useful for its original purpose.

It could be handy to someone assembling a distribution system for vintage analogue TVs who wants to equalise the outputs from five modulators tuned to those channels, to give the authentic effect of analogue TV in East Anglia in the late 1990s, but otherwise it's probably mostly good for parts or as a historical artefact!

Chris
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Old 8th Jan 2017, 4:13 pm   #5
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Default Re: What's this Labgear device?

The lines look to be tuneable, would a communial aerial system for digital TV not require an equaliser?
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Old 8th Jan 2017, 4:48 pm   #6
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Default Re: What's this Labgear device?

As far as I am aware, digital receivers are much more tolerant of signal strength than analogue (after all, you only have to distinguish between a "one" and a "zero").

It probably would have been necessary in a distribution system -- especially a multi-aerial setup where BBC was received from one direction and ITV / CH4 / CH5 from a different direction, as might be required in problem areas* -- to prevent the strongest signals from overloading any downstream amplifiers.


* Without naming names, I have seen housing estates built with apparently scant consideration being given to essential infrastructure -- I mean even main roads, never mind utilities -- so it's unlikely that anyone ever rejected a plot of otherwise-prime home building land on the strength of TV receivability .....
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Old 8th Jan 2017, 4:54 pm   #7
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Default Re: What's this Labgear device?

It is as you say, an equaliser. They are used to present the end user with channels all in the same range of levels, compensating for varied input levels and "tilt" in the long cables.
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Old 8th Jan 2017, 5:20 pm   #8
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Default Re: What's this Labgear device?

Yes we used them at Philips with our distribution system. It was essential that the showroom (open to the public by appointment) used to demonstrate new model TV's was presented with good clean signals on all channels. One of these gadgets was even more essential when Channel 5 started since it was the weakest of all. We cheated a bit with the C5 signal since it had it's own aerial up on the roof (pointing in a slightly different direction from the Crystal Palace aerial) but we still had to equalise the signal in the showroom.

We had another in the test lab for the same reason. We set them up very accurately with the aid of a spectrum analyser.
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Old 8th Jan 2017, 6:05 pm   #9
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Default Re: What's this Labgear device?

Quote:
Originally Posted by julie_m View Post
As far as I am aware, digital receivers are much more tolerant of signal strength than analogue (after all, you only have to distinguish between a "one" and a "zero").
The transmitted signal of a DVB-T system is very analogue indeed.

First of all there are thousands of carrier frequencies (Orthogonal Frequency Division Multiplex = OFDM, though the orthogonal bit seems redundant because different frequencies are implicitly orthogonal)

Each of those carriers has 4 or more bits of data applied as QAM = Quadrature amplitude modulation. Think of a carrier as being a vector, the length and phase angel get switched so that it points to one of 16 targets representing the combination of 4 bits. Satellite and cable DVB have more uniform transmission characteristics and can get away with more points in the constellation for each carrier... EG 256 points representing 8 bits of data at once.

Any distortion in the transmitter, path and receiver will intermodulate some carriers into the frequency space of others, and the outer points will have their amplitudes compressed making them more easily confused with their neighbours, putting the error raw rate up. The error correction system can fix this up to a point, then as is traditional in digital systems, everything falls straight on its face.

When you think of the gyrations of the instantaneous voltage of all those complex amplitude modulated carriers added together, the result is definitely analogue.

There is a theorem which says that any channel carrying the maximum information flow its bandwidth can support will look like it is filled with white noise. This applies to looking at its spectrum as well as its waveform.

One of my favourite jokes when giving a lecture is to leap around like a loony, jump up and clap my cupped hands together. I peer into them and announce that I've caught an electron. I offer to show it to a member of the audience and then ask him whether it's a digital one or an analogue one Electrons and voltages and currents are real. Analogue and digital relate to the assignment of meaning to those electrons and voltages and currents.

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Old 16th Jan 2017, 10:45 pm   #10
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Default Re: What's this Labgear device?

Many thanks for all the posts.

Mods - this topic may be closed.

Thank you

Dave
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