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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 14th Feb 2016, 7:32 pm   #1
eddie_ce
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Default VSB Filter and Aurora

Hi,

I am saving up for an Aurora. The general consensus seems to be that this is a superb piece of kit but that its output is rich in harmonics and also dsb. Would it be feasible to construct a vsb / low pass filter to clean up the output?

Just a thought, any advice from those more knowledgeable than I am is greatly appreciated.

Eddie
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Old 14th Feb 2016, 8:33 pm   #2
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Default Re: VSB Filter and Aurora

A lowpass filter could be made to pass a number of channels and to remove their harmonics. This would be very important if a signal was to be transmitted. But if the output of the Aurora is fed to the TV receiver by screened cable, like co-ax, then the RF tuned circuits in the tuner will exclude the harmonics anyway.

A VSB filter would be very difficult at the RF frequency and it would only be useable on one channel. The purpose of a VSB filter on a broadcast transmitter was to stop one of its sidebands overlapping with the adjacent channel. In a receiver, the IF selectivity must be shaped to fit the VSB signal so that it does not receive any of the adjacent channel. So there is a VSB filter in each receiver. Unless you are making many channels and want to populate adjacent channels, you should not need a VSB filter.

David
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Old 14th Feb 2016, 10:28 pm   #3
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Default Re: VSB Filter and Aurora

I believe the TV IF filtering is such that it can cope with either a VSB or DSB signal. As David says if you did fit a VSB filter it would only be useable on one channel.

If you do want to filter out the harmonics, which is not really necessary if feeding a TV or TVs via coax, then you could try feeding the Aurora output through a Triplexer designed to combine, or separate, VHF, DAB and UHF signals. This should at least attenuate the harmonics.

Keith
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Old 15th Feb 2016, 5:51 am   #4
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Default Re: VSB Filter and Aurora

Hi Keith,
I think most triplexers are just highpass networks for the UHF port, so any reduction of harmonics would be due to roll-off from strays.

The difficulty with harmonic filters at these frequencies is that stray resonances and even just the lengths of components will cause a filter's attenuation to come back up in scattered peaks. getting good attenuation up to daylight is one of those bottomless tasks, and some active devices can give significant harmonic output over a very troublesome amount of spectrum.

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Old 7th Mar 2016, 7:51 pm   #5
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Default Re: VSB Filter and Aurora

Eddie, you say you are saving up for an Aurora - singular.

Are you planning to distribute any other signals on your network?

If so, what?

If you are planning to ultimately have a large multi-channel network (rare) you might want to consider some filtering but, with careful planning, it shouldn't be necessary.

You definitely don't need a VSB filter unless you are planning to operate on adjacent channels but, again that would be a rare event, I think!

I've attached a table showing where the harmonics, up to the 10th harmonic of the vision carrier, fall for all 13 VHF channels. However, to make it easier to use, I've deleted all harmonics which do not fall in Bands I, III, IV & V.

As you can see, for most of the usual channel pairings such as 1 & 9, 2 & 10, 3 & 11, etc, there is no problem. If you want to add a UHF signal, you can see from the list that there are plenty of options available.

One advantage of UHF is that, unlike VHF Turret Tuners, there are no sets with precise channel numbers so, if the original UHF channel is likely to cause a problem, you should be able to find an alternative which looks as if it is correct.

A case in point here is the distribution system I designed for the Dulwich museum which replicated the 60s Crystal Palace/Croydon allocations using channels 1, 9 & 33. This had to be changed after a short whike when a new mux was introduced on Ch. 33 from Crystal Palace, just down the road!

BBC2 was moved to Ch.32 and normal service was restored but, to the casual observer, the sets still appear to be tuned to Ch.33!

If you use RF amplifiers, there is a greater chance of problems and filtering might be needed. At Dulwich, a CATV Forward/Reverse diplexer was used for Band I/III combining. This has a sharp cut-over between 65 and 85MHz, so would be no use where Ch. 5 was required, but that is not a problem here.

UHF/VHF filters can be a problem now that BI has been vacated throughout Europe. A Vision UHF/VHF diplexer bought for the museum was found to only be specified down to 85MHz whereas it used to cover all of Band I down to 40MHz. However, Vision aligned a filter especially for the museum, for which we are very grateful!

The bottom line is: don't worry!

A final note: If you are including Band II in your network, the 2nd harmonic of channels 1 & 2 vision carriers fall on 90MHz and 103.5MHz respectively and the 2nd harmonic of the channel 2 & 3 sound carriers fall on 96.5 & 106.5MHz respectively.
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Old 12th Mar 2016, 2:13 pm   #6
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Default Re: VSB Filter and Aurora

Hi Terry,

many thanks for your comprehensive reply. Much appreciated. The query regarding VSB Filter was more out of interest than anything else.

However, the question regarding spectral purity is relevant. I would like to use the Aurora as a mini free-space transmitter using a small indoor antenna and some amplification. I only want to enable reception in the house and not the whole village . But being under the flight path of the approach to a major airport I am a bit worried about the 2nd and 3rd harmonics which depending on channel will fall within the VHF airband. I am considering experimenting with low-pass filters designed for the 6m amateur band.

Am I being over cautious?

Thanks to all,
Eddie
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Old 12th Mar 2016, 10:43 pm   #7
terrykc
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Default Re: VSB Filter and Aurora

The real question to ask yourself here is why? The Aurora can cover quite a few outlets without any amplification with fully screened cable and no chance of causing any interference.

Presumably you are relying on broadcasters having vacated the VHF bands to give you a clear run? How can you be certain that those frequencies are not being re-used? In the UK, the old B1 channel is currently allocated to the Ministry of Defence who would not take kindly to such antics. Those frequencies were never used for TV in Germany so anybody could officially be using them.

The bit which really makes my blood run cold, though, is "being under the flight path of the approach to a major airport'

Please don't give this entirely unnecessary plan any further thought.

Quite frankly, having just looked at your occupation in your profile, I am appalled that you should even consider such a mad stunt.
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Old 13th Mar 2016, 9:20 am   #8
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Default Re: VSB Filter and Aurora

I fear that perhaps I didn't express myself clearly enough. Terry has shared my original concerns. I will kick this idea into the long grass.

Mods this thread can please be closed.
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Old 13th Mar 2016, 2:39 pm   #9
terrykc
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Default Re: VSB Filter and Aurora

Sorry to give you such a hard time in my previous post but you'd got me seriously worried ...

Here's an example of how unpredictable things can happen when you let RF out into the wild. It happened the best part of 55 years ago ...

I picked the next set off the pile and put it on my bench. The ticket said "RF interference - BBC only". I plugged it in and switched on, then tuned to Channel 9 - just to make sure.

Sure enough, there was a perfect ITV picture so I retuned to Channel 1 - where a perfect Test Card C greeted me! So I wiggled and waggled, poked and prodded, but zilch. In desperation I lifted one side of the cabinet a couple of centimetres off the bench and let go. Still nothing!

So I put the set to one side and got on with the next one.

Eventually, Norman put in an appearance so I asked him about the problem.

He'd done virtually the same as me in the customer's house - barring the drop test, of course! - to try and get some indication of what was causing the fault but the patterning had persisted.

We left the set on soak for couple of days before returning it to the customer, whereupon the interference returned! There wasn't much more to be done, except suggest to the customer that they raise the issue with the Interference Investigation Department of the GPO.

The customer did precisely that and kindly dropped in to tell of the result a week or so later.

The GPO had traced to source to an RF Heating system used to dry paint by a local timber company who manufactured large numbers of doors. The GPO had fitted the customer's aerial feed with a special filter which had cured the problem.

We were quite amazed by this as the customer lives about three kilometres away from the factory as the crow flies and, in a well built up area, there must have been thousands of TVs within a 3km radius, so why was this the only one affected?

A number of frequencies are reserved for medical and industrial purposes, 40.68MHz being the most likely culprit in this instance but it still seems a bit tenuous as the only obvious beat product is Ch.1 Sound 41.5MHz - 40.68 = 820kHz. However, I can't see this finding its way into the video stages unless it beats again with the Sound carrier to produce a further beat 820kHz higher, which (just) falls within the vision passband, 2.68MHz below vision carrier - not the most noticeable of frequencies, I think.

Unfortunately I never saw the original problem so have no idea what visual form it took and now, 55 years on, I suppose I never will!

It's still one of life's unexplained little mysteries, though ...

Incidentally, returning to the original subject, I do have an idea for producing VSB signals but the catch is that you will have to build your own IF modulator and output converter to go with it! Anyone interested?
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