ITV Channel 11 Test Transmitter.

[Terry]

With my wonderful powers of hindsight it occurs to me that large numbers of sophisticated modulators would, presumably, have been scrapped by the cable companies in the changeover to digital.

VSB UHF/VHF System I, but I'm sure the VHF ones could have been modified for +ve video mod and AM sound.

Given that these modulators usually had an output of +60dBmV (1 volt!), three of these and a passive combiner could have run the entire Dulwich museum RF system without using an amplifier!

[Murphyv310]

Hi Terry.
Yes the output is quite high and indeed would easily run the system at the museum. The unit is bereft of harmonics according to my spectrum analyser, another plus.
This unit though is pure system A it's certainly not designed around negative modulation 

[Terry]

... This unit though is pure system A it's certainly not designed around negative modulation ...

I wouldn't expect anything different, Trevor, as that is the system it was obviously designed for!

As for modifying a System I modulator for System A, this should be reasonably easy - and much easier than  than an A to I conversion, because of the FM sound.

In fact, the sound modulator in the System I modulator is no longer required.

Modifying the Vision modulator for positive mod shouldn't pose many problems and an external AM sound modulator working on 3.5MHz should be reasonably easy to build.

Assuming the IF SAW in the modulator uses the TAC frequencies of 39.5MHz and 33.5MHz for sound and vision respectively, the ideal response of the SAW should be as shown.

(Note that some System I SAWs follow European practice, which would mean the frequencies shown will be 600kHz high, but it doesn't affect the basic argument.)

If we move the Vision modulator to 34.75MHz  and feed it with both the System A video and the AM sound mixed together* the output will be a composite System A VSB IF, for translation to any required output channel.  It will have the correct 0.75MHz vestigial sideband and, provided there are not unduly high levels of video components above 4.5MHz, would be highly suited to systems requiring adjacent channel operation.

* It may be necessary to inject the 3.5MHz sound signal directly into the modulator section to avoid any problems with video clamping circuitry.

[Geordie McBoyne]

It's an attractive idea but is it possible on System A to insert the AM sound in the form of a sub-carrier? That is simply modulate a 3.5Mhz carrier and supply the vision modulator with both the baseband video and the modulated 3.5Mhz.
It's one of those things I've often considered but never put in practise.
The modulated sound carrier will be above the vision carrier which is incorrect for Sytem A.

Geordie McBoyne.

[Terry]

The modulated sound carrier will be above the vision carrier which is incorrect for Sytem A.

No! This IF, remember! With a local oscillator running at 79.75MHz, for example, Fv = 79.75 - 34.75 = 45MHz and Fs = 79.75 - 38.25 = 41.5MHz.

As for generating the AM sound at 3.5MHz, 34.75 + 3.5 = 38.25MHz, which is the required sound IF. It might complicate things if the sound IF was generated at 38.25MHz which means that it would have to be introduced between the vision modulator and the SAW - my idea is to keep the modifications as simple as possible and mixing both before the vision modulator is much easier - particularly if the schematic for the original modulator is not available! 

SAWs can do unpredictable things if you upset the impedance matching!

[ppppenguin]

Perhaps I'm being a bit thick but some simple (awkward?) questions.

There are 2 metal boxes. Have we established that the bigger one is the modulator while the smaller one is a VSB filter?

If the smaller one is a VSB filter why does it have 3 RF connections? Where does the 3rd one go?

Is it established that the modulator is indeed an IF one with frequency translation? Or is it direct modulation to Ch11?

Is there a sound modulator already in the thing? Possibly fed via one of the contacts on the edge conenctor.

[Brianc]

I would suggest that the smaller box is the modulator, the third output, which goes to the front panel, being a monitor output . The larger box is the filter as the output goes directly to the rear of the module and I assume that there is a coaxial connector under the "tongue" that can be seen to the rear of the box - say!
Cheers
Brian

[Murphyv310]

The larger box is the oscillator and multiplier. The small box is the ring modulator, with video in, RF in and output. There is also an output on the oscillator at Crystal frequency.

[Murphyv310]

Hi.
Sorry my last reply was done on my phone awaiting Anne's return to the car!
The multi connector on the rear of the unit only uses ground and one other connection for Negative DC input, the Supply Is positive earth. Makes me think it could have been for field use in a positive ground car? So it begs the question is this older than the late 60's I've already thought it is.

[ppppenguin]

+ve earth could be related to telephone exchange practice.