"If you were a crank...." - UK Vintage Radio Repair and Restoration Discussion Forum

Brigham · 15th Apr 2006, 11:39 pm

Hello,
Just been reading about the 'Vest Pocket' Aurora. One of my many 'impossible' schoolboy inventions was a tobacco-tin sized 'plug-in' which went between the aerial downlead and the set, and let you get BBC2, like the posh kids! Yesterday's science fiction.......!
More seriously, If you were a crank, and you had the correct period recorded programmes from both BBC and ITV, and seperate machines to play them simultaneously, would it be possible to use a pair of the new budget-priced Auroras, together with attenuators and a band I/III diplexer, to create the effect of 'turning over' to see what was on the 'other side'?
You would have to be a crank, of course!

Cheers. B.

ppppenguin · 16th Apr 2006, 7:27 am

Quote:

Originally Posted by Brigham

would it be possible to use a pair of the new budget-priced Auroras, together with attenuators and a band I/III diplexer, to create the effect of 'turning over' to see what was on the 'other side'?.

Yes, with qualifications. I haven't seen and measured one yet, but there are probably harmonics from the modulator. This means that some band 3 channels may be fouled up by harmonics from the band 1 modulator. Many pairs of channels will be OK. A proper selective diplexer band 1/3 will fix this completely.

oldeurope · 16th Apr 2006, 11:49 am

Hi,
guess why I use these complicated filters at the output of my modulators.

But I sadly learned that noise in the picture is well accepted on an old set.

Kind regards
Darius

Kat Manton · 16th Apr 2006, 1:08 pm

Hi,

Flying dangerously close to drifting OT, here; but still staying within the general theme of "having several 405-line VHF channels rather than just one" I have a moderately insane idea...

With an indecently rapid PC and some hacking about with the software it wouldn't be impossible to fit three tuner cards in a PC. Theoretically one can then arrange to have the three image streams from the cards greyscaled then combined into one colour image such that one channel is in red, another is in green, and the third in blue. Then you just need to add the syncs seperately to the three RGB card outputs; feed each of these three now composite monochrome outputs to three modulators set to different channels...

Don't expect me to demonstrate this anytime soon; I have plenty of other mad ideas to be getting on with for now and there's only one of me, only 24 hours in a day, etc., etc.

Regards, Kat

ppppenguin · 16th Apr 2006, 1:28 pm

Kat, doesn't sound too implausible to me.

The approach that Darryl and I discussed some time ago (and I think some of it was in the forum too) was direct digital synthsis at the final RF carrier frequency. This can do multiple channels without any fundamental problem except that as you get above band 1 the digital clock frequency gets a bit impractical. I suppose you could synthesise at a suitable IF and heterodyne the whole lot up to RF. The conclusion we reached was that it was almost certainly technically feasible but a whole lot more work than any of us reckoned was worth it.

oldeurope · 16th Apr 2006, 6:25 pm

https://www.vintage-radio.net/forum/...ead.php?t=2837

Good evening,
I think the London- Birmingham modulator is the ultimate solution.
It makes all 405 sets working without any switching.
The only complicated thing is the filter, but until now I haven't seen
any modulator that don't need a filter. The axing modulator uses the
MC44B_373 and has a switched filter banch at its output. Not good but
much better than nothing.

Darius

Sean Williams · 16th Apr 2006, 6:34 pm

The DDS idea is a good one, and with a fixed crystal oscillator and mixer on the output you could generate band 1 and 3 signals fairly easily, you do still need filtering, which becomes complicated when you have to change frequencies so far apart - simple at up to 30Mhz, but into VHF the additional inductance and loss is not so acceptable.

There is no reason why the modulator Darius has designed cannot be converted for ch2 or 3, the one I have built does channels 1 and 3 at the same time, however, there are some very strange mixing products on the output!

No real need for things to be so complicated

Cheers
Sean

Steve · 19th Apr 2006, 3:30 pm

I wonder if it would be possible to use one of these - http://www.gnu.org/software/gnuradio/ - to generate the signals? With some very clever software could you can take an off air signal with one daughterboard (maybe a couple of channels in a digital boquet?), convert them to 405 and then generate the relevant Band I/III signals?

15th Apr 2006, 11:39 pm	#1
Brigham Octode Join Date: Mar 2006 Location: Co. Durham, UK. Posts: 1,115	"If you were a crank...." Hello, Just been reading about the 'Vest Pocket' Aurora. One of my many 'impossible' schoolboy inventions was a tobacco-tin sized 'plug-in' which went between the aerial downlead and the set, and let you get BBC2, like the posh kids! Yesterday's science fiction.......! More seriously, If you were a crank, and you had the correct period recorded programmes from both BBC and ITV, and seperate machines to play them simultaneously, would it be possible to use a pair of the new budget-priced Auroras, together with attenuators and a band I/III diplexer, to create the effect of 'turning over' to see what was on the 'other side'? You would have to be a crank, of course! Cheers. B.

16th Apr 2006, 11:49 am	#3
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Re: "If you were a crank...." Hi, guess why I use these complicated filters at the output of my modulators. But I sadly learned that noise in the picture is well accepted on an old set. Kind regards Darius

16th Apr 2006, 1:08 pm	#4
Kat Manton Retired Dormant Member Join Date: Jul 2005 Location: West Yorkshire, UK. Posts: 1,700	Re: "If you were a crank...." Hi, Flying dangerously close to drifting OT, here; but still staying within the general theme of "having several 405-line VHF channels rather than just one" I have a moderately insane idea... With an indecently rapid PC and some hacking about with the software it wouldn't be impossible to fit three tuner cards in a PC. Theoretically one can then arrange to have the three image streams from the cards greyscaled then combined into one colour image such that one channel is in red, another is in green, and the third in blue. Then you just need to add the syncs seperately to the three RGB card outputs; feed each of these three now composite monochrome outputs to three modulators set to different channels... Don't expect me to demonstrate this anytime soon; I have plenty of other mad ideas to be getting on with for now and there's only one of me, only 24 hours in a day, etc., etc. Regards, Kat

16th Apr 2006, 1:28 pm	#5
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: "If you were a crank...." Kat, doesn't sound too implausible to me. The approach that Darryl and I discussed some time ago (and I think some of it was in the forum too) was direct digital synthsis at the final RF carrier frequency. This can do multiple channels without any fundamental problem except that as you get above band 1 the digital clock frequency gets a bit impractical. I suppose you could synthesise at a suitable IF and heterodyne the whole lot up to RF. The conclusion we reached was that it was almost certainly technically feasible but a whole lot more work than any of us reckoned was worth it.

16th Apr 2006, 6:25 pm	#6
oldeurope Retired Dormant Member Join Date: Dec 2003 Location: Solingen, Germany Posts: 727	Re: "If you were a crank...." https://www.vintage-radio.net/forum/...ead.php?t=2837 Good evening, I think the London- Birmingham modulator is the ultimate solution. It makes all 405 sets working without any switching. The only complicated thing is the filter, but until now I haven't seen any modulator that don't need a filter. The axing modulator uses the MC44B_373 and has a switched filter banch at its output. Not good but much better than nothing. Darius

16th Apr 2006, 6:34 pm	#7
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Re: "If you were a crank...." The DDS idea is a good one, and with a fixed crystal oscillator and mixer on the output you could generate band 1 and 3 signals fairly easily, you do still need filtering, which becomes complicated when you have to change frequencies so far apart - simple at up to 30Mhz, but into VHF the additional inductance and loss is not so acceptable. There is no reason why the modulator Darius has designed cannot be converted for ch2 or 3, the one I have built does channels 1 and 3 at the same time, however, there are some very strange mixing products on the output! No real need for things to be so complicated Cheers Sean __________________ Engineers make things work and have spare bits when finished

19th Apr 2006, 3:30 pm	#8
Steve Tetrode Join Date: Aug 2003 Location: Cambridge, Cambridgeshire, UK. Posts: 91	Re: "If you were a crank...." I wonder if it would be possible to use one of these - http://www.gnu.org/software/gnuradio/ - to generate the signals? With some very clever software could you can take an off air signal with one daughterboard (maybe a couple of channels in a digital boquet?), convert them to 405 and then generate the relevant Band I/III signals?