Locking the TPG11 from the 625 line field sync pulse.
The Murphy pattern generator uses the mains frequency as the reference to lock the master oscillator to the correct frequency. We need something more stable and the frame sync pulse of the 625 system is the answer. The counted down frame sync envelope pulse from the divider chain is coincident with the zero crossing point of the mains sine waveform and operates in a limited range about zero. To use the 625 sync pulse I'm considering using a variant of the circuit used in my standards converter. A CMOS 555 timer can be employed. The device is operated as a mutivibrator and triggered by the 625 field sync on the reset pin 4. The waveform across the ramp forming capacitor in the timing circuit can serve as the reference for the discriminator circuit. The CMOS 555 can operate at VCC 15volts so the waveform across the capacitor will be 1/3rd of VCC and used as a substitute for the reference derived from the 6.3V heaters supply.
Till Eulenspiegel.
Someone got a bargain this morning when a Murphy TPGII sold. It looks slightly different to mine as well as having many more of the 6SN7 in place of the ECC34`s
But will it be restored or simply used a source of Audiophool double-triode valves?
Till Eulensiegel.
My guess is stripped for its value.
I reckon only a dyed in the wool engineer would appreciate the design intricacies of the Murphy TPG11 pattern generator. As I have often mentioned before it wasn't easy designing high performance circuits in the old days before transistors, TTL and programmable devices.
Till Eulenspiegel.
Couldn`t resist the temptation to aquire a second TPGII, this chassis was re auctioned because the original buyer ony wanted the ECC34 valves.
Now to try and source quite a few used 6SN7GT valves !
Slightly off-topic, but does anyone know what form an EIA (525 line) to CCIR (625 line) standards converter would have taken in 1969? Optical or electronic?
Whatever you do, don't buy the Russian versions - they really don't work very well at all in it. A friend has done done some very clever (reversible) mods using the wire-mended 70-series valves (sometimes using the original octal bases) and it has transformed his TPG11 into something with rock-solid pulses.
@irob2345 I'm not sure if it was in service in 1969 but the BBC built their own electronic 525-625 standards converter for the 1970 Mexico World Cup.
@cathovisor Do you know what was used for the 1968 Mexico Olympics, the pictures were excellent albeit with the slightly reduced video bandwidth. On the CTV’s available for domestic use it wasn’t noticeable.
Frank
A friend has done done some very clever (reversible) mods using the wire-mended 70-series valves (sometimes using the original octal bases) and it has transformed his TPG11 into something with rock-solid pulses.
That sounds very interesting, would you be able to get any further details ?
Just wondering what would have been used to convert the Apollo 11 moon landing from 525 to 625.
Of course, it originated from a much lower resolution and was "optically" converted to 525 (long persistence phosphor, vidicon camera) before being linked to the US and beyond. In Oz, where the signal originated, the networks took the 525 feed from Parkes and did their own conversion.
Apparently, there was a tape recording made of the native signal from the dish (prior to any conversion) but despite an extensive search the tape has never been found.
@nuvistor There's info on the CO6/506 here: https://www.bbc.co.uk/rd/publications/rdreport_1970_37 and there is some information on the TV Camera Museum too.
See also: https://www.bbc.co.uk/rd/search?query=standards+conversion&submit=
That sounds very interesting, would you be able to get any further details ?
I can ask, but it's unlikely.
It's agreed the TPG11 will be an excellent source of 405 line sync pulses. The method of synchronising the frame sync to the mains is clever but doesn't lend itself to locking to the 625 line frame sync pulse.
My take of the Murphy frame sync lock is the 50Hz reference pulse from the count down circuits is coincident with the zero crossing point of the AC waveform.
To synchronise the two standards the input frame sync pulse will have to be altered in a manner to resemble the AC waveform. A suggested circuit can be found on page three of this topic. Some adjustment of the position of the zero cross-over point will be required in order that it is co-incident with the 625 frame sync pulse.
I should be possible to improve the TPG11 by adding definition bars into the display. Having decided the position of the bars the frequency of the bars can be controlled by some type of logic employing vintage techniques.
Till Eulespiegel.
@till If memory serves, the TPG11 - with BBC mods, of course! - was used as a sync pulse generator for OB vehicles at one point; it gets mentioned in the BBC ACO folders - Apparatus in Code Order.
