Digital TV Signal to the old UK 405 line standard?

[spec]

In 1960s UK, Bush 14 inch black and white, 405 line TVs were dirt cheap. I had two of these sets. The first was in the RADAR trailer rest room when I was in the Royal Air Force (RAF), and the second was a bit later when I got married.

Just for old times sake, I have often thought about getting an old Bush TV again and completely refurbishing it- the cabinets were in a nice wood veneer. The bush TV would make a good conversation point in the lounge.

I was also thinking about building a digital TV signal to 405 line analog signal converter so that the set could display a picture- is this a practical proposition?

spec

 

[throbscottle]

Thinking about the proliferation of STB's which are now rattling around in drawers since people have got new digital TV's, you are partway there with one of those.
I'm sure there must have been some 625 to 405 converters produced back in the day too, might be worth looking for. Then you would have a 2 box system without needing to invent anything. But where's the fun in that?
I know there were dual-standard sets, I dismantled a few when I was a kid! Possibly worth finding schematics for those for inspiration.
I was only taught about 625 in college and I've forgotten most of it since I never did electronics professionally. I would guess the conversion is mostly about throwing some lines away. Something along the lines (pardon the pun) of a sawtooth generator so the ramp allows 5 out of every 6 lines through, then the down-stroke is similar to flyback in principle and that line is lost.
How the z axis information differs between the systems I don't know. I have vague feeling bit of the 625 signal are negative going, where the 405 equivalents are positive?
(edit - with 5 out of 6 lines you get a bit of a fudge factor, but it's near enough considering the few lines that were given to ceefax etc anyway)

 

[spec]

Thinking about the proliferation of STB's which are now rattling around in drawers since people have got new digital TV's, you are partway there with one of those.
I'm sure there must have been some 625 to 405 converters produced back in the day too, might be worth looking for. Then you would have a 2 box system without needing to invent anything. But where's the fun in that?
I know there were dual-standard sets, I dismantled a few when I was a kid! Possibly worth finding schematics for those for inspiration.
I was only taught about 625 in college and I've forgotten most of it since I never did electronics professionally. I would guess the conversion is mostly about throwing some lines away. Something along the lines (pardon the pun) of a sawtooth generator so the ramp allows 5 out of every 6 lines through, then the down-stroke is similar to flyback in principle and that line is lost.
How the z axis information differs between the systems I don't know. I have vague feeling bit of the 625 signal are negative going, where the 405 equivalents are positive?
(edit - with 5 out of 6 lines you get a bit of a fudge factor, but it's near enough considering the few lines that were given to ceefax etc anyway)

Some good info TC,

I hadn't thought about pirating some of the old designs.

My initial thoughts were to have a virtual picture written to RAM from the digital signal over HDMI and read out for the analog signal.

I thought this could possibly be done with a single board computer, like a Raspberry Pi for example.

spec

 

[throbscottle]

My initial thoughts were to have a virtual picture written to RAM from the digital signal over HDMI and read out for the analog signal.
spec

Ahh bloo bloo blah?
I still speak analogue, sorry :/

 

[spec]

Vat es das analog?

spec

 

[AnalogKid]

I'm sure there must have been some 625 to 405 converters produced back in the day too,

Nope. The 625-line broadcast standard, later named PAL when color was introduced on top of it, was introduced in 1964. At that time transistors in TV receivers were rare; they were all-analog, all valve devices. Since by definition all-electronic scan conversion requires the storage of at least one full frame of video, such technology was a decade away at the station and two decades away in the home. The scan converters at the time were a high-quality video monitor of the input scan standard pointed at the lens of a tv camera of the output scan standard.

ak

 

[spec]

Nope. The 625-line broadcast standard, later named PAL when color was introduced on top of it, was introduced in 1964. At that time transistors in TV receivers were rare; they were all-analog, all valve devices. Since by definition all-electronic scan conversion requires the storage of at least one full frame of video, such technology was a decade away at the station and two decades away in the home. The scan converters at the time were a high-quality video monitor of the input scan standard pointed at the lens of a tv camera of the output scan standard.

Very nicely put AK. After your comments I now remember how difficult and expensive scan converters were.

spec

 

[Les Jones]

Hi spec,
This thread must be my fault with my silly comment about TV transmisions. We did not have a TV before the 405 line standard. My father built our first TV. It was built fom goverment surplus parts. The sound and vision receivers were totaly separate from each other and were I think modified radar receivers. The display was a 6" diameter tube (VCR97) with a green phosphor. I think this must have been in the late 1940s as I remember watching a childrens program program called Muffin the mule. Another strange thing is I still remember the names of two of the program presenters / news readers. Here is a story about a "fault" one night with this or possibly a later version which had a 9" (I think) tube with white phosphor and magnetic deflection. The top of the cabinet was just a plywood board and it normaly had newspapers and books piled on top. Back to the "fault". There was no display on the tube so my father started looking for the fault. At some point he must have removed the plywood cover with all the items still piled on top of it. He got it working again and the board was put back on top. The fault had come back. I think this happend a few times before all the things that had been piled on the board were removed. It turned out that I had left a powerful magnet from a magnetron on the board which was deflecting the electron beam in the tube so much it did not reach the screen. I don't know if I remember this event from when it happened or from being told about it at some later time.

Les.

 

[ericgibbs]

hi spec and Les,

Maybe a little before your time, but I built a TV set using the VCR97 CRT [Green phosphor] ~6 inch dia and EF50 valves from a Radar unit during the 1950's.
Used copper pipe to make the dipole.

Eric

BTW: the VCR97 made a great little Scope, about 10kHz TB with a Neon TB generator.

 

[spec]

Hi Les,

This thread must be my fault with my silly comment about TV transmissions.

Not silly at all- I thought it was funny.

We did not have a TV before the 405 line standard. My father built our first TV. It was built from government surplus parts. The sound and vision receivers were totally separate from each other and were I think modified radar receivers. The display was a 6" diameter tube (VCR97) with a green phosphor. I think this must have been in the late 1940s

How strange, my father did roughly the same thing in about 1950. But he built the set for a newsagent- we could not afford even a government surplus TV kit.

I remember watching a children's program program called Muffin the mule. Another strange thing is I still remember the names of two of the program presenters / news readers.

Again, same here, but I didn't remember the names.

There was a well-off household in our area and they actually had a TV. Some afternoons the lady of the house would let about eight of us kids watch Muffin the Mule at about 2:30 in the afternoon. We all sat silently in a darkened room and watched the 'potters wheel' expectantly for about ten minutes before Muffin came on. Us kids where totally enthralled by whole experience.

spec

 

[spec]

Hi Eric

...I built a TV set using the VCR97 CRT [Green phosphor] ~6 inch dia and EF50 valves from a Radar unit during the 1950's.

That sounds similar to the TV set that my Dad built.

the VCR97 made a great little Scope, about 10kHz TB with a Neon TB generator.

Yes, I remember the VCR97 well. My first scope used a CV138 (or similar number) 3 inch green tube, with valves. All parts, apart from the tube, which was government surplus from Edgware Road, London, were scrounged from old TVs.

I think that scope had a bandwidth of around 10Khz too.

spec

 

[Nigel Goodwin]

As far as I'm aware, you can buy 625 to 405 converters, either ready made, or as kits - they are popular with the vintage TV crowd.


I seem to recall seeing a PIC based one on the web a while back, with all the details to build your own?.

Here's a commercially available one:

https://www.tech-retro.com/Aurora_Design/Single_Converter.html

 

[spec]

As far as I'm aware, you can buy 625 to 405 converters, either ready made, or as kits - they are popular with the vintage TV crowd.


I seem to recall seeing a PIC based one on the web a while back, with all the details to build your own?.

Here's a commercially available one:

https://www.tech-retro.com/Aurora_Design/Single_Converter.html

I wondered if you would contribute Nigel- thanks for info.

spec

 

[throbscottle]

So my idea is impossible then? Pity, I quite liked it...

 

[spec]

So my idea is impossible then? Pity, I quite liked it...

The important thing is to have an idea.

spec

 

[throbscottle]

Yeah, I suppose it stops my brain from seizing up

 

[AnalogKid]

As far as I'm aware, you can buy 625 to 405 converters, either ready made, or as kits

An interesting and succinct way to summarize the last 50 years of electronic circuit development. As an old video geezer, I'm glad to see there is enough interest in "the old ways" to foster a market for such a device.

ak

 

[Nigel Goodwin]

So my idea is impossible then? Pity, I quite liked it...

Sorry, but it made no sense in any way