AV on a 405 line set?

[19Seventy7]

Hi all

I just had a thought, how easy would it be to inject composite video into a 405 line TV, bypassing the tuner. Would it even be possible due to the difference in resolutions from the TV and the source (like a STB for example.) It's not something I'd plan to do but was just curious if it's possible and if so the obstacles that'd get in the way.

Thanks
'77

[peter_scott]

If you are thinking of AC/DC TVs you'd be better just to build a modulator and connect via the aerial input.

Peter

[19Seventy7]

I personally would go the aerial route, as it makes the most sense. I was more curious on the technical side of it and it's possibilities. I think I'm thinking more of the 405 line system itself than any particular sets

'77

[jayceebee]

Not a difficult job to convert most sets to composite input but needs an isolation TX. Don't quite understand what you attempting to feed in, no STBs or DVD with 405 lines. Some line oscillators might just reach 625 with a bit of a twiddle or a couple of component changes but the LOPT won't be happy running at that frequency.

[19Seventy7]

It's not something I plan on actually putting into practice, it was more just a curiosity of if it was possible or not, as I know 625 line sets can be converted but didn't know about 405 line sets. Didn't know if composite output on a set resolution or was just the direct video signal itself either so that's something learnt

I did think the oscillators would've been adjusted myself, but wondered if it'd have been just as simple as that or had other technical hurdles to pass

'77

[jayceebee]

Unless you're going way back most video output stages would handle the 625 video bandwidth without too many issues. A cable TV company called Rediffusion modified lots of their cable 405 only sets to 405/625 operation but I suspect all will have needed an upgraded LOPT.

Actually a lot of people modified sets to baseband video in and out recording the 405 video on to VHS recorders then playing them back. This was before standard converters like the Aurora became affordable. Recorders like the Ferguson 3V29/30 could record 405 no problem but some issues could be seen on playback due to the 64uS drop out compensation.

[19Seventy7]

Funny you should mention Rediffusion as it's my Rediffusion (MK7, c1963) that made me think of all this. It's an aerial set but is missing the tuner, and so it brought up the idea, not that I plan to do it as I'd rather it was running on original circuitry.

Thats interesting about recording 405 line footage onto tape, I have actually read about it briefly before. Also funny you should mention the 3V29 as I've got one of those too! Maybe when I get a little more advanced I can try record 405 line to VHS with it and see for myself.

It's an interesting subject really

'77

[raditechman]

I did not think there were any Rediffusion MK7.s left. I worked on them at Rediffusion
in the 1960's. Not my favorite, set especially the aerial version. So pleased you have saved one.
Could you not get a different tuner to work and then feed a signal in.

John

[Graham G3ZVT]

When I did the initial tests with the PC graphics card, it was before I had built an RF modulator. I simply applied the baseband +ve video directly onto the grid of the video output valve.
I think I mention it here.
https://youtu.be/T2oNculjoQc

PS it's essential to use a mains isolation transformer.

[Dave Moll]

Originally Posted by 19Seventy7

Thats interesting about recording 405 line footage onto tape, I have actually read about it briefly before.

As long as the VCR doesn't attempt anything clever, such as the aforementioned dropout compensation, it is indifferent to what is recorded within a frame of video, which it records as a single helically-recorded stripe of 20mS (the frame period of both 625 and 405). Of course, once it replaces a 64μS line that it determines has dropped out, this will fail to match the line structure of 405 lines and cause various glitches.

[Lloyd 1985]

As has been said, it can be done, I did it during the restoration of my Ekco TMB272, which is a mains isolated 405 only TV, I wouldn’t try it on anything that wasn’t isolated. I used the composite output of the Aurora standards converter, fed via a capacitor onto the grid of the video amplifier valve, the picture was actually pretty good! I was trying it as the tuner/IF stages were not letting anything through at the time.

Regards
Lloyd

[Graham G3ZVT]

Originally Posted by 19Seventy7

Thats interesting about recording 405 line footage onto tape, I have actually read about it briefly before. Also funny you should mention the 3V29 as I've got one of those too! Maybe when I get a little more advanced I can try record 405 line to VHS with it and see for myself.

It's an interesting subject really

'77

It's sometimes said that old technology top-loaders from the 80s are more likely to give good results with 405, however I was presently surprised when I tried my relatively modern Panasonic NV-HV61EB. Judge for yourself:
https://youtu.be/P8MHUcEM0ts

Mind you, it may be a different story if the tape is played back on a different machine (I must try that sometime).

[19Seventy7]

Originally Posted by raditechman

I did not think there were any Rediffusion MK7.s left. I worked on them at Rediffusion
in the 1960's. Not my favorite, set especially the aerial version. So pleased you have saved one.
Could you not get a different tuner to work and then feed a signal in.

John

I'm surprised there's some left too, from what I've read they don't seem to have been well liked, so I imagine were scrapped at any given chance. Have I got myself confused and called the channel selector the tuner thinking they're the same thing when they're not?

When I did the initial tests with the PC graphics card, it was before I had built an RF modulator. I simply applied the baseband +ve video directly onto the grid of the video output valve.
I think I mention it here.
https://youtu.be/T2oNculjoQc

PS it's essential to use a mains isolation transformer.

That looks good, surprised it works as well as that considering it's doing something it was never designed to do. Seems very easy to do too if thats how it's done?

As long as the VCR doesn't attempt anything clever, such as the aforementioned dropout compensation, it is indifferent to what is recorded within a frame of video, which it records as a single helically-recorded stripe of 20mS (the frame period of both 625 and 405). Of course, once it replaces a 64μS line that it determines has dropped out, this will fail to match the line structure of 405 lines and cause various glitches.

I did always wonder about why they couldnt, i just thought if it's recording a 405 line video onto tape, it'd only store those 405 lines, therefore showing a 405 line image, but it makes sense now.

Does anyone have an example of what the glitches look like? Sounds interesting

As has been said, it can be done, I did it during the restoration of my Ekco TMB272, which is a mains isolated 405 only TV, I wouldn’t try it on anything that wasn’t isolated. I used the composite output of the Aurora standards converter, fed via a capacitor onto the grid of the video amplifier valve, the picture was actually pretty good! I was trying it as the tuner/IF stages were not letting anything through at the time.

Those Ekcos seem to be handy little sets. I'm starting to consider it as a possibility should I not get this working as hoped (I think it may be isolated, it has two transformers in the power supply) It just seems a shame to modify what i guess is a rare set, though it's easily reversible.

It's sometimes said that old technology top-loaders from the 80s are more likely to give good results with 405, however I was presently surprised when I tried my relatively modern Panasonic NV-HV61EB. Judge for yourself:
https://youtu.be/P8MHUcEM0ts

Mind you, it may be a different story if the tape is played back on a different machine (I must try that sometime).

Wow that looks good, especially for VHS. I'd be more than happy to have a result like that from VHS, 405 or 625 line. Would be interesting to see how it plays on a different machine.

(Sorry for the late replies, had poor internet for a while)

Thanks for the replies

'77

[peter_scott]

I don't know the set but I'd have thought it unlikely that a 1963 Redifusion would have a mains transformer.

Peter

[peter_scott]

Originally Posted by 19Seventy7

Does anyone have an example of what the glitches look like? Sounds interesting
'77

I don't know about the Mk7 but the Mk8 does not have a mains transformer.

I recorded a short piece of 405 line VHS. I am only playing it back on my Panasonic NV-FS90 and its drop out compensator causes a slight ghosting of the image which you should ignore and just concentrate on the actual dropped lines which are simply occasional horizontal flashes.

https://youtu.be/Z46N7imx-O0

Peter

[FRANK.C]

That is not bad at all Peter.
I was expecting to see something a lot worse.

Frank

[peter_scott]

Hi Frank,

Yes, the VHS video bandwidth shows up quite well at 405 line speeds. I'm playing it on my Sony 9-90 which on VHF suffers from a lack of DC restoration but I just used the UHF output of the VHS machine because the 9-90 can play UHF 405 with much better DC response.

Peter

[19Seventy7]

I don't know about the Mk7 but the Mk8 does not have a mains transformer.

I recorded a short piece of 405 line VHS. I am only playing it back on my Panasonic NV-FS90 and its drop out compensator causes a slight ghosting of the image which you should ignore and just concentrate on the actual dropped lines which are simply occasional horizontal flashes.

https://youtu.be/Z46N7imx-O0

I'm not 100% sure either, I could be totally wrong of course, I don't really know how to tell the difference between an isolating transformer or an ordinary one (if there is a difference?) At the risk of this thread focusing on the set I've posted a couple pictures below for anyone interested.

I agree with Frank.C and have to say it's much better than I expected. I was expecting a mess but that looks perfectly watchable to me, and I expect it looks better in person too?

Thank you for uploading that video, that was interesting to see, and see it performed so well at that

'77

[19Seventy7]

Forgot the photos

Oops!

(I was surprised at how big the cabinet is for a 19" set. That's it sitting on top of a 26" set for a size comparison)

[Lloyd 1985]

I’d say that’s not mains isolated, just going by the large green dropper resistor, those transformers are too small to be mains isolating types, more likely one is a choke and the other could be audio output or frame output transformers. Safest to assume it’s live chassis until proven otherwise!

Regards
Lloyd