405 to 625 conversion

[`FRANK.C]

Thanks Trevor and Peter.

Whether converting from 625 to 405 or 405 to 626 the horizontal resolution don't really change. In either case 720 samples(pixels) are taken per line.

Frank

[ppppenguin]

It's a question of convenience when converting 405<>625 not to change the number of pixels along a line. For 625 line systems there are 702 visible pixels out of 720 active and 864 total. For 525 it's 704/720/852 respectively. 720 was chosen as part of the original CCIR 601 standard for digital video, along with a 13.5MHz pixel clock. This allowed 625 and 525 systems to have the same pixel clock and number of active pixels. It also allowed a margin for H blanking artefacts. What it didn't give was square pixels, something that became a minor nuisance in the computer age. HD standards all have square pixels.

For 405 we simply adapt the "601" standard by reducing the pixel clock by 405/625.

[television1936]

Thanks Frank for the 625-405-625 line demo really impressive. I will have to get round to building a Hedgehog II. I understand Hedgehog 1 is not compatible.

[`FRANK.C]

Hi television1936
You are correct 405 to 625 firmware will only work with a Hedghog II.

Frank.

[Jac Janssen]

Frank's design works fine when the input is a 405 line signal from a very steady source (like a 625 - 405 line converter ), but unfortunately the 405 line video signal from my VCR won't work.
Especially, as probably expected, the horizontal sync goes haywire.

Jac

[`FRANK.C]

Hi Jac
Thanks for testing and letting us know.
I had feared that might be the case if there is jitter on the line sync from the VCR.
I have no VCR myself to test

Frank

[ppppenguin]

Before Covid-19 I was planning to do some trials with 405 line tapes recorded on Sony CV2000. The guys with the CV2000 are in the archive recovery business and their route from 405 on CV2000 at the moment is both complex and awkward. All on hold at the moment.

[Mike Watterson]

I remember that with 625 Videotape jitter someone discovered that if you didn't have a super expensive new shiny TBC (Time Base Corrector) but had access to a Sound In Syncs system that it "fixed" jittery tapes. Mid 1970s BBC CTA. People would ring and ask us to route it. Those were the days just before Betamax and VHS. Richer people (Ad agencies) and News had the Sony-Umatic and poorer people (colleges) had Philips N1500, N1700, EIAJ 1/2 reel to reel and the cartridge variants. I don't know who owned the mad Akai colour 1/4" reel to reel. Saw that once. Real VTRs used 2" tape and didn't generally move. News was often on film.

[`FRANK.C]

Jeffrey I will be interested to hear how those trials go.

Frank

[ppppenguin]

Decoder chips, as used in many devices, have had to cope with VTR jitter. Now there is less need for this but I doubt if the capability has been dropped. The TVP150 as used on Hedghog and Aurora is a fairly modern device. The Aurora copes with VT replay. I don't know if anyone has tested Hedghog in normal 625>405 to see.

I suppose I could dig out a VHS machine, record some 405 on it and replay via my 405>625 converter. I would expect it to be OK but that's not the main purpose which is to recover CV2000 material. This has a nasty skip field system with non-standard V sync pulses. The other purpose is recovering archive Quadruplex where the jitter is very low and the waveform is accurate.

I didn't know that SIS had implicit timebase correction. I remember the revelation of first having a TBC (a CVS517) available in a small studio, c1976. Seemed miraculous.

The VR3000 Quadruplex (record only) was a backpack portable. If you were strong. Others Quads were on castors. Forklift and/or taill lift to move them more than a short distance. Up and down stairs was a job for specialists though I once arranged for members of the WIlliam Ellis School 1st XV to shift an RCA TR4 up one floor. They may not have been actual rugger types but they needed to be built that way. The TR4 was one of the smallest Quads. In the end we barely used it.