VTR-Project:Umatic DUB to Y/C conversion - diyAudio
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Old 30th December 2001, 03:11 PM   #1
diyAudio Member
Join Date: Dec 2001
Post VTR-Project:Umatic DUB to Y/C conversion

Hi !

Since I have a couple of Umatic-VTRs around, I wanted to integrate them in my Y/C-video setup.

The S-Video Y/C-format uses Y & C signals, where the C-part is encoded @ 4.43 MHz (PAL)

To those who don't know - the 2 Umatic - studio VTRs came with DUB-connectors. That was a 7-pin port, that provides

1. separate Y-line (the luminance-signal), which is fully S-Video Y compatible (4.43 MHz)

2. a "color-under" coded C-line, which,depending on the tape-format works at different frequencies (<-and this is the trouble-maker - it's running at 688 - 924 kHz)

The Umatics provided these ports for better-quality machine to machine dubs, in order to circumvent the composite en/de-coding circuitary, which generates quite a loss in vid. quality.
The signals present at those dub-ports are basically as they get recorded on tape..

So I made a bit of investigation :

* According to my handy TV-technician-Cookbook, *all* home VTR are using (recording with) the color-under technique.
A VHS (PAL) for instance 626.9kHz, a Betamax 685.5 kHz & 689.4 kHz

* All VHS & Betamax VTRs are treating it's signal in the following manner:
1. they split it up into the Y & C-components - using BP/notch-filters
2. they modulate the C-component down to 626.9kHz

So all I need to do is use the same technique that a VHS-VTR uses to de/-modulate the FBAS 4.43 into it's internal subcarrier-format & vice versa - then I'm done right ?

* Reading further on, I found out, that the C-conversion is done rather quite simple - by applying some average FM-voodoo.

1. the C-signal gets filtered trough a BP-filter, to ensure we have a clean 4.43 MHz *modulated* signal present.
2. then the signal gets converted DOWN by simply mixing it with
a) a 4.434MHz color-carrier
b) an Oscillator, which is running exactly with a difference of 626.9kHz above the C-frequency (5.061 MHz for instance)
- which leads to a new subcarrier @ 626.9kHz !

* This voodoo is reversible - by simply applying a 5.061 MHz oscillator to the 626.9kHz color-under-coded (modulated) signal !

(- and maybe cleaning it up a bit with a filter - though in theory it's ready to use !)


- since I'll start to build this converter firstly for Y/C (S-Video)-ports, I don't need to worry about filtering, since the C-signal is already present in a "clean format".

- So the only thing I'd need to build is a precise 5.0xx MHz oscillator...

Am I correct about my conversion theory ?

Anyone made a Y/C-to composite AND composite to Y/C-converter - or something alike ?

Anyone out there using umatics, wanting to do (or have) the same thing ?

Anyone made an *precise* 5.00xxx MHz (and I mean precise at the "x"-ses ) oscillator ?

Should I go the PLL-route - or fiddle around with a quartz ?

How about building a low(er)-frequency oscillator & using clock-multipliers ?

Thanks for any hints in advance & happy holydays to everyone !

In the meantime, I'll play around a little bit with my oscilloscope and some transistor HF-oscillators..
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Old 3rd January 2006, 04:27 PM   #2
filmcan is offline filmcan  United Kingdom
diyAudio Member
Join Date: Jan 2006
Location: Lincolnshire UK
Default U-matic dub cable

Hello JanK!

I've just come across your ancient posting about a dub cable from U-matic to S video, [located vcia Google] as I'm wanting just such a thing to transfer many tapes to DV.

Did you ever make this device, and if so, did it work?

Are you able to provide me with necessary info or sell the cable/box to accomplish this? (I'm not as tech savvy as you!).

I look forward to hearing from you!

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Old 15th March 2015, 01:22 PM   #3
dhaen is offline dhaen  Europe
diyAudio Moderator Emeritus
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Join Date: Jun 2002
Location: U.K.
Default A solution

The archive I work for asked me for a solution to allow U-matic component dubbing. The unit I made has been successfully deployed to digitise some important collections at a better quality than previously possible using composite connection. It was apparent that no off-the-shelf solution exists so I made some more. The are available at the Keystrobe web site called "Dub-optimiser": KeyStrobe quality essentials
It's not rocket science, but it does have carefully tailored filtering for Y and C components for minimal artefacts.

Moderators, please move this post if it better suited to a different forum. I posted it here because it directly related to the thread. Thanks.
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