Hi !
Since I have a couple of Umatic-VTRs around, I wanted to integrate them in my Y/C-video setup.
INTRO:
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 !)
CONCLUSIONS:
- 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...
QUESTIONS:
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..
VTR-Project:Umatic DUB to Y/C conversion
) oscillator ?
