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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions. |
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7th Sep 2008, 11:46 pm | #1 |
Pentode
Join Date: Aug 2005
Location: Northwich, Cheshire, UK.
Posts: 206
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Sync Insertion
Hi,
There's a host of problems assoc with the home made converter, not least its (my) poor ability to mix the sync pulses into the picture info coming off the Op Amp which takes the o/p of the DAC. Is there a sensible way to do this without the two video and the sync fighting seeming to fight each other ? It does work sometimes, but is sensitive to picture content. The general idea of the attached dodgy circuit in use is that when the sync pulse comes along, it fires the trannie which shorts the video to ground, in effect. My problems may in reality relate to coupling and black level clamping, ofcourse….. If there’s a better cct I could copy I’d be pleased to see it ! Pip pip JM |
8th Sep 2008, 8:13 am | #2 |
Retired Dormant Member
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
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Re: Sync Insertion
Not sure where to start here. 100u coupling caps what I presume to be a 75 ohm video path are going to damage the signal very badly. The transistor can pull down to give a reasonable falling edge of sync but the rising edge is going to be abysmal. Add in the effects of AC coupling and I realy don't want to predict what the waveform will look like. If you want to amuse us, how about a photo of the waveform.
The simplest approach is just a resistive mixing network. Not 100% ideal but let's assume you've got about 1.5Vp-p of video from the opamp and 5V of syncs from logic. Try around 560R from the sync source to the output and 82R from the video opamp tothe output. This will give you about 1Vp-p of video with about 300mV sync into a 75 ohm load. There will be a DC offset but this shouldn't matter if black level is below about 1V DC. Sync edges will be too steep but again this won't matter for simple experiments. Best method of all is to add the syncs digitally and use the DAC to convert the whole waveform. |
8th Sep 2008, 8:23 am | #3 |
Octode
Join Date: Jan 2003
Location: Birmingham, West Midlands, UK.
Posts: 1,268
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Re: Sync Insertion
Brute force isn't the way. What you really need is a switch.
Attached is a basic circuit (sorry, monday morning and haven't finished cuppa yet). Bias base volts so that "no video" gives 30% of your output signal. Pulse base high to turn it off -> 0v at collector = sync. Video source needs to be low impeadance and you need to watch what you drive the tranny with (volts need to be stable outside of sync period so watch out if AC coupling. Also, it is a common base configuration so that'll help keep the bandwidth up (though watch out for coupling of sync pulse into collector circuit, don't switch it too fast). AC coupling the input isn't good as there is no black level. However the cct attached is just to give example of using tranny as a switch. TTFN, Jon TTFN, Jon |
8th Sep 2008, 8:27 am | #4 |
Retired Dormant Member
Join Date: Dec 2003
Location: Solingen, Germany
Posts: 727
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Re: Sync Insertion
Hi, you can find some solutions in the MK II converter:
https://www.vintage-radio.net/forum/...ead.php?t=1757 The 625 line sync is eliminated at the input. The new 405 sync ist added at the output. I tested two different ways: https://www.vintage-radio.net/forum/...3&d=1108233028 sync constant current added to the non clamped vision signal (Q24), DC restorer (Q29) https://www.vintage-radio.net/forum/...7&d=1119087114 vision clamped on blanc level (Q36) sync add (Q39) The MK III converter replaces only the V-sync. https://www.vintage-radio.net/forum/...?t=7795&page=3 https://www.vintage-radio.net/forum/...5&d=1150622399 Kind regards, Darius |
8th Sep 2008, 8:40 am | #5 |
Retired Dormant Member
Join Date: Dec 2003
Location: North London, UK.
Posts: 6,168
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Re: Sync Insertion
Why bother to switch when you can add? Switching requires that black levels are matched. If the video is correctly blanked, sync can simply be added, either at the DAC output or at the final output. If the video isn't correctly blanked, then do that digitally before the DAC. If the sync waveform is mucky then clean it up with a dedicated HCMOS gate before adding it to the video.
Jon's circuit would need an output buffer unless the collector resistor is 75 ohms. |
8th Sep 2008, 10:03 am | #6 |
Pentode
Join Date: Aug 2005
Location: Northwich, Cheshire, UK.
Posts: 206
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Re: Sync Insertion
Chaps
Blimey ! Plenty to think about there. Many thanks for the sketches and web links More anon.... John |
8th Sep 2008, 10:57 am | #7 | |
Retired Dormant Member
Join Date: Dec 2003
Location: Solingen, Germany
Posts: 727
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Re: Sync Insertion
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