View Full Version : Color subcarrier visible in retrace on early TVs


Gleb
03-24-2015, 05:05 PM
Hello colleagues!

When I tried to connect a DVD player to one of my antique TVs, I saw a strange interference on the screen:

http://videokarma.org/attachment.php?attachmentid=186312&stc=1&d=1427233336

I attached that player to a scope and found the source of interference. In the back porch of the horizontal blanking interval there is some oscillation burst, next to the sync pulse:

http://videokarma.org/attachment.php?attachmentid=186310&stc=1&d=1427233336


http://videokarma.org/attachment.php?attachmentid=186311&stc=1&d=1427233336

According to the frequency (4,43MHz), that's the "colorburst" signal used to synchronize color decoders. But it's amplitude is enormously big, exceeding the black level and causing it's visibility on the retrace. Of course, later TVs have built-in retrace blanking circuits. Depending on it, modern video signals often use "unnecessary" blanking intervals as a free space to carry any additional information. First ones were color signals, then they added teletext, copy protections etc. But very early TVs lack their own retrace blanking, depending only on blanking pulses of standardized video signal. So, any shit stuffed to blanking intervals will be shown by retrace lines.

Anybody knows the solution?

Eric H
03-24-2015, 05:50 PM
Get a Progressive scan DVD player and only connect the Green video connector, there's no color signal on it.

Electronic M
03-24-2015, 06:20 PM
A 12 cycle burst at 4.43MHz? This ain't NTSC standard video is it.

Gleb
03-24-2015, 06:32 PM
A 12 cycle burst at 4.43MHz? This ain't NTSC standard video is it.

Yes, it's PAL standard. I live in Russia where TV broadcasting uses SECAM standard but DVD, VHS and others usually use European PAL standard. By the way, I've never met such problems while receiving usual SECAM broadcasting.

Get a Progressive scan DVD player and only connect the Green video connector, there's no color signal on it

Does the Green out contain full brigtness signal ?

NoPegs
03-24-2015, 07:19 PM
Yes, it's PAL standard. I live in Russia where the TV broadcasting uses SECAM standard but DVD, VHS and others usually use European PAL standard. By the way, I've never met such problems while receiving usual SECAM broadcasting.



Does the Green out contain full brigtness signal ?

Indeed it does. It contains the "4" in 4:2:2 encoding. The Blue and Red jacks are just the difference signals, Green contains the whole signal. You may have to fiddle with the DVD player menus a bit to get it to run non-progressive video out of the component outputs. If you set it to 480P it'll look like your vertical sync is off by 2x.

Gleb
03-25-2015, 01:45 AM
You may have to fiddle with the DVD player menus a bit to get it to run non-progressive video out of the component outputs. If you set it to 480P it'll look like your vertical sync is off by 2x.

I need 576i (625/50).

I'm interested in a more general solution like some kind of filter or blanking pulse shaper. It seems like the days of analog broadcasting are numbered, and the problem may reappear with digital TV receivers. Who knows how they will decide to f..ck up that blanking intervals in the future...

I've aldeady messed with visibility of teletext signals in vertical retrace. I developed a retrace blanking circuit and installed some sort of it in each antique TV. It's just a simple differentiating circuit which forms rectangular pulses from the vertical sawtooth fall and injects them into the brightness control. Of course, I made it out of vintage components so they are almost invisible in the original circuits.

old_tv_nut
03-25-2015, 10:16 PM
As was posted above, if you can switch the player to component mode, you will have the Y (luminance) on the green output. Another possibility is if your player has an "S-video" output (don't know if you have such on a PAL player), one output will be clean luma and a separate one will be subcarrier, which you don't need.

Phil Nelson
06-08-2015, 01:59 AM
I developed a retrace blanking circuit and installed some sort of it in each antique TV. It's just a simple differentiating circuit which forms rectangular pulses from the vertical sawtooth fall and injects them into the brightness control. Of course, I made it out of vintage components so they are almost invisible in the original circuits.Would you mind sharing the details of this circuit?

Just curious.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Gleb
02-04-2017, 09:25 AM
Would you mind sharing the details of this circuit?

Phil, better late than never, sorry for delay. If a CRT is driven the 'classical' way (the signal to the cathode, the brightness control to the 'grid'), it comes pretty simple:

http://videokarma.org/attachment.php?attachmentid=193635&d=1486214957


Just cut up the wire from the CRT's grid, and connect a 150-200k potentiometer to the gap. Then take a signal from the vertical oscillator tube's plate, and feed it to the CRT's grid through a capacitor of a few thousand picofarads. Balance the potentiometer to blank the retrace lines completely and not to let the picture get distorted. Then measure the actual resistance of the potentiometer, and install a resistor of the closest value instead of it.
It gets more tricky if the CRT is driven the 'odd' way (the signal to the grid, the brightness control to the cathode). In such a case, the flyback pulses from the vertical oscillator have the 'wrong' polarity to feed them to the cathode of the CRT. So you have to get them from the plate of the vertical output tube where the flyback pulses are dirty and distorted by the reactance of the vertical deflection coils and/or transformer, and need to be extra cleaned and shaped before feeding them to the CRT.

Phil Nelson
02-05-2017, 02:44 AM
Thanks, Gleb.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html