Interference that looks like Macrovision but isn't
 

I'm trying to figure out whether this is a problem with our cable installation or something else. I have hooked up one of our (Comcast/Xfinity) cable boxes to an agile modulator to broadcast cable throughout the house. Recently, I noticed that many of the lower channels (under 150 or so) are showing what looks interference with the vertical signal:

http://antiqueradio.org/art/temp/RCA...FadingBars.jpg

The interference bars fade in and out over a period of several seconds, and they seem to come down from the top of the screen. The interference only appears on vintage TVs (both B/W and color); modern TVs are immune.

I have seen similar interference when playing Macrovision-protected DVDs on old TVs, as in the following photo:

http://antiqueradio.org/art/MacrovisionInterference.jpg

I can eliminate the Macrovision interference by running the signal through a video stabilizer, but I tried the stabilizer on this, just for kicks, and it had no effect.

The problem has nothing to do with my agile modulator (it appears when I connect the cable box directly to a TV with an RF modulator). I also swapped cable boxes and tried different locations in the house, to eliminate boxes and locations from the equation.

I don't have a photographic memory, so I can't swear that this interference never happened before, but it seems to be a recent development. Again, this is only present on some of the cable channels below about channel 150.

Any ideas about what's going on? At one point I wondered if it might be closed-captioning data, which is more likely to be present on our lower cable channels (largely local and network broadcasting) than on higher channels (HBO and the like). But I have no easy way to tell which broadcasts have closed captions.

I put my oscilloscope at a couple of spots in the vertical sweep circuit of my CTC-11, which is currently on the workbench, and didn't see any obvious signal distortions when the interference fades in and out.

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

If it fades in and out, it is not data, but must be a form of copy protection. It must be on different lines than your stabilizer eliminates. It may be possible to modify your stabilizer so it blanks these lines.

Anyway it's some sort of modern shit stuffed to the vertical blanking intervals. Sometimes it exceeds the blank level causing visibility and luminance modulation of retrace lines. I prevent any problems of that kind by adding simple vertical blanking circuits to my TVs.

Do you mean a blanking circuit along these lines?

http://www.earlytelevision.org/vertical_blanking.html

I have added that type of circuit in a couple of my B/W sets (Admiral 19A12 and Philco 49-1240).

Out of curiosity, I walked around and turned on a few sets to see if all of the old ones show this interference. Here's the score card:

DuMont RA-103: no
DuMont RA-113: no
Philco 49-1240: yes (despite vertical blanking mod)
Philco Predicta Siesta H3412L: yes
Philco Miss America F4626M: very very faint
RCA CTC-7: yes
RCA CTC-11: yes
unknown 16" TV in Scott 800B combo: no

Now I'm at least as puzzled as before. The interference only shows up on certain cable channels. Some sets show it, and others don't.

Phil Nelson

Supposedly there was an 'upgraded' form of macrovision implemented in the last few years that some broadcasters can use to allow the signal to be only copied once by more modern equipment (so you can time shift it, but not dub copies of that to sell).

I wonder if this is that.

Of course, the quick way to get unpuzzled is to look at a baseband video output with a scope.

Thats a good question.

I would not put it pass them if they are doing it.With todays DTV/cable they can do anything.Like the old Twilight Zone shows.They can control anything.
It sounds scary .

I remember some of the older TV dont like prerecorded movie tapes with the macro crap imbedded in the signals.Back in the day in the repair shop.The customers were complaining about this and the manufacturers were sending out bulletins with the fix for this problem.

Bear with me while I ask a stupid question: by baseband, do you mean the video signal coming directly from the cable box?

Here's the video signal from a high channel (521) showing a dark letterboxed vampire movie.

http://antiqueradio.org/art/temp/Cab...ighChannel.jpg

Here's the video signal (nothing changed on scope) from a low channel (4) showing the Wheel of Fortune stage full-screen.

http://antiqueradio.org/art/temp/Cab...LowChannel.jpg

They sure look different, but I'm at a loss how to interpret . . . .

Phil Nelson

On the first trace, the scope is synced at horizontal rate. On the second, if you didn't change the sweep rate, the scope has lost sync.

You need to set the sweep rate to vertical rate, and look for the pulses in the first few scan lines of the signal after vertical sync. You can compare which lines the pulses are on from the cable box to which ones they are on from a DVD player, for example.

[edited a few typos]

Okay, at that speed I can see it happening.

The first trace shows the video when the interference is absent:

http://antiqueradio.org/art/temp/InterferenceAbsent.jpg

The second shows the culprit when interference is at its peak:

http://antiqueradio.org/art/temp/Int...ncePresent.jpg

The circled pip starts at the bottom of the trace and glides straight up, pauses at the top, then moves down again. Inteference is worst when it's at the top, absent when it's at the bottom.

The interference exactly matches the movement of this thing . . . whatever it is.

Edit: I added a link to a brief video that shows the demon pip in motion. Watch the left edge of the trace, and note the audio buzzing that accompanies the interference:

http://antiqueradio.org/art/temp/Ver...terference.mpg

Phil

On page two of this pdf, it shows a 608 caption data waveform as it would be on an analog composite video signal. No caption data, just the static waveform.

https://wiki.millersville.edu/downlo...=1393615787752

(The other info in the pdf is related to 708 Hi-def captions, and does not apply here.)


Your images could show a signal from some closed-caption encoder run amok.
If captions, the waveform should be on line 21, field 1. There are synchronizing run-in clock pulses on the left, and data bits on the right. The waveform should be from zero to 50 on the waveform scale. An active closed-captioning waveform will have smaller 'dashes' in the right hand area that change every frame.

If the encoder is unterminated, it would be roughly twice that level, and could cause other downstream interference.

Some cable and satellite boxes can have internal caption encoding chips, that re-constitute line 21 captions from a data stream, rather than from the station's video signal. This encoder could lose termination thru internal component failure, or lack of termination in the external connection to another device. Or, it could be fed an unterminated signal. Keep in mind that Sony, for example, had (mechanical) auto-switching BNC termination setup that occasionally didn't work.

The varying levels you see may be due to automatic gain control function, or malfunction.

So, set your scope to show you line 21, field 1, and see what you've got. (Hopefully you have a line-select scope.)

Other things can can populate the vertical blanking interval are:

VITS (vertical interval test signals -- color bars, etc. on a single line)
VITC (vert. int. timecode -- which will have smaller dashes than you see)
Nielsen rating information codes
Macrovision (basic, early version)
and stuff like that.


Chip

The regularity of variation and going to greater than 100 IRE definitely mean it's a deliberate copy protection. Not accidental, no doubt whatsoever. The same techniques as used for stabilization of other sources will work, but the timing has to be modified to eliminate the pulses on these particular scan lines.

Sorry, I'm using a basic old (1980s) analog scope. I don't understand everything in that captioning article, but I did notice that caption coding may be different for HD. On our cable system, channels below 100 are all non-HD and some low channels have their HD versions 100 channels higher (for instance, channel 5 non-HD is repeated as channel 105 HD). I had hoped that I could avoid this problem by always selecting the HD channels, but unfortunately, if the interference is present on the non-HD channel, it's also present on the HD (channel+100) version. I still don't understand why the coding is totally absent on the higher channels (about 150 and above), which includes all of the premium content and both HD and non-HD programming.

Whether it's copy protection or caption encoding, I'm getting the idea that I can't fix the problem by phoning the cable company (i.e., the signal is intentionally present, not some malfunction or glitch on their side).

Retroactively adding blanking circuits to all of my restored TVs (a couple of dozen) ain't gonna happen, particularly considering that the rudimentary circuit I added to my Philco 49-1240 did not help with this interference.

My video stabilizer is a cheap item, bought several years ago from a company that no longer exists. No chance of getting a schematic for it, and I don't have the skills or equipment to dive in and hotrod this sort of thing bareback. Here is a peek at the board for those who are curious.

http://antiqueradio.org/art/temp/Vid...lizerBoard.jpg

Possibly there is some other stabilizer or signal cleaner out there that could eliminate the interference. The question is how to find it?

Phil Nelson

I don't see any capacitors of a type that would indicate they are used for timing, so if the circuit uses a digital countdown, it may not be adjustable - sorry. The two pots may adjust some levels rather than timing, I couldn't say for sure.

Could it be a trigger signal to enable DTAs to pass certain channels to the lowest tier cable customers... might be revealing to compare the channels that the "interference" occurs on to a list of the "basic" tier channels.

Just a WAG,
jr