PAL TV - white peaks during vertical blanking

[benta]

While trying to troubleshoot some video instability in an analogue cable-TV net, I observed the following:

During the vertical front porch period, there is a white (100% level) white spike approximately 2.5 lines before the vertical sync pulse.

Additionally, during the vertical blanking period after the VSYNC, Videotext data is present (this is normal). However, lines 9, 10 an 11 of the Videotext data look strange. Lines 9 and 10 start with a white peak, then dropping to black and stepping up to white again in a staircase fashion. In terms of amplitude, it looks something like this:

Sync, colourburst, 100%, 0%, 20%, 40, 60%, 80% 100%, sync.

Line 11 looks like data, but not at the normal Videotext levels.

Is this normal?
Or is our cable provider injecting some non-standard signal here?

By the way, this is taken directly from the tuner, so it's not VHS-Macrovision related or something like that.

Thanks for any help you can give.

Benta.

 

[benta]

What? None of the experts here can help?

Anyway, I think I've found a partial answer. At least the front porch white spike is an "anti-ghost" signal for PALplus.

The other part of the PALplus data should be in line 23, so that doesn't fit my findings.

Half the problem solved...

Benta.

 

[MacGyverS2000]

I don't do PAL ;-)

Dan - Owner

http://www.Hi-TecDesigns.com

 

[benta]

Yeah, I see from your website that you are in Florida.
With the switch to digital, I suppose NTSC is as dead as a dodo now?

Benta.

 

[MacGyverS2000]

Keep meaning to change that... I've been just north of D.C. for over 3 years now ;-)

I've been letting the LED business languish while I push more towards the lasering side of the business. The entire site needs a complete revamp. I imagine the LED side of things will disappear completely within the next few years.

I spent quite a few years banging on NTSC-related video projects, but I have to pull out notes these days if I wanted to get back into it.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

Crazy old Wiki has a lot of pointers here:

http://en.wikipedia.org/wiki/Teletext

"The nice thing about standards is that there are so many to choose from."

 

[benta]

Just a comeback.
After buying the book "Video Demystified" (I can recommend it), I found the answer.
Broadcasters actually do send test signals on lines 18, 19, 330 and 331. That's what I'm seeing, so everything is normal.

Cheers,

Benta.

 

[zappedagain]

Thanks for the follow up!

Z