Additional Data on an Analog Video Signal - What is it? - Page 1
Forum section: electronics / general

author: Halcyon
author_rank: Unknown
timestamp: November 24, 2015, 01:45:16 pm
content: This is something that I've wondered for a long time; Analog video aficionados will probably be able to answer this one easily.If I set my display to underscan mode, above the visible image is a collection of flashing and static white dashes which clearly represents data of some kind. My question is, what is it used for? Does this form part of the Vertical Blanking Interval?I've attached a photo for those playing along at home.

author: BradC
author_rank: Super Contributor
timestamp: November 24, 2015, 01:50:03 pm
content: I've seen a few variants over the years, but most I've seen have been one or another format of time code.

author: Mechanical Menace
author_rank: Super Contributor
timestamp: November 24, 2015, 01:53:09 pm
content: Closed captioning maybe? I can remember Betamax tapes with them and know VHS could even if it wasn't widely available in PAL regions.

author: mauroh
author_rank: Frequent Contributor
timestamp: November 24, 2015, 02:06:45 pm
content: If you know what is it you are old.... Ok I'm oldIt is the teletext.https://en.wikipedia.org/wiki/TeletextI think it can be explained to a young guy as a sort of Internet where we used to search for information trough the TVThere was just a limited amount of available "pages" organized on topics like TV Schedules, News, Sport, Airplane arrivals/Departures (only for the biggest airport)...It was very useful but worthless compare to googleMauro

author: Halcyon
author_rank: Unknown
timestamp: November 24, 2015, 02:10:48 pm
content: Quote from: mauroh on November 24, 2015, 02:06:45 pmI remember Teletext and I know it used the VBI to hide the data, but this is playing back a purchased cassette tape so Teletext wouldn't have anything to do with this recording. I didn't think Teletext is something that could be recorded onto tape anyway (as it was normally stripped out of the signal before making it to tape). Mechanical Menace mentioned closed captioning but I've never seen this as an option on any VCR sold in Australia.I think BradC might be onto something with the timecode theory. I might be able to test this with the VTR I have, by generating its own TC (or I can feed one externally) and seeing what comes out on playback.

author: mauroh
author_rank: Frequent Contributor
timestamp: November 24, 2015, 02:26:55 pm
content: Yes, I think it was better to say that what you are seeing when you set your display to underscan mode is part of the VBI information, the place that teletext belongs...On CCTV analog camera output or analog cassette recorder output, that is just part of the VBI like the frame number....If you notice part of what you are seeing in thous few lines is fix and something else is like binary counting...Mauro

author: SeanB
author_rank: Super Contributor
timestamp: November 24, 2015, 05:56:26 pm
content: On a prerecorded video tape that will be an incrementing counter, showing frame count, time in seconds, minutes and hours, and possibly some other information like a title. Used in the professional world to have a correct timing and also so they can cut at the correct frame when needed, so that a commercial can be automatically inserted into the video when used for over the air broadcast TV, where they set up a few machines with tapes, and then the automation would set up commercial breaks according to the schedule as the video plays, allowing an entirte day or night's programming to be done with nobody there other than a single machine minder, who possibly would be handling a whole set of different TV station programmes.With non broadcast it is also used to do editing, as you set up the whole sequence by frame number, and then the edit suite will do the crossfades and other effects as needed, while the master recorder gets the data from the 2 or more source machines.

author: rollatorwieltje
author_rank: Unknown
timestamp: November 24, 2015, 05:57:03 pm
content: Smells like Macrovision copy protection. They used a pattern that would screw up the gain control of a the recording device.

author: TerraHertz
author_rank: Super Contributor
timestamp: November 25, 2015, 12:09:42 am
content: Since it looks like it's on two adjacent lines, and doesn't contain much data, and you said it seems to be binary counting, it's most likely VITS.https://en.wikipedia.org/wiki/Vertical_interval_timecodeUsed with old tape-based video editing systems, to code the tape with elapsed time, frame number and PAL/NTSC multi-frame sequencing for editing synchronization.

author: miguelvp
author_rank: Super Contributor
timestamp: November 25, 2015, 12:27:10 am
content: could be closed captioning or teletex or extended data serviceshttps://en.wikipedia.org/wiki/Line_21different countries use a different scan line, not sure what you use, PAL, SECAM, NTSC?But it's pretty much the same concept of using the scan lines during vertical refresh to store digital information.

author: BradC
author_rank: Super Contributor
timestamp: November 25, 2015, 03:39:26 am
content: Quote from: TerraHertz on November 25, 2015, 12:09:42 amOn the old Panasonic VHS editing decks we used at school we had to "format" the tape if we were planning to use insert editing for exactly that reason. Nothing quite as exciting as writing out an hour of black before you could get started on a project. Clever little machines those, they could switch between reading and writing fast enough to overwrite the horizontal line without disturbing the sync pulses. Pretty advanced for sub-broadcast machines in the 80's. Certainly a lot easier to edit on than the 2 inch Quad's.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 25, 2015, 07:24:07 am
content: Quote from: TerraHertz on November 25, 2015, 12:09:42 amActually,it'sVITC-------VITS are Vertical Interval Test Signals,which allow a user to examine:-Transient Response & Line "tilt" (2T Pulse & Bar)Chroma /Luma Gain & Delay (20T Pulse)Differential Gain & Phase (Staircase with chroma on it)Line time Luma Non-Linearity( plain Staircase)Rough check of frequency response (Multiburst)Apart from VITS & Timecode,as mentioned it was common to use other lines in the Vertical Blanking Interval for Teletext,& other purposes.

author: Emo
author_rank: Regular Contributor
timestamp: November 25, 2015, 08:15:51 am
content: There is also data for automatic image sizing, called WSS; wide screen signaling in line 23 in order to have the screen switch between 3:4 and 9:16 and all the possible other formats.This document from the ETSI(European Telecommunications Standards Institute) has an overview of most options;http://www.etsi.org/deliver/etsi_tr/101200_101299/101233/01.01.01_60/tr_101233v010101p.pdfHowever there exists a number of country specific alterations.

author: tooki
author_rank: Super Contributor
timestamp: November 25, 2015, 10:57:23 am
content: Precisely what it is depends on the video source. If it's off the air, it's teletext and the widescreen trigger (in the PAL world) or Closed Captioning (NTSC world). If it's on a studio tape, then it's one of the timecode systems mentioned above.What itisn'tis an inherent part of analog video, nor Macrovision.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 28, 2015, 02:46:13 pm
content: We still have teletext around. It even made it to the internet.http://nos.nl/teletekst