Why we have overscan

As you can see around the black bars, sometimes there are things that we should not see on our TVs. Inconsistent frame areas are the main reason and although these screenshots are 1994, this is still much of an issue as it was then.

Having overscan off is only a problem on SD where white and black flashing bars appear, but on HD is not a problem as the picture fills the whole frame. So I always try to turn overscan off.

Overscan was an issue that came from the way older televisions would display their image. Because the overscan existed, this is why lazy and rushed designers and producers would not take the care to ensure graphics touched the edge, and alignment of video was precise.

The poster seemed to mix up the result of having overscan, to the reason for the overscan.

And also the way tubed-based cameras originated their images as well contributed to the slightly nasty edges of frame.

Blanking can still be an issue even with HD cameras these days - as some models use analogue triax connections between the camera head and the camera control unit, and there can be some variation as a result.

There are also issues in SD digital TV between 702x576 (the 4:3 or 16:9 picture area) and 720x576 (the total active line length which is a bit wider) when handling analogue or HD downconverted sources. (HD sources are 1920x1080 or 1280x720 16:9 sources, which should be remapped to 702x576 not 720x576 - but if you have a display that incorrectly maps 720x576 to 1920x1080 you will then see black bars - and a slightly stretched picture)

Can you explain your reference to :

"Blanking can still be an issue even with HD cameras these days - as some models use analogue triax connections between the camera head and the camera control unit, and there can be some variation as a result."

Whether using triax or fibre optic cabling between the head and and CCU/BSU, the encoded signals are still digital all the way - where do you believe this 'variation' is being introduced?

Err - except they're not digital all the way. On most HD triax systems they are carried as wideband analogue signals NOT digital signals down the camera cable, with digital/analogue and analogue/digital conversion at either end.

Sony have only recently introduced their first digital triax HD system (as used by the HSC 300 cameras being used by The One Show, EastEnders and at Broadcasting House) - which uses video compression to get the bitrate down to something manageable for a decent length of triax.

However the previous generations of HD triax systems for Sony cameras, relied on using fibre cameras (like the 1500s) and external fibre<->triax converters, used analogue triax techniques (i.e. the digital video on the fibre is converted to analogue for triax carriage, and then converted back to digital, put back on fibre to go into the fibre CCU) Not only does this introduce some potential timing issues - it can also introduce a bit of extra video noise, and knock the edges off the pictures a bit...

My understanding is that the Philips (now GrassValley) LDK range use similar analogue techniques when run triax. (If you read the spec sheets there is NEVER any claim of digital carriage over triax...)

** EDIT : http://www.sony.co.uk/biz/content/id/1237479584530/section/broadcast?articlesection=2 mentions that the BBC went for the new Sony HD digital triax system to avoid using analogue HD triax with fibre/triax converters

And an earlier article here : http://broadcastengineering.com/mag/broadcasting_hd_triax/

And a 2012 white paper from GrassValley here : http://www.grassvalley.com/docs/WhitePapers/broadcast/cameras/ldk8000_elite/CAM-4080M_3G_Transmission_Whitepaper.pdf

GrassValley have only just introduced digital triax (using OFDM modulation) with their newest range of cameras introduced over the last 12 months, as they couldn't get 1080/50p or 1080/60p (aka 3G) signals down analogue triax as they have with 1080/50i and 1080/60i (and the similar 576/150i and 480/180i Super Slow Mo SD systems).

I'm always surprised by the number of people who think HD=Digital...

Interesting, thanks very much for taking the time to find those references.

No problem. Anyone who has used Sony 'toasters' to run HDC1500s over triax is very well aware of their limitations...

We've only ever really used these for getting the signals through the Technocrane heads and the occasional circuit through the infrastructure of the building, no really long cables lengths.

I always thought the Sony HDTX/FX genuinely converted the digital signals - hence my surprise at learning they don't. From your links it would seem I was 2/3 years ahead of the game with my thinking!

More probably. HD over triax has been happening since the late 90s when the US went HD. Most sporting venues were pre-wired with triax, but Sony wanted to run fibre only. Philips offered a triax option, and then started selling a lot of cameras... So Sony kludged up the 'toaster' solution (Philips have a triax back option)

Don't worry - you're not alone in thinking triax is digital. Lots of people do. In fact AIUI not all fibre solutions are digital either - they're just very clean analogue.