|
Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
|
Thread Tools |
27th Dec 2004, 9:18 pm | #1 |
Administrator
Join Date: Dec 2002
Location: Cardiff
Posts: 9,073
|
Interference to TV cameras (1980s)
I am watching the Live Aid DVDs (Xmas pressie). On some of the UK tracks there is horizontal colour ripple (red/green) rolling down the screen on the close shots taken with cameras on stage. The sleeve notes say:
"The horizontal banding that appears on some of the UK performances is called microphany and occurs when the camera lens is affected by great volume. Whenever you see microphany you know that whatever is going on, for the cameraman closets to the action, it is really LOUD." I think they are simplifying it somewhat here, and would be interested to know what the cause really is... Bu the way, it's a good DVD set! |
27th Dec 2004, 9:36 pm | #2 | |
Retired Dormant Member
Join Date: Dec 2002
Location: London, UK.
Posts: 69
|
Re: Interference to TV cameras (1980s)
Quote:
You can see this on old tapes of Old Grey Whistle Test and Top of the Pops as well as some Dr. Who episodes with pyro effects - more likely OGWT as I worked on the last series of that (coincidentally, in the studio I'm in charge of now) and believe me, it was LOUD. In the days before the AIMS "Minim" too - the thing that limited sound level dosages so effectively that it led to a four-letter outburst from the Gallagher brothers when they exceeded the dosage level on "The Late Show" down Lime Grove and killed the power to the PA live on national television... |
|
27th Dec 2004, 9:48 pm | #3 |
Administrator
Join Date: Dec 2002
Location: Cardiff
Posts: 9,073
|
Re: Interference to TV cameras (1980s)
Thanks Mike. That sounds much more logical to me.
In a few shots (in particular some of the Queen performances) there is the smearing effect (remaining for many seconds) caused by bright lights too. Maybe that's a clue as to what cameras were being used (BBC, 1985). |
28th Dec 2004, 3:38 am | #4 |
Retired Dormant Member
Join Date: Dec 2002
Location: London, UK.
Posts: 69
|
Re: Interference to TV cameras (1980s)
Smearing effects (known as Comet Tails in the BBC) are caused by target overloads; where the charge on the tube's target accumulated by exposure to light cannot be discharged by a single sweep of the electron beam as would normally happen. This effect was counteracted in later generations of tubes by briefly flooding the target face during blanking with a relatively high beam current (about a milliamp from memory - IIRC signal current was typically 260 nanoamps for red/green and 130 for blue) to remove the offending charge - these were known as ACT (Anti Comet Tail) tubes and had to be *very* carefully set up else you'd strip the cathode in days - hours, even. This was one part of the set-up procedure for the Link 125 you ignored at your peril, lest you really wanted to explain why that £1400 tube lasted just ten days to Tube Test...
Typically the cameras used by BBC OBs at the time were Philips LDK5s, but on this event I would imagine that if it produced a picture, it was pressed in to service! Again, you'd see this effect more on first-generation cameras like the revered EMI 2001 which used four 30mm non-ACT tubes. Look at the comet tails on some dramas or episodes of Dr Who to see the effect. It could be counteracted to some slight effect by winding up the beam current on a particular tube to stop it, which led to the old engineering board joke: Q: What do you do if there's a fire in your studio? A: Put in two notches of red beam! All right, I'll get me coat... Last edited by mjizycky; 28th Dec 2004 at 3:51 am. |