Interference to TV cameras (1980s) - UK Vintage Radio Repair and Restoration Discussion Forum

Paul Stenning · 27th Dec 2004, 9:18 pm

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!

mjizycky · 27th Dec 2004, 9:36 pm

Quote:

Originally Posted by Paul Stenning

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...

By the way, it's a good DVD set!

It's a simplification, but it's wrong. What's actually happening is not the lens being affected by the noise, but the electron gun assembly of the pickup tube vibrating (a "Plumbicon" if they were Philips, or a "Leddicon" if they were made by English Electric). The output from the blue tube is usually too low to see: remember, it's only 11% of the total luminance output in the NTSC equation. So what's happening is the electron stream is being mechanically modulated by the vibration from the PA. Of course, this effect doesn't exist with modern CCD cameras.

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...

Paul Stenning · 27th Dec 2004, 9:48 pm

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).

mjizycky · 28th Dec 2004, 3:38 am

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...

27th Dec 2004, 9:18 pm	#1
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,072	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! __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

27th Dec 2004, 9:48 pm	#3
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,072	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). __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster

28th Dec 2004, 3:38 am	#4
mjizycky 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.