TV studio broadcasting

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Desmond
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TV studio broadcasting

Post by Desmond » Thu Feb 19, 2015 2:15 am

Hi This is really a question for someone who has worked in TV studio broadcasting if there are any of you. I remember being involved radio and TV. It is to do with the very large lengths of cable. A line of a television broadcast was 64 us complete with front and back porch. But with these cables the lengths are the same due to delays in the cable caused by capacitance and other factors. Is there any way this is documented so I can prove that I did not make this statement up. Any URL links to a website would help.

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Jim Sofonia
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Re: TV studio broadcasting

Post by Jim Sofonia » Thu Feb 19, 2015 9:46 am

Desmond wrote: Is there any way this is documented so I can prove that I did not make this statement up. Any URL links to a website would help.
Yes, it's the "Cable Velocity Factor" that you are seeing the effect. Here is a link that explains it.

http://www.picwire.com/technical/velocity_factor.php

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PID_Stop
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Re: TV studio broadcasting

Post by PID_Stop » Thu Feb 19, 2015 9:48 am

In NTSC, 1H is 63.5µS, typically measured between the midpoint of the falling edge of horizontal sync from one line to the next line. The color subcarrier frequency is 3.579545 MHz, which means that one cycle takes about 279 nanoseconds.

I think the issue you are raising has to do with the propagation delay through a given length of cable. The most commonly used cable for analog broadcast video was Belden 8281, and just shy of 182 feet of it would create 279 nanoseconds of delay, the same as delaying by 360° of subcarrier. Where people get hung up is on the assumption that electrical signals travel at the speed of light, but in reality signals travel slower through coaxial cable because it represents an equivalent chain of LC networks. The proportion of speed through a given cable, versus the speed of light in a vacuum, is called the Velocity of Propagation; for Belden 8281 it happens to be 66% (you will find this on the second page of the spec sheet). Different types of cable have a different VP, so the length of cable needed to produce a specific delay will vary by type.

If you are curious about how the VP figure is derived, you can read more about it here.

Before the days of digital, every studio installation I did required painstaking effort to match cable lengths between distribution amps and routers, distribution amps and production switchers, and distribution amps and master control switchers. Since six inches of error was enough to mis-time the chroma by a degree, the work was both tedious and exacting. Nowadays with digital gear that at least auto-times inputs within a line or so, things are so much easier!

Did this answer what you were looking for?

Regards,

-- Jeff


(p.s.: you know you've been at it for a while when your quickest reference is the coffee cup on your desk!)

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Desmond
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Re: TV studio broadcasting

Post by Desmond » Thu Feb 19, 2015 9:53 am

Hi Thanks. The problem is that when I told people that the cables connected to the cameras had to be the same length they did not believe me. This link is a bit too tech for them. I could do with something that would confirm it if there is anything. Failing that

Question: Do the cables on broadcast cameras have to be the same length and what lengths of cable arte used?

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KPJL FM
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Re: TV studio broadcasting

Post by KPJL FM » Thu Feb 19, 2015 10:16 am

If the cameras are genlocked (or lockable), and the camera has adjustments to set the phase of the gunlock signal, and the downstream equipment the camera output connects to has timing adjustments for the input(s), then no, the lengths don't have to be the same. Although some really odd combinations of lengths can present nearly impossible correction adjustments without something in line to correct, say 180 degrees worth of match, that would be in a plant that had enough mismatched brands/types of gear (like my old cable company studio in the '80s). Then you might consider cable length instead of buying more gear.

In short, you shouldn't *have* to match lengths anymore, even in an older NTSC plant.
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w9wi
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Re: TV studio broadcasting

Post by w9wi » Thu Feb 19, 2015 3:54 pm

Theoretically, the cables *don't* need to all be the same length. If you have only one control room involved, you can adjust the timing controls on the various video devices to match up the signal timing between devices.

Where you need equal lengths is if you have more than one control room. Cameras 1, 2, and 3's video signals must all reach Control Room A at the same time -- and they must also reach Control Room B at the same time.

If you just pick random lengths of cable to connect each camera to each control room, you're going to find that adjusting Camera 2 to match Camera 1 at Control Room A results in the timing not matching in Control Room B. It will be impossible to get the timing to match in both control rooms at the same time.

The *difference* between the length of the line connecting each camera to Control Room A, and that of the line connecting that camera to Control Room B, must be the same.

The easiest way of accomplishing this is if each line connecting a camera to Control Room A is the *same* length, and each line connecting a camera to Control Room B is the *same* length.

(my first job in TV consisted of preparing several dozen identical cables to wire a production control room and a master control room in Madison, Wis...)
--
Doug Smith W9WI
Pleasant View, TN EM66

Desmond
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Re: TV studio broadcasting

Post by Desmond » Fri Feb 20, 2015 1:52 am

Thanks that was the answerer I was looking for. I am not a TV engineer at your levels. I used to be radio officer in the merchant navy and had to study radio at all levels from radar frequencies down to 500khz. We understood how higher frequencies would propagate from an aerial faster. Also there is a transmitter in rugby transmitting at 16khz but uses an enormous amount of power. My ship radio was fully synthesised and could go down that far. We were trained as design engineers because we had to fix it or improvise if needed. As a child I helped my dad fix TVs and our first colour TV had a manual. In it the TV had a luminance delay and a chrominance delay. I didn’t understand then because I wasn’t trained. These are delays for the propagation of the areal not the transmission as each TV would have to have different delays depending on how far the TV was from the transmitter. My problems are understanding Digital TVs and channels. Emley Moor transmits 35 programs (we call channels) on 714 megs only Chanel 51. I do understand Mpeg though.

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