Why 75 Ohms not 50 Ohms?

The dBu used in audio isn't 'impedance referenced', so it's entirely legitimate to say a 1:2 transformer has 6dB of voltage gain.

Graham

And you are breathing under fasle pretenses.

"Arny Krueger"

Yawn - usual pile of useless crap from Arny.

.......... Phil

"Tam/WB2TT".

............. Phil

You said.... " For dB to make any sense whatsoever, you should be working in the same impedance. "

And I pointed out that to be untrue.

Transformers voltage gain or loss can be perfectly legitimately represented as dB.

Graham

John, Phil is obviously talking about himself. ATT long lines distributed baseband video via coaxial cable for years, and a dedicated telephone pair carried the audio for the programs. I knew someone who had worked for ATT long lines for over 10 years to maintain the coaxial video system. I have a HP video waveform monitor that was customized for the telephone industry to monitor the video networking. It has the WE coaxial jacks on the front so they could use their standard video patch cables to connect it into the system for troubleshooting and monitoring.

The video leaving the network control room was 75 ohms, and the input impedance of the video proc amp at the TV station end was 75 ohms. A 4% error at each end of a thousand miles of coax, amplifiers and equalizers isn't going to cause any real problems, is it?

Anyway, 75 ohm is a group of compatible cable types with slight impedance variations from one RG number to another. The TV and Video production trades simply refer to all of it as 75 ohms. Newer 75 ohm coaxial cables are closer to the 75 ohm designation, and have less variation in quality.

I read in sci.electronics.design that "RST Engineering (jw)" wrote (in ) about 'Why 75 Ohms not 50 Ohms?', on Tue, 13 Sep 2005:

Have a King Canute Award. Decibels have been used in non-constant impedance contexts almost from the time they were invented. A 1:2 transformer has '6 dB of voltage gain', and '-6 dB of current gain'.

There was once an attempt to use the term 'decilog' for voltages, but it failed.

You are a lazy and sarcastic ass.

The AT&T system was designated "L Carrier," and it was in use coast-to-coast for many years. Long Lines did not own and operate all of it, however. Pacific Telephone & Telegraph company owned, operated, and maintained Its own L Carrier from Los Angeles to Seattle, with drop points along the way. L Carrier used rigid 72 Ohm coax. Line amplifiers, regulators, equalizers, and test equipment were all 72 Ohms. The only 75 Ohms was following a transformer shift at the handoff to the Television Operating Center (TOC).

I guess there are at least two people who know of it.

That is total nonsense, a.k.a you don't know what you're talking about.

It is a puzzle to me that so many people, such as yourself, are so willing to spew forth their absolute ignorance and then get upset with people who DO know what they are talking about. Is it a congenital problem? A medication you are taking or should take, but aren't?

Look it up yourself dipshit. I don't need a URL for details..... I was there.

Here is my best guess. 50 Ohms is used as a compromise between power rating and attenuation. That IO am pretty sure of.

For video work, attenuation is important, whereas power rating is not. So the impedance is moved to a one where the attenuation is less for a given amount of copper.

1936/37.

Isn't it about time you re-wrote Newton's Laws for a change?

.......... Phil

It would take a fool to think it was done in anything but

Hate to tell you this, but as best I can find out TV signals were transmitted by ATT on (mostly) L3 systems as an upper sideband VSB signal, with a reduced carrier at 4.139 MHz. They probably wanted to avoid going near the edge of the L3 frequency range to reduce delay distortion.

Tam

= 52.324324324324324324324324324324

In any case, I said " does not (ie normally) have to and NOT " cannot possibly be made to ".

............. Phil

"Michael A. Terrell"

Or are you the only person alive aware of it ?

........... Phil