Re: [telecom] Walter's Telephones [Telecom]

W

Wesrock 16 years ago

None of the stations I worked with put their control on their program

> line. The program lines were generally driven with a WE 111C
> transformer to drop the 600 ohm source resistance (studio equipment)
> down to 150 ohms to drive the line. Another 111C converted it back to
> 600 ohms at the transmitter.

Just a matter of curiosity--what was the advantage of 150 ohm lines?

600 ohms was the standard for telco lines, including program grade lines. Wes Leatherock snipped-for-privacy@aol.com snipped-for-privacy@yahoo.com
***** Moderator's Note *****

I'm always tempted to answer these sorts of questions myself - I worked on the "Radio" board, after all - but then I realize that a lot of the things I "know" about program audio lines are just memories of the way we did things, and I'm not sure of the technical basis.

Let's see: impedance varies with frequency, so (I'm guessing) I think

150 ohms is closer to the impedance at the upper frequencies, thus giving a boost to the "high end" tones, and making equalization easier. Come to think of it, that would be _part_ of the equalization, wouldn't it?

Vote

D

David Clayton 16 years ago

Lower impedance lines/equipment are less susceptible to picking up noise (especially hum) than higher impedance stuff, in audio you would generally go for the lowest impedance possible to reduce the line's noise.

As well various types of audio cabling is made with a specific impedance, so 150 ohms may well match those cables along with the interfaces.

Vote

S

Steve Hayes 16 years ago

My understanding is that the original open-wire telephone lines did have a characteristic impedance of about 600 ohms but, from having worked on analogue carrier systems many years ago, twisted pair lines are about 135 ohms. We designed our carrier equipment to match this impedance. This would give a reasonably flat response up to several hundred kilohertz but losses at all frequencies were very high. That didn't matter much to the carrier system - it could tolerate up to 40 dB of loss. I'd assume the same approach is used for T1/E1 and DSL systems.

That sort of loss would be intolerable for ordinary wireline phones (the circuit is duplex and adding much gain makes it oscillate) so they are still designed to match 600 to 900 ohms. The mismatch doesn't matter much at low frequencies or on short lines and the loss is reduced but there is a lot of high-frequency rolloff on longer lines. This can be mitigated by adding loading coils. Along with the line capacitance, they make the circuit into a low-pass LC filter with reasonably flat response to around 3 kHz but little or no transmission above that.

I'd guess that it's easier to use 150 ohms for landline feeds to broadcast facilities where perhaps a 15 kHz bandwidth is required and just crank up the gain and tweak the high frequency response a bit than it would be to use

600 ohms with or without loading coils and have to equalise the resulting horrible rolloff. Not that I've ever worked on them...

Vote

H

harold 16 years ago

You've got it! Driving and loading the line with a lower impedance makes the capacity of the cable pair have less effect. In many case, no equalization was required at all. Just a transformer at each end. There's a paper on the frequency response of a cable pair (not a telco pair) with various source and load impedances at

formatting link

.

Harold

Vote

Re: [telecom] Walter's Telephones [Telecom]

Join the Discussion

Didn't find your answer?