Posted by
Mike Harris-9 on
Jul 02, 2004; 4:59pm
URL: http://elecraft.85.s1.nabble.com/OT-Line-level-tp368401p368415.html
G'day,
My interpretation of the point that G3VVT was making is that a blanket
dismissal of the 600 ohm reference is hazardous depending upon the
circumstances. I also know nothing about pro audio, however, like G3VVT I
have had a long and varied career in telecommunications. 600 ohms is the
general impedance of 0.5mm diameter conductor poly insulated twisted pair
telephone cable at audio frequencies. This drops to about 150 ohms at
T1/E1 frequencies over the same pair. All the telco test equipment I have
worked with at audio level offers 600 ohm source and terminations as well
as high impedance bridging. AF attenuators were all 600 ohm balanced
input/output.
Impedances in transmission systems commonly change along the line. In our
first earth station, built 20 years ago, we went from 600 ohm input to the
PCM channel equipment, from there 75 ohm to the up/down converters and 50
ohm for the rest of the SHF link. It was essential to make sure you had
the right matching transformers in your test set-up when making
measurements.
The USA and Europe has a history of doing things differently, a totally
different coding law for PCM, different baseband data rates and
hierarchy..... and on.
Standards are great, so many to choose from!
Regards,
Mike VP8NO
(G3VUI)
Ex. GPO/PO/Telecomm and C&W
----- Original Message -----
From: "Jim Brown" <
[hidden email]>
To: "Elecraft List" <
[hidden email]>
Sent: Friday, July 02, 2004 10:26 AM
Subject: Re: [Elecraft] OT: "Line level"
> On Fri, 02 Jul 2004 06:29:33 -0400, Mike S wrote:
>
> >At 11:32 PM 7/1/2004, Jim Brown wrote...
> >>BTW -- forget all that ancient stuff about 600 ohms. Pro audio hasn't
used a 600 ohm
> reference for at
> >>least four decades.
> >
> >That may perhaps be true for a limited range of "pro audio"
applications,
> >but 600 ohm audio transmission lines are still VERY common both in
> >telephone and PA applications. If you forget that stuff, you WILL end
up
> >with real problems.
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