[Elecraft] Loading Coils For Audio Transmission Lines

[Jim Brown]

On 3/31/2014 7:52 AM, George Danner wrote:
> One of the things that Bell Labs found in adding loading coils to phone
> lines (to reduce the high frequencies) was that in an audio system if you
> reduce the high frequencies then you needed to also reduce the low
> frequencies to keep the intelligibility constant. Since a phone system
> needed intelligibility above fidelity; Bell Labs just decreased the coupling
> capacitors to reduce the low end while using the line loading coils to
> reduce the high frequencies.

Sorry George, but that's not quite right. The attenuation, velocity 
factor, and characteristic impedance of ALL transmission lines vary 
drastically through the audio spectrum. All transmission lines behave to 
some extent as a low pass filter (because attenuation increases with 
frequency), and the variation in Vf causes the higher frequencies to 
arrive before the lower frequencies. Loading coils were added to phone 
lines to compensate for those variations. The result was to "flatten" 
both the amplitude response and the time response in the audio passband, 
then allow it to drop sharply above audio.

There's a tutorial discussion of this on my website. It was an appendix 
to the materials I prepared for a 3-day "Hum, Buzz, and RFI" workshop 
for audio professionals in 2005.

http://k9yc.com/TransLines-LowFreq.pdf

73, Jim K9YC