Re: Phone line interface for DTMF

On Fri, 22 Jun 2007 21:54:24 GMT, in sci.electronics.design Eeyore
<rabbitsfriendsandrelations@xxxxxxxxxxx> wrote:



Tim Wescott wrote:

The quickest answer, if you can find one, would be an 8 ohm to 600 ohm
coupling transformer. This will be a better match to your computer audio.

Why would a 1:8.6 step-up transformer be better ?

Can you explain that ?

Do you seriously want to send several watts of audio down the phone line ? The
telco might have something to say about that ! I think the recommended level is
about -6 to -10dBm. i.e no more than 400mV rms.

Graham


I used to use a Quad 303 poweramp on OB's as a line driver. Seemed to
work/minimised xtalk onto our circuit

But even more impressive, we ( capital radio in the 70's) got
broadcast TX line organised, and working, to France in less than 3
hours from first contact, from next to a famous pedestrian crossing
in Abbey Road.
We almost had talkback, well, by the time the show had ended.

I think the paperwork must have been worse than the show


martin
.

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