On Wed, 20 Jan 1999, Wing Kent Kwan wrote:

> but I can not connect the DTMF SPK output pin to the phone line
> directness
Careful! Telephone off-hook operation requires at least several megs to
earth ground otherwise you will get massive 60hz (or maybe 50hz for you)
hum.

> I use the oscilloscope to watch the DTMF is a Squre Waves, but another
> DTMF IC is the Sin Waves,So how can I exchange the PWM to Sin
> Waves???calculus circuit is need??????? how can I do it????????
So for example, you can't hook a bench scope to the telco. (Unless you are
interested in observing how it is affected by ground fault :) A portable
scope would work, though.