[TriLUG] RE: Phone to use with PC

[sjackson at radarfind.com]

Scott

Actually, as a long time ham and hardware guy (30+ years), I know exactly
what you want.

Trouble is, the hardware you have doesn't get you where you want, without
some work.  While you are correct that the modem has the needed circuitry,
that circuitry isn't wired to provide the functionality you ask for.  For
example, you wrote:

"I want to use the same headphones I listen to music from the PC as my
earpieces, and the microphone over the monitor as the mouthpiece"

If you somehow could wire into the modem to make those physical connections
and get the levels right, how would you null out the sidetone, if you have
both the mic and the speaker 'live' at the same time?

(BTW, ASCII just doesn't cut it for these kind of descriptions.  We need a
white board.)

Anyway, it might be possible for you to hack into your modem card to make
what you are describing, but the task may be quite a challenge unless you
have an older ISA style modem card, with schematic and parts layout
diagrams.  And, even if you can make it work as you wish, to make the
"speakerphone" or "phone patch" as you describe, you would have to access
parts of the echo canceller that may not be pin available in all ICs.

Note that with a traditional ham 'phone patch' in the old days, one used
headphones, not a speaker ... And a manual 'null' control for the sidetone
balance.  Right?

In the broadcast studios where I used to work, same applies:  during call-in
shows, the studio staff listen to their air monitor through headphones, and
the phone patch null is done with a DSP (made by Eventide, also usually with
a delay line).

My suggestion to use the Meridian phone was to avoid having to fiddle with
audio hardware, and give the computer the control over the dialing and so
forth.  If you want to gut a modem card to do this instead, it's a good
hardware experiment and I encourage you to try it.

Steve Jackson



-----Original Message-----
From: Scott G. Hall [mailto:ScottGHall at BellSouth.Net] 
Sent: Thursday, May 05, 2005 4:21 PM
To: Triangle Linux Users Group discussion group
Cc: sjackson at radarfind.com
Subject: Re: Phone to use with PC


sjackson at radarfind.com wrote:
> Easiest way I know is to use a Nortel/Aastra Meridian 9617 USB phone.
> Plugs right in to the PC's USB port, and either the phone or the computer
> has full control over the comings and goings.

I don't want a phone handset!!  Quite the contrary, I want to use the same
headphones I listen to music from the PC as my earpieces, and the microphone
over the monitor as the mouthpiece.  In ham-radio, this is called a "phone
patch".  In the broadcast industry, it is called a telephone hybrid (based
on the use of hybrid transformers), such as those used for talk-radio shows.

On a standard telephone POTS line, there are several signals on it at the
same time: the incoming audio, the outgoing audio, a high-voltage ring
signal, and a DC voltage used to power handsets.  After blocking the DC
voltage, and shunting the ring voltage, a hybrid transformer set separates
the incoming from the outgoing audio (they are blended or "mixed" together).

A standard modem already has the circuitry to "hybrid" the signal, and DSP's
to generate DTMF and modem negotiating tones, to receive and understand DTMF
and modem neg. tones, and to separate the various narrow frequency bands
that make up the communications channels in both directions used to pass
digital information.

A lot of these same modems' DSP's also are capable of the wide spectrum
(relatively) bidirectional streaming of regular audio.  Evidently my modem
is one of these, as the manufacturer included software that utilizes it to
act as an audio in-and-out device.

Since my modem is not unusual, and since there are several pieces of
software in the MS-Windows world that do this function and work with my
modem, and knowing that advanced state of Linux today, I thought for sure
that there was software in Linux to do it as well.

I am unsure if my modem is one of those "winmodems" that uses the host's CPU
to act as a DSP to process the signals.  If so, the likelihood of software
to do this in Linux may even be more readily available.  My other box is
running a winmodem and works just fine with my Debian-based 2.6 kernel as a
regular modem.

--
Scott G. Hall
Raleigh, NC, USA
ScottGHall at BellSouth.Net