Re: routing VoIP through a regular phone line

From: P Gentry (rdgentry1_at_cablelynx.com)
Date: 09/13/04 

Date: 12 Sep 2004 19:18:18 -0700

"C3" <_> wrote in message news:<4140e475$0$30576$afc38c87@news.optusnet.com.au>...
> The only news to me is that voice modems are not capable of performing the
> very limited task needed of them.

Depends on what you mean by "limited". The "voice" in voice modems
refers to the dsp and codec firmware that _digitizes_ the analog voice
stream (and vice versa). It's the software that actually provides
what we, as users, think of as the functionality. Ie., voice mail,
answering machine, video phone (haven't seen one of those in a while),
etc. Voice mail and fax are probobaly the two most used "extras"
beyond simple data linking for dial-up networking.

> I always believed that voice modems are simply modems that can receive a
> signal through a 3.5" minijack and pass it onto the phone line, and also to
> send out the sound from the phoneline onto another 3.5" minijack plug. These
> two can be connected to the input and output of a soundcard.

"Everyone" (me too) has tried this at one time or other -- these lines
(sound card to modem) won't "match" for one thing and the other
problem is that the input/mic jack on the modem is just to _record_ a
message. Obviates the need for a separate sound card for input _or_ a
"speakerphone" (mic and speackers). A person-in-the-flesh doesn't
really need a modem to "speak" on the wire -- that's what POTS and
handsets are for.

> The only thing left is to dial the number, that's just an ATDT away, isn't
> it?

All modems _can_ dial out. The problem is that without human
intervention (lifting up the handset, eg.) or some software app the
modem will try to contact another _data_ modem. If you lift the
handset before the modem/modem handshake begins, the passthrough
circuit for the handset "shorts out" the modem (ie., modulator and
other data circuits) and you can carry on a normal voice conversation.
 In this case you're just using the modem as a phone dialer.

Googled about the past two days to catch up on modem developments and
not much has changed regarding "garden variety" modems for home/office
use. The "voice" capabilities (beyond the raw signal processing) are
mostly found in software and all I've looked at are still file based
-- ie., record/playback a compressed audio file. Check here for some
software examples:
http://www.imptec.com/callstation.htm

FWIW, you _can_ sometimes hear some innocent person speaking at the
other end of a data dial-out -- usually cursing about the strange
noise ;-)

> Aren't voice modems capable of this?
>
> cheers,

I think you might be able to build an app that would accept a voice
input stream from another app and send it out a "voice connection" on
the modem. That is I _think_ the "on chip" hook for voice apps is
generic enough to allow this, but for the life of me I wouldn't want
to wrestle with the timing, dialing, codec, and data transfer issues
involved -- I would just pick up a phone ;-)

Doing this with an app which handles incoming VoIP (which is h.323
based -- like netmeeting) and translating(?)/transfering that voice
stream to a modem app that decodes/relays the analog voice content
onto a POTS line (which is already dialed out and connected) seems
like a lot of work. Maybe that's why there are commercial companies
that offer this sort of VoIP-POTS interconnection. High quality
digital PBX networks can too. Eventually, the phone lines will likely
be 100% digital, but we're not there yet -- maybe not for many years
as no one sees enough $ in such an expensive upgrade.

hth,
prg
email above disabled

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