[asterisk-users] Walkie talkie to sip phone interfacere:

[Dave Platt]

> I've been trying to find a solution that would allow our sip phones to 
> communication with walkie talkies.  Our setup is that we have sip phones 
> setup in 2 locations, headquarters and dome.  We can communication from 
> headquarters and dome through sip phones, but within the dome we have 
> technicians that use walkie talkies to communicate as they go about 
> their work.  Our hope is to allow 2 way communications from our sip 
> phones at headquarters (or within the dome) with our technicians using 
> their walkie talkies as they are working throughout the dome.  Not sure 
> if this is possible but I would appreciate any suggestions.

It's definitely possible.  How practical it is, remains to be seen and
is probably a "how well do the details work out?" issue.

The simplest approach is probably this:  you'll need a walkie talkie
"base station" which will serve as the transmit/receive point for
the dome.  The most straightforward would be to use one of the
actual walkie talkie radios, with a well-filtered DC power supply
in place of a battery pack.

You would need to hook up the W/T's "speaker out" and "mic in", and
perhaps the "push to talk" line, to suitable audio and control
I/Os on some sort of Asterisk end-point.  The least expensive way
would probably be to use the Asterisk server itself (or some PC
running a soft-phone client), and use the PC/server's sound card
jacks.  You would need some sort of level-adjusting (padding)
system for the signal being fed into the walkie talkie's "mic"
input (these generally require much less voltage than a sound
card's line-level output), and it would probably be a good
idea to have an audio isolation transformer in each audio path
to prevent ground loops and hum and RF pickup.

You'd need some way of keying the radio's push-to-talk when someone
on the phone starts to speak, and then release PTT when the
voice stops.  Some walkie talkies have a VOX (voice-operated
switch) which will do the job.  Others do not, and you would need
a separate VOX circuit (not difficult).

One possible hardware device which might save you trouble is
the Tigertronics SignaLink USB - it's primarily designed for and
sold to amateur-radio operators but has multiple uses.  It consists
of a USB "sound card", isolation transformers, an adjustable
VOX/PTT circuit, and a very flexible radio-interconnect-cable system
which you could adapt to the speaker/mic needs of your walkie talkie.
You'd simply plug it into the server's USB jack, and it would become
a secondary audio interface.

On the Asterisk side, you'd want to use the ALSA channel driver,
and create an inbound extension which would simply "dial" the
ALSA channel.  Or, you might decide to use one of the various
Asterisk bridging/conference applications, and have the ALSA
walkie-talkie channel be perpetually signed into a conference
bridge which one or more other users could phone into.

You'll need to select and implement a suitable security policy,
to control who can access your dome audio link, and decide
whether someone in the dome can use a walkie talkie with
DTMF capability to dial calls or otherwise control the Asterisk
system via radio.

Finally, you need to make sure that all of this is legal in your
particular jurisdiction.  Here in the U.S., there are quite
a few personal and mobile-radio services for which it is *not*
legal to create a connection to the wired telephony system.
This is probably something you should determine first, rather
than at the end.