Digital DECT Phone Audio Tap

[kolbep]

Hi All.

One of my customers has an 8 Channel Audio Recording PCI card. It accepts Line Level Inputs or Direct connection to an Analogue Telephone Line (Selectable per port).

We have it connected to several Analogue Extensions on the PABX (Paralleled with the Phones), and that works fine.
They also want to record their conversations on their main switchboard phone. But that is a Digital Dect phone. With it being a digital line, we cant just tap into the PABX Extension, because you get the digital hash noise.

Before they upgraded their PABX and Switchboard Phone, all I did was tapped the recording interface onto the Speaker signal that goes from the phone base to the handset. That worked fine.  However with their new PABX/Switchboard combination, it is not working too well.
They say it is drops the volume down too much, and it is hard to hear.

The Recording Interface has a high input impedance (>10kohm) and the Telephone Speaker is 8 Ohm, so I did not expect that to be a cause of the problem.  I then thought it could be a problem between different ground potentials, so I took a small choke out of a CCTV Balun, put one winding in Parallel with the phone speaker (also used a 10k resistor in series, just to make sure it did not load down too much). The 2nd Winding I ran to the Interface.  Even then they say that it drops the volume.

I now have 2 thoughts about how to do this (Not to load down the existing phone, and to provide isolation), and could use some advice :

1, Get a small coil with Multiple turns, run the handset speaker wire through it. Then feed an opamp or small preamp. That way it is electrically isolated from the phones wiring itself. The Problem is, where to find a suitable coil, with lots of turns on it that would be suitable. or
2. Get a Mic with Preamp (eg Max9814 preamp with AGC). I then install the mic in the handset in a position where it can pick up the sound from the speaker.

I prefer no 1, as I can possibly put the coil and opamp/preamp in the phone base itself. That will mean neater wiring as I can bring separate power, and audio out direct to the phone base. Minimizing wiring all over. Without the acousitc pickup of a microphone the signal should be clearer.

With no 2, by putting a mic in the handset itself, it means that I will need to run another wire to the handset itself (in addition to the curly cord for the phones mic and speaker). This will be messy, and get in the way.

I did discuss with them that their New Panasonic PABX has a facility for Automatic 2 Way Recording, as they already have a DSP Card, and the Voicemail SD card, but they were quoted over R40000.00 just for a Supervisor licence to enable the recording, which caused the manager to nearly fall off his chair.

Any Suggestions.

Thanks

[texaspyro]

Get the coil from a relay... some relay coils can have thousands of turns.