Recording TX and RX audio from a DM4000

Question

We have Motorola DM4601 radio station with external mic (not connected on the front). Is it possible to record audio from the radio station and microphone? I know that there is no output on the radio (for mic part) if it would be connected in the front, but what if the signal for recording microphone would be taken directly from the microphone and audio of the radio station from the back pin - I think it's 14 or 13.

Answer

The is no means to extract transmit audio on another external connector. 

Pin 13 is used for an external PA. There is an accessory available that will allow the user to use the radio like a public address system. They would press a button then press PTT and their voice would be routed through this pin to the accessory (external audio amplifier) and on to a horn speaker (think Sheriff Buford T. Justice!). While this feature is turned on, the radio will not transmit voice and any PTT press will result in the audio being sent to the PA system.

The same accessory setup allows for off-air received audio to be sent to the PA system.

TX Audio on pin 11 is not connected to the microphone socket on the front so it would not be possible to tap-off audio in this way. The microphone input on the front and TX audio have their own audio buffering circuits.

I've never seen or worked with this accessory and Motorola Solutions don't even have it setup to sell in EMEA, as far as I can tell. If there was any requirement, the dealer probably procured something locally or the radio was integrated with an existing siren system.

The only way recording microphone audio from a transmitting radio would be to use an external amplifier and splitter in series with the microphone. This circuit is needed to overcome the impedance mismatch; insertion loss and ensures the audio levels are correct for the connected equipment.

In essence, the abovementioned circuit will look like this.

The resistor value depends on the required input voltage and impedance of the recording equipment as well as the output level of the amplifier and input level on the DM4000 series radio.

Simply wiring the radio and recording equipment in parallel would create an impedance mismatch and there would be insufficient drive from the microphone to both devices. The splitter solves this but also introduced some insertion loss - the amplifier is there to overcome this but also to provide electrical isolation.

Since this circuit will be in proximity to RF, it makes sense to include RF filtering on all incoming and outgoing lines (i.e. capacitors; chokes and screening).

Formulas and sample circuits for the above are available on the internet. Connecting unknown; non-Motorola equipment to the radio carries some responsibility in terms of regulatory approval and safety so if you decide to go through with this, please pay careful attention to my disclaimer.