CDM audio out

[Spiffy50]

Hey gang. I've got a customer (small volunteer FD) who needs the ability to record one of their VHF channels on an infrequent basis.

They are looking at a PC based solutuon. My question is, using a spare CDM750, I can program a pin to be Filtered RX audio. A few questions:

1) Is this audio only passed if it has the proper PL or does all channel audio get sent?

2) Can I adjust the volume of this audio?

3) Is it safe to send this audio directly into a sound card or does it need to be handled the same as external speaker (as in, balanced audio, no grounding)?

Many thanks!

[Josh]

1) Is this audio only passed if it has the proper PL or does all channel audio get sent?

Filtered audio only passes with proper PL.

2) Can I adjust the volume of this audio?

No.

3) Is it safe to send this audio directly into a sound card or does it need to be handled the same as external speaker (as in, balanced audio, no grounding)?

This is debatable. I've run directly into my computer's sound card without consequence, although the levels are a little high so I have some attenuation in-line to lower it more. I connect to the "digital ground" of the radio though, not the chassis ground if that makes any difference. Others argue a transformer is required or you'll fry the audio circuitry.

[Jim202]

As Josh stated, this is a good source of RX audio. I would suggest that you put a 0.1 uf cap in
series with the line to remove any of the DC bias that is there. The RX filtered audio is always
there. You don't have to program anything. Use pin 11 on the accessory connector.

Jim

Hey gang. I've got a customer (small volunteer FD) who needs the ability to record one of their VHF channels on an infrequent basis.

They are looking at a PC based solutuon. My question is, using a spare CDM750, I can program a pin to be Filtered RX audio. A few questions:

1) Is this audio only passed if it has the proper PL or does all channel audio get sent?

2) Can I adjust the volume of this audio?

3) Is it safe to send this audio directly into a sound card or does it need to be handled the same as external speaker (as in, balanced audio, no grounding)?

Many thanks!

[Will]

I would suggest that you put a 0.1 uf cap in
series with the line to remove any of the DC bias that is there. The RX filtered audio is always
there. You don't have to program anything. Use pin 11 on the accessory connector.

Correct, adding a capacitor of at least 1uF in the high side of the filtered audio. Route this to the LINE IN of the sound card.
You can vary the volume control on the radio without affecting the RX Filtered audio level.

[Bill_G]

On CDM's I've used pin 11 rx audio, and pin 7 ground, for single ended fixed level logging audio. I added 4.7k resistor to get the level down. In Radio Config / Accy Config I set the rx to filtered audio so it doesn't pass squelch noise all the time. Worked well. There's a 47uF cap into a 560 ohm resistor in the radio on pin 11 driven by U211-2. I don't think you need to add a second cap to get dc isolation.

[Spiffy50]

Perfect thanks guys! I added a 4.7k resister on the high side of the audio and wow does it ever put out decent audio. Makes their recording program very happy! Thanks!