MCC7500 clipping audio

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MCC7500 clipping audio

Post by pfish »

Trying to troubleshoot an issue that has been occurring for over a year in our environment with not much progress from the service provider. On our analog VHF channels, we've been having an issue where sometimes dispatch misses an entire transmission, or more frequently misses pieces of a transmission. Field units listening via RF have no problem hearing these transmissions.

The missing audio is absent from the recorder and all the consoles (10 positions). To me it seems like the audio is being gated or not meeting some audio squelch level. On the field side, I can always hear the carrier when someone is keyed up, even if they are not speaking. On the logger and console side, it will go completely silent. I would expect, at the logger/console side, to never drop or gate the audio if it doesn't meet a certain threshold.

Here is some example audio of the issue: - lots of clipping here.
Here is the same transmission recorded from a scanner:

Other examples of clipping: - clipped after "engine 73", transmission resumes - on the field side, the audio was never clipped - clipped after "county road 24", transmission resumes - on the field side, the audio was never clipped - audio seems to be clipped as there is no trailing "splash"
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Re: MCC7500 clipping audio

Post by alex »

I am not well versed in MCC7500 so this is going to focus a bit more on the system with a couple of suggestions based on responses:

1) Is the system single site or is it a voted channel?
2) What are you using for voting? JPS, Spectrac, Digitac? MLC? GRV? Ethernet? Tone? E&M?
2) Is it happening on multiple channels or just one?
3) How is the system connected to the MCC? 4 wire? E&M? IP?
4) What is your system transport like (e.g. microwave, phone company (ugh), fiber, metro-e, etc...)

Let's assume it's analog voted and controlled by console via 4 wire.

Can you plug in a CPI Tone remote in line with the console feed to the CCGW? DO you experience the same drops? (note - if you "tap" the audio line you may need to change impedance on either end or add an audio bridge (and once you use an audio bridge you will never go back...)).

If you don't experience the drop (which since the field does not hear it I'll assume the CPI won't either) then it's clearly something in the MCC that's causing the issue. I assume you are basically at the point of eliminating everything else and blaming the MCC.

If this is analog, voted, and connected via 4 wire - it wouldn't shock me if you had an audio level issue. Has someone been out with measured the audio levels with a TIMS set? Using the TIMS you should be able to re-create the drop and look at how to bring down or up the levels on the comparator to match what does not drop. This reads like you have a level issue - or this could be as simple as a broken or bad copper jumper between the pieces of hardware. As funny as it sounds - have you replaced jumpers or removed and screwed them all back down? If it's a spectra tac - have you worked some of the pots in the last year? I think it's the console level screw you have to work back and forth a bit on it to clean up issues on this - if you never had the issue you would never know a left and right swing once a year would clean an audio issue up.

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