Audio Problem

[Video]

I'm experiencing an audio problem that has me puzzled. We have a classroom that holds about 20 people. There are 10 push to talk microphones in the room, a lav microphone, a VCR and a compter that feed into a Mackie audio board and finally to the CODEC - a VCON. There is a vacum-type sound that is just annoying! All of the mics are OFF. When we mute the system from the CODEC, the sound quits, and it also quits when we mute it from the MCU (bridge).

We previously had a problem with receiving a radio signal and had to get another audio mixer in to prevent the system from receiving the signal.

Any ideas or help would be appreciated.

[Gary Miyakawa]

Sounds like the Vcon is driving the AGC up trying to hear something when you mute from the microphones. Try disabling the AGC on the VCON if you can.

Gary Miyakawa

[Video]

Thanks Gary.

Yeah, I have checked the AGC before and it didn't change anything. We have other rooms in other towns that are wired similarly but do not have the problem.

Looks the like problem occurrs in the early part of the day (8 am - Noon), but does not seem to be noticeable during an evening class that goes from 6 pm - 9:00 PM. A wireless lav mic is used in the evening - the only difference I know of. The lav mic receiver and level are up all the time, but the microphone is only turned on during the evening.

I think that the radio signal may still be the problem. Any ideas? Any ideas on how to shield it from picking up the radio signal?

[Oliver Black]

Could it be the echo canceller trying to cancel a non-existent echo, ie creating the noise. When the lav is on it, the echo canceller can do its thing because it has two-way audio, when the lav is off, it has only one-way audio and is perhaps injecting its own artifacting. Also, could the lav receiver be creating noise when the lav is off? Is the squelch adjusted correctly (if there is one?).

[AVGeek]

To carry on Oliver's train of thought, some echo cancellers need something to reference all the time so perhaps leave one of the push-to-talk mics on all the time and just use the codec mute when privacy is needed?