Tandberg to Tandberg Audio Problem

[NickC]

Hi, Im new to this forum. I have been asked by my boss to come up with a solution for an audio problem weve experienced in VCs with our other office. Both systems are Tandberg T6000 codecs and they are connected at 384 Kbps.

One of the regular participants has a tendancy to get heated and shout in meetings which causes distortion at the far end. We have turned the gain on the mic down to 3dB and still the problem persists. The metering on the VC did indicate the level was backed off a bit and the volume was healthy (spikes to 0dB) or slightly higher if yelling. We have tried to reproduce the fault in tests but cannot.

On the last test we noticed the AGC was turned off and switched this on as well as adding another mic for better room coverage. I am definite the speaker is not placing the mic to close as I have been assured by technicians that this is not the case and the room size is set to the size we normally use when installing the equipment. Does any one have any suggestions how to improve the audio experience as I fear the AGC wont be powerful enough to keep the audio from distorting.

The receiving AGC is turned off at the end receiving distortion could this help.

Any help would be much appreciated!

[robertk]

I've had this problem numerous times. However,

It might be small network (isdn and IP) errors that produce the shout, but I really haven't traced the problem down. (Usally you get feedback right after one of this shouts!?)

I also belive that newer software is better at correcting this audio-errors. One of our customers have had enourmous problem with this, but upgrading to E4 software and resettting the audio settings to defaults helped alot. Maybe the network has gone better too... but that I can't verify.


//Robert

[Glen Sykes]

Proximity to the microphone may have something to do with it. If you place a sensitive mic (such as the audio technical AT751 as shipped with the T6000) close to a very loud audio source, it can sometimes cause distortions in the mic itself.

The most hilarious example of this was one year at christmas time when my company participated in the videoconference broadcast of a pantomime to a childrens hospital. Keith Chegwin (a well known and slightly annoying childrens TV presenter in the UK) was starring, and a highlight for the hospitalised kids was a chat with Keith after the performance.

Keith, who wasn't best known for how quiet he was, when arriving in the projection room in the theatre where the VC kit was set up, scanned the room for a microphone to speak into (in true TV presenter fashion), spotted the 'odd-looking' AT751, picked it up and yelled 'how are ya kids??!'....

Suffice to say the kids weren't that startled at the intelligible noise emanating from the TV, it was Keith Chegwin after all :D

[George]

NickC

Slap us with some more details. What Codec, Audio eqipment, and software versions are you using?

[NickC]

Thanks for the info this is a great site

T600 MXP with F1.2 1 x AT-871R mics at end A (receiving distorted audio) and T6000 (E4.0) and 2 x AT-871R boundary mics at end B (sending distorted audio). The following tests have been performed.

1. The AGC was turned on at end B

2. A second mic was added at end B

We then performed a test with the person who speaks loud and it seemed to resolve the issue ie the clipping and distortion was apparent with AGC off but not with AGC on. However I can not be sure if this will be the case in an actual meeting where there are more participants and background noise. All mics are set at 3dB.

We often have problems with network issues as the call is connected through our PABX which is notoriuosly unreliable. Also because of cost restraints the calls are usually connected at 128kbps.

Any further suggestions to improve user experience would be great. Is it better to have the mic at the end of the table or the middle? The table is about 2m x 1 m wide.

[MACC the AVCOtek]

The AT-871R is a unidirectional mic so anything behind it won't be picked up. Therefore, if you've got people on three sides of the table you'll need to place the mic at the closest end of the table to the camera to be able to pick everybody up. If you put it in the middle of the table, everybody sitting behind it won't be heard. The reason the mic is unidirectional is to reduce the possibility of picking up sound from your speakers.

This assumes, of course, that you're using the codec in a typical 'roll-about' configuration - codec and camera with a monitor or two on a cart at one end of the conference table.

More information about the mic can be found here: http://www.audiotechnica.com/prodpro/profiles/AT871R.html

[trapehzoid]

sounds like simply overdriving the microphone and input. Are you sure there is no mic mixer involved? If the mic is just plugged into the codec, this shouldn't be an issue with the setting you mentioned. Always keep AGC on on the mic inputs. Only reason to turn it off is when using an off-board mixer with its own AGC controls.

Also, you don't mention which output device you are using. Make sure you aren't overdriving your outputs with higher gain settings.

[Video]

one of these?

http://www.fmsystems-inc.com/prod_list.cfm?cat=none&industry=c

The ALM 771 is an audio limiter that is can be attached to the outgoing audio source.