AccessGrid.org

General Clearone Echo Canceller Information and tips and tricks!

The following information has been provided by Dean Gray (AV Technical Services – Deakin), which provide some tips and tricks with getting the most out of your ClearOne Devices, especially the XAP and Converge systems.

Mic audio gain - get this right and you can solve a lot of problems before they occur. Open up the level meter in G-Ware or the Converge Console and look at the levels, you should talk in a normal voice and adjust so the post gain level peaks at around 0 db. This will also give the processor the best possible signal to work with, and decrease the amount of processing noise that will appear in the output. If you find you need a large amount of attenuation with some mics such as 20db, don’t be afraid to turn the coarse gain down to 25db instead of 55db (with the XAP devices). Using the 25db setting, even with a few db of fine gain, will sound better and be far less prone to distortion than a coarse gain of 55db with a stack of attenuation. This comes back to the basic audio principles, keep it even throughout the system, no large gains, no large attenuations.

Mic muting/gating - there are some mics out there that have a built in on/off switch or a built in gate. These should be disabled. The ClearOne units "listens" to the room constantly to adjust itself for echo cancellation, noise cancellation, and to obtain an ambient noise level of the room for the adaptive mic gating to work. If you need mic muting functionality at the table, mics such as the Shure MX392 & MX393 can have their push buttons and leds wired to the logic i/o port on the XAP. This way, the XAP can still listen to the mic but it is muted internally from being sent to the far end.

Mic gating – It has been found that the factory defaults for this pretty soft, I like to lift the gate ratio a bit to say 18 or 20db, the mic shouldn’t gate open with just background noise, adjust the gate threshold until it only opens with speech. This will solve some noise issues before the noise canceller has to deal with them. I also like to increase the off attenuation, as the factory default is generally only a 12db reduction when the mic is off. Turning on first mic priority will also help, because this stops one sound opening two mics, which increases noise and decreases intelligibility. Also, don’t be afraid to set the maximum number of mics on to 2 or 3. This ensures only 2 or three mics can gate open at a time. There should never be a need to have all 8 mics open.

Noise cancellation - is a great feature of the XAP and Converge units, but used incorrectly the XAP can run out of DSP power (Not sure about the Converge units). By setting the input gain and gating correctly you can help avoid this situation. Don’t use it unless you have to, and don’t automatically set it to the 15db maximum, try less settings first. When the unit runs out of processing power either its echo cancellation function can be compromised and bursts of echo can leak through, or the "gentner garble" effect can creep in, where the audio in the local room can sound like it is underwater for a second or so.

Using the system as a PA - in some environments, such as a lecture theatre, you might need to send the lectern/presenter mic to the speakers. Don’t be tempted to “just” route the mic straight to the speakers. Doing this will cause the echo canceller to cancel the local room audio. To do this you need to use a much more complex method which involves using a virtual echo reference.

Compression - Compression is your friend, it is not a bad thing like in the PC world, such as MP3 compression. What compression does is to reduce the dynamic range of the audio. Quiet bits become louder and louder bits become quieter. This is a principle used in every large PA system and every TV & Radio broadcast. To use it, route all your mics to a processing bus rather than straight out to the PC. Then route the output of the processing bus to the PC, making sure that the mics are not routed directly to the PC. Go into the processing bus options and enable the compressor, a threshold of around -5 db with a ratio of 10:1. If compressing music, a ratio of no more than 4:1 would be recommended, but speech works well compressed hard. If you get this right, you can say goodbye to transmitting distorted audio when the person speaking “shouts”. I find a using another compressor between the output from the PC and the input to the power amp works well, to reduce the level from excessively loud sites that dont have their compression operating.

Output audio gain to speakers - set this to around the 0db mark, on most power amps this results in a really loud output. Turn down the power amp to get the correct listening volume. This helps the echo canceller enormously as in its relative terms it only has to content with a small amount echo. Never use an equalizer or any other audio processor between the “ClearOne ” device and the power amp.

Output audio gain to the PC - typical PC soundcards have an input level of either -10db or -20db. The output of the Clearone is at the hotter level used by professional gear of +4db. This means you need at least 14db of attenuation between the ClearOne output and the input to the PC. In a purists world you would do this with a resistive pad in the cable between the ClearOne and the PC. The realist says it’s probably OK to run -14db of attenuation on the output of the ClearOne because the typical poor quality soundcard will add more noise than is produced by a bit of attenuation in the XAP or Converge. Remember your compression, a hard compressor will stop the input of the PC from distorting.

Input audio gain from the PC - set the PC output level fairly high, say around 80% and do the volume adjustment in the XAP. This makes sure that the signal to noise ratio is as high as it can be. PC soundcards typically have a lot of background noise in them, by running them loud you maximise the difference between the signal you want and the noise that is inherent in the card. The volume control doesn’t adjust the noise, just the wanted signal on top of the noise. If you use a compressor on the input from the PC you will get rid of excessively loud sites.

Hopefully by now you should have some information to get the most out of your ClearOne devices for the Access Grid.

A special thanks should also go to:

Dean Gray - AV Technical Services, Deakin University, Australia

For providing all of the information.

 

Revision History

Date

Revised by

Changes

18/March/2008

Jason Bell

Created Document