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Calibrating a distance conferencing system

Although there’s no magic bullet to make teleconferencing or videoconferencing systems work properly, there are a couple of quick things we can do to get closer to an ideal system. The tips in this article are not meant to supplant proper system calibration, but they are a good a starting point. 

First steps

  1. Make certain you have peak meters connected to your input blocks and RMS meters before the output blocks.
  2. Check signal routing to ensure that the proper signals are routed to the AEC reference block(s). In general, incoming signals from the far end (Telephone interface, CODEC and/or VoIP interface) should be routed to the AEC reference. Optionally, program material can be also be sent to the AEC reference.  Microphones should almost never be routed to the reference.
  3. Double-click to open both the AEC Input block and the Peak Meter block that is connected directly to it.  Speak into the microphone from a normal distance and at a normal volume, and increase the "Gain In" on the AEC Input block until the corresponding Peak Meter reads about 0dBu.
  4. Make a test call and have the far end talk. Adjust the input level of the interface (phone, VoIP or line input from CODEC) until that signal reads 0dB on the input meters. Repeat with every conferencing interface in your system.
  5. The signal to the AEC Reference Block should now be reading near 0dB as well. If not, make adjustments so it does.
  6. Adjust the amplifier input attenuators to get a proper volume in the room.  Alternately, you can adjust the "Full Scale Out" in the Output block to adjust the volume.  Both of these methods are always preferable to using a digital Level Control block to make drastic level adjustments.  
  7. Make sure that there aren't any digital level adjustments (like Level Control blocks, or level controls built into processing blocks) that are permanently set too far from 0dB.  If you find that there are digital levels adjustments that are adding a lot of gain, consider lowering them and adding gain to the "Gain In" of your Input block instead.  If you find that there are digital level adjustments that are taking away a lot of gain, consider raising them and taking away gain at the "Full Scale Out" of your Output block or turn down the amplifier.

After these adjustments, minor tweaks might be needed to dial in the system a bit better.

Extra recommendations

These are some changes you can consider to tweak the system:

  • Add a bit of noise reduction (NR In) in the AEC Input block. Values between 6 and 10 work well in most cases, if the far end is hearing some constant background noise.
  • Use High Pass filters on microphone inputs. The default value works but if there's a lot of air handling noise, increase the filter to 120Hz or 150Hz and increase the slope to BW24.
  • If the configuration has an Automixer, consider limiting the Number of Open Microphones (NOM limit).
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