Video Transcript
AEC & Q-SYS Conferencing System (Part 3)
5m 26s
00:07
Welcome back.
00:08
Now that we’ve covered the more generalized description of what AEC is and what it does,
00:14
let’s look at the specific application in the Q-SYS environment.
00:17
The most basic signal flow for systems without local reinforcement have two distinct signal paths.
00:23
The path to the far end includes the microphone input block, the AEC processing,
00:29
an automix of the conferencing mics and finally the transmit block to the far end.
00:34
In this example it’s a softphone transmit block, but this could be a USB transmit
00:39
or a line level output fed to an outboard codec.
00:42
In some systems it may be a combination of or all of these.
00:46
Remember the best practice is to have the ‘last mic on’ setting enabled
00:52
in the automixer so that some room noise is always fed to the far end.
00:56
This helps keep the far end participants from feeling like they’ve been disconnected.
01:01
The path to the near end mixes the far end receive signal
01:04
or signals with any program sources such as those from a presenter’s computer.
01:08
After the mix, these may be fed to a line output and on to an amplifier and speakers,
01:14
or directly to a Q-SYS amplifier via Q-LAN.
01:17
Of course in many cases we’d like to process the incoming microphone signals
01:21
to maximize intelligibility and make them sound as good as possible.
01:24
As a best practice, we’d like to apply that processing AFTER the AEC block in the signal chain.
01:31
We’d like the AEC to operate on the signal before any processing,
01:35
especially any processing that is non-linear in nature such as compression.
01:39
This makes it much harder for the AEC to converge.
01:43
Another consideration here is the privacy muting of the conferencing mics.
01:48
When possible, this should be done in processing AFTER the AEC processing block.
01:53
When the far end is talking the AEC block will still hear the reference
01:58
and the resultant echo even when the mics are muted back to them.
02:01
This keeps the block converged, avoiding echo when the mic or mics are unmuted.
02:07
There’s one case where it is acceptable to apply some processing ahead of the AEC block.
02:13
When multi-microphone beamforming arrays are being used such as the SHURE MXA910,
02:19
it's desirable to follow the published guidelines for EQ ahead of the AEC to remove excess low frequency energy.
02:26
This can often be done in the microphone processing itself,
02:29
but can also be done in the Q-SYS environment as shown.
02:33
Even in this case we NEVER place microphone dynamic objects, linear or non-linear before the AEC block.
02:41
As outlined in the Quantum level one topic on gain structure and signal flow,
02:45
all microphones should be calibrated to a -20dBFS nominal input level.
02:51
From there the system should be able to operate at unity gain structure through the signal flow.
02:56
If user level control is required, it’s best to place
03:00
an overall transmit level control just ahead of the conferencing transmit block,
03:05
as giving level control of individual conferencing microphones can cause inbalances in the system.
03:11
You’ll want to also make sure you set reasonable upper and lower limits for this level control such that
03:17
the user can’t send a clipped or distorted signal to the far end.
03:21
Those things considered, the most important factor in having a functional conferencing system
03:26
with AEC is composing a proper AEC reference.
03:30
To recap here, we’re looking to tell the algorithm what signal it is we’re trying
03:35
to remove from the microphone being processed.
03:38
In cases with no local reinforcement, the reference signal should be:
03:42
All far end signals, as it’s obvious that's our goal, to keep these signals from going back to the far ends.
03:48
And any program feeds.
03:51
If we’re not sending program sources to the far end directly,
03:54
we don’t want them to hear it passing through the microphones.
03:57
If we are, we don’t want them to hear direct program feeds
04:00
and a delayed version that comes back through the conferencing mics.
04:03
In short, as long as there’s no voicelift in the room,
04:06
the AEC reference signal should simply be the signal that’s going to the room speakers.
04:12
Notice: we NEVER put a microphone signal into its own reference either directly or indirectly.
04:19
This results in a pumping, underwater like sound that reaches the far end.
04:24
When placing user controls in the system,
04:27
it often seems simple to place a single gain control directly before the output block.
04:32
Imagine a user in a call.
04:34
Let’s say the softphone receive signal is quiet, so the user increases the gain.
04:38
Then a program source starts and it’s incredibly loud because the single output gain control is set so high.
04:45
So, it’s best practice to place user gain controls with reasonable limits ahead of the mixer so that the user
04:52
is controlling the level of ONLY the source that needs to be changed.
04:56
In addition, this is great for AEC systems.
05:00
The red tag in this diagram represents the reference signal.
05:04
If I increase the level of the softphone receive in the room,
05:08
I also increase the level to the AEC reference by the same amount.
05:11
Placing controls in this manner reduces the amount of reconvergence required
05:16
by the AEC algorithm as compared to changing only the level in the room.
05:20
Alright, let's take a quick break right here and come back whenever you're ready.
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AEC & Q-SYS Conferencing System (Part 3)
5m 26s