The AEC default control panel is ordered in two groups of controls for every input channel. The first group of controls are identical to the standard Soundweb London input cards and function in the same manner. These controls are the audio input meter (configurable as Pre or Post-AEC), input meter controls - Attack, Release, Reference, and Phantom Power - for each input channel. The second group of controls are the AEC controls.
The basic AEC control panel allows enabling and disabling of AEC and AGC, and allows setting levels for noise cancellation, non-linear processing and signal threshold. The AEC control panel for the BLU-101 , BLU-102 and BLU-103 AEC processing object differs from that of the AEC input card since it does not show the analog input meters and input controls which can be found on the control panel for the analog input card for those devices.
This button enables or disables AEC processing for each channel. When this button is enabled the AEC algorithm will remove the acoustic echo from the audio channel with linear processing and with a specified amount of non-linear processing. (See NLP Level below.)
The Echo Return Loss (ERL) meter is a measure of the room's natural attenuation of the far-side audio as it leaves the speaker(s) and re-enters the microphone(s). This parameter is controlled by proper gain structure setup (ensuring a good signal to noise ratio and reasonable headroom for the AEC input signal). A proper gain structure is critical for distortion free sound and optimal performance for AEC. This is the single most important parameter when setting up the AEC system.
The AEC algorithm will only be able to recognize and remove echo to its best extent when this meter is displaying in the 'green' range. The 'green' range is indicated on the control panel below 0dB. The algorithm will continue to converge over 0dB, but the convergence rate will decrease in that range. If the ERL meter is over 10dB, convergence should not be expected.
This meter will not update during double-talk. It is updated based on far-side speech only.
The Echo Return Loss Enhancement (ERLE) Meter measures how much acoustic echo is being removed from the signal path. This measurement consists of the natural room attenuation as indicated by the ERL meter and the amount of echo removed by the AEC algorithm. A lower signal indicates more echo being removed. The lower the meter, the better.
NOTE: As dictated by industry standards, NLP contributions are not included in this reading. NLP contributions are made in addition to this meter's reading.
The Non-Linear Processing (NLP) setting determines the amount of non-linear suppression that will be applied in conjunction with the AEC algorithm for each channel. NLP will remove the residual echo not removed by the linear part of the AEC algorithm.
This parameter represents a trade-off between achieving good double-talk performance, with no suppression of the local speech signal, and very robust echo suppression, with no echo audible on the far side. At its most aggressive setting (NLP at 100%), the non-linear processing will remove any of the residual far-side echo picked up by the microphone. However, this is done with an increased risk that some of the near-side speech will be degraded as well, especially during double-talk. At its least aggressive setting (NLP at 0%), the non-linear processing is effectively disabled, which may let some echo through, but will allow for a more natural double-talk performance.
The best setting for this parameter may depend on several factors, including the acoustic properties of the room and user preference. The default value of 50% may give a good balance between these two competing goals.
The Noise-Cancellation (NC) setting will determine the amount of noise cancellation that will be applied to each channel. The noise cancellation algorithm is a very advanced algorithm that will remove steady-state noise without compromising the quality of speech passing through the channel. This algorithm is great for removing projector noise, HVAC, and other unwanted background noise that can compromise speech intelligibility.
In a conferencing system, some microphones may have a mute or push-to-talk feature built in. If a mic goes into or comes out of mute, then the characteristics of the conferencing system change instantly, and echo may leak through as the AEC re-converges. A signal threshold is defined to allow mics with mute or push-to-talk features to work seamlessly with AEC. Using the threshold, a level can be defined that is below the normal, ambient noise floor of the room. If the mic level goes below this level, then the AEC algorithm will treat the microphone as muted, and minimize any echo that would have occurred otherwise. The "Active" LED indicates that the microphone level is over the threshold, and the mic is not treated as being muted. When the LED is off, the mic level is below the threshold, and the mic will be treated as being muted.
To set the threshold:
Put the microphone in its muted mode.
Set the threshold to a level where the LED turns itself on and off randomly.
Raise the threshold from this level by 3 to 6 dB. The LED should now be off with no flickering.
Take the microphone out of its muted mode and the LED should illuminate.
This process may need to be repeated if the microphone's preamp gain is adjusted. To disable the mute feature based on signal threshold, simply set the threshold to its minimum value.
The Automatic Gain Control is designed for voice applications. It is designed to compensate for varying distances between the speaker and their microphone as well as speech level variances at the near end. This provides the far end with a signal that will automatically be increased or decreased to maintain a consistent audio level. Setting the maximum gain too high can cause inconsistent gain structures and bring up the noise floor. The AGC will adjust the gain during near side speech only. This means that during pauses in near side speech, the noise floor will maintain a constant level, and will not grow to hit a target gain output. Only near side speech signals are used to control the gain.
To use the AGC, first define target levels for the transmitted speech signal. The default target levels for AGC are a maximum of 6dBu and a minimum of -10dBu, which define a target window with 16dB of dynamic range. If the speech level is within the target window already, then the AGC-applied gain will go to 0 dB.
If the speech signal is below the target window (i.e., below the minimum target level), then the AGC will increase the gain (to a limit) so that the signal level meets the minimum target level. The AGC will limit the gain it can add to a signal by a maximum gain setting. Once the AGC has adjusted its gain high enough to meet the maximum gain setting, it will stop adding gain, even if the minimum target level is not reached. This is useful to stop very weak speech signals, such as whispering, from driving the gain too high. Setting the maximum gain too high can cause inconsistent gain structures and bring up the noise floor.
Similarly, if the level of the speech signal is higher than the maximum target level, then the AGC will reduce the gain, by as much as the minimum gain setting, in an attempt to bring the speech level down to the maximum target level.
A generous range for the maximum gain and the minimum gain have been provided. Care should be taken, particularly with the maximum gain setting, to avoid extreme levels. Situations where the maximum gain setting should be set over 10dB will be rare. The maximum gain setting has the potential to break a gain structure, so set it carefully, especially if the setting is to be used over 10dB.
The attack and release rates for the AGC describe how fast it will adjust its gain. Because the AGC only adjusts gain during near-side speech signals (and not during unvoiced consonants like t, s, p, and f), the attack and release rates should be set higher than other typical AGC implementations.
The AGC meter shows the current amount of gain being applied to the signal.
Here is a summary of the AGC settings:
AGC On : On/Off switch to enable/disable automatic gain control
AGC Max Target : Range -20 dBu to +20 dBu (default is +6 dBu)
AGC Min Target : Range -20 dBu to +20 dBu (default is -10 dBu)
AGC Max Gain : Range 0 dB to +20 dB (default is +6 dB)
AGC Min Gain : Range -20 dB to 0 dB (default is -6 dB)
AGC Max Rise Rate : Range 0.1 to 100 dB/s (default is 13.33 dB/s)
AGC Max Fall Rate : Range 0.1 to 100 dB/s (default is 15.62 dB/s)
Parameters
Reference - as per standard analogue input card
Attack - as per standard analogue input card
Release - as per standard analogue input card
Gain - as per standard analogue input card
Phantom Switch - as per standard analogue input card
AEC Enable - Appears on the default panel, described in 'AEC Card Control Panel' section.
NLP Enable - Non-linear processing is on by default.
NLP Level - Appears on the default panel, described in 'AEC Card Control Panel' section.
NC Enable - Noise cancellation is on by default. This parameter is for advanced use and is accessible via the design tree.
NC Level - Appears on the default panel, described in 'AEC Card Control Panel' section.
Signal Threshold - Appears on the default panel, described in 'AEC Card Control Panel' section.
AGC Enable - Appears on the default panel, described in 'AEC Card Control Panel' section.
AGC Max Gain - This parameter sets the maximum gain for AGC operation. It is for advanced use and is accessible via the design tree.
AGC Min Gain - This parameter sets the minimum gain for AGC operation. It is for advanced use and is accessible via the design tree.
AGC Max Target - This parameter sets the maximum target for AGC operation. It is for advanced use and is accessible via the design tree.
AGC Min Target - This parameter sets the minimum target for AGC operation. It is for advanced use and is accessible via the design tree.
AGC Attack - This parameter sets the maximum rise rate (dB/s) for AGC operation. It is for advanced use and is accessible via the design tree.
AGC Release - This parameter sets the maximum fall rate (dB/s) for AGC operation. It is for advanced use and is accessible via the design tree.
For each recognized AEC card in a Soundweb London unit the following configuration symbols will appear in the Default Configuration view:
The left hand 'AEC Input Card' block functions like a standard Soundweb London input card. This block contains the 4 channels of processed AEC audio as well as the 4 channels of 'dry' (unprocessed) input audio being fed into the Soundweb London AEC card. The right hand block 'AEC Input Ref (REFERENCE) Return' is used to provide the REFERENCE signal for each AEC algorithm. The Reference signal is the signal that will be removed by the AEC algorithm from the signal path. The Reference signal should be taken from as close to the output as possible. This will provide the AEC algorithm with the most accurate representation of the signal (to be cancelled) and will provide the best AEC performance.