Ambient Level Control
Purpose
This device is designed to take the mystery and complexity out of setting up a responsive and comprehensive ambience sensitive level control system, which until now has been relegated to expensive external boxes or using our functional (yet limited) Gap Ambient Level Control algorithm.
We have provided some form of ambient sensing within MediaMatrix since the early pay-for-feature days of MWare 2.0 (circa 1996). However, this has always been limited to being able to measure the zone ambience during the (sometimes rare) periods of silence in the program material― hence the name Gap Ambient Level Sensor― meaning that the device only works during periods of time in which there is a gap in the program material.
With the introduction of the Ambient Level Control, you now have a powerful and cost-effective tool in your arsenal of MediaMatrix devices. It has the ability to "learn" the characteristics of the zone ambience by effectively comparing its expected output with the signal arriving at the sense mic. Measuring the expected output and comparing it with the sense mic input allows the device to arrive with a calculated ambient level without waiting for a gap in the source material.
Input Nodes
Program
This is where you connect the audio meant to be sent to the zone which will have its level adjusted depending on the detected ambience in that zone. This input signal is typically background music and/or paging audio. Any gain added or subtracted from this source starts from 0dB (unity gain) which is what you will get when the ALC (Ambient Level Control) is bypassed.
Speaker
This input is the return of the audio you would expect to hear out of the speaker. In other words, the audio going into this node should be a duplicate of what the system is delivering to the amplifier that drives the zone. The most likely place to return the audio from is just before the DSP Audio Output Block as illustrated in the diagram below.
Of course, when a device's output is connected into one of its inputs, a feedback loop is created. In the case of the ALC device, we need to tell the compiler to allow the feedback. To do this, it is necessary to use a Via device which allows feedback in the view file. The Via device is located in Miscellaneous\Other.
NOTE: The Speaker audio input is only used for measurement and will not pass audio through to the ALC's output node.
Sense
This input is where you connect the microphone used to sense ambience in the zone. In its simplest form, the microphone audio is not processed by any equalizers or filters before being fed into the ALC. However, in some advanced applications, this input may have additional processing.
NOTE: When properly calibrated, the ALC device will effectively ignore most program material coming into the microphone. Anomalies may occur as a result of having the sense mic too close to the speakers. The ALC will work best if the microphone is not in the direct or indirect path of any speakers.
Controls
There are two different groups of controls: Calibration/Monitoring and Gain Response. The Calibration and Monitoring Controls affect how the audio at the inputs of the device are measured, calculated and utilized. The Gain response controls affect how and when the ALC takes action on the results of the calculations.
Calibration and Monitoring Controls
Speaker Level Meter
RMS Level of Speaker audio input.
Sense Mic Meter
RMS Level of Sense audio input.
Calculated Ambient Level (RMS)
A Calculated RMS value of the ambient level of the zone. This calculation takes into account that the speakers may influence the sense mic level and modifies its calculations accordingly. This meter is for reference only and will not be accurate before the ALC device is properly calibrated.
Ambient RMS Time
RMS time constant for the level detectors. This is basically the window of opportunity the ALC will use to perform its calculations to derive the calculated ambient level. If this control is set to 1 second, for example, the ALC will only use events that occur during a 1 second period to derive the calculated ambient level. The smaller the value, the more responsive the system will be to changes. In an industrial or public transportation environment, the time should be short (somewhere in the area of 2 seconds). In other applications, such as casinos and dining rooms, the time should be longer (around 30 seconds). 10 seconds is the default.
Modes of Operation Block
Bypass Gain Updates
When engaged, the ALC provides unity gain (0dB) at the output.
NOTE: The gain level meter will continue to update the [projected] calculated gain of the circuit as a gauge for updates that will occur when the device is taken out of bypass.
Begin Learning
Used to calibrate the device for the system application. Please read the section below on the proper calibration procedure
Calculated Gain of Return Signal
This control is for reference only. The calculated value of gain from the output of the device (through the speakers) to the sense mic input. In other words, this value is the volume level of program material that the ALC hears coming into the sense mic. If the sense mic is in the direct field of the speaker, this value will, most likely, be high. As the distance between the speaker and sense mic increases, this value will decrease.
NOTE: This control will only update at the end of the calibration process and in the best case scenario will be a very low value.
Gain Response Controls
Attack
This control sets the reaction time for the gain circuit to make changes when the calculated ambient level rises above the ambient threshold level.
Release
This control sets the reaction time for the gain circuit to return from changes made when the calculated ambient level drops below the ambient threshold level.
Ambient Threshold Level
Sets the ambient level above which gain changes will occur. When ambient levels are below this threshold, the device gain is set to the minimum value. As the ambient level increases above this level, gain is added as determined by ratio and limited by the Max gain value.
Maximum Gain
This is the maximum gain that the ALC is allowed to add to the system when the calculated ambient level is above the Ambient Threshold Level. This should be set greater than Minimum Gain control.
Minimum Gain
Determines the gain of the ALC when the calculated ambient level is either at the ambient threshold or below. This control should always be set less than Maximum Gain.
Ratio
For every decibel that the Ambient Level exceeds the Threshold, gain is added. Minimum Gain sets the Base Value, while Ratio sets the decibel gain increase for each decibel increase in Ambient Level above the Threshold. Care must be taken to ensure that Ratio, Minimum Gain, and Maximum Gain are set to avoid clipping of any signal in the path from the device output to the Sense and Speaker inputs
Gain Level
The Gain Level meter's function is to inform the operator of the current gain of the ALC device, in dB. This meter will continue to update if the ALC device is bypassed, however, the actual output of the device will be unity, or 0dB gain.
Overview
Careful sense microphone placement is essential for optimum performance of the ALC device. Optimum microphone placement is achieved when the output of the zone's speakers do not leak back into the sense microphone. Obviously, in some facilities, this is not practical, so to deal with this real-world problem, the ALC must first go through a system calibration phase before it can accurately estimate the ambient level separately from the speaker leakage (returned through the sense mic).
Qualifications
Calibration should be performed after all speakers and microphones are installed and set at their final positions and settings. Moving a microphone or changing the level control on the power amp (or any device after the ALC in the circuit) will require a new calibration. Although it is not necessary to have a completely quiet room for calibration, a constant ambient level is required.
Steps
1. Verify the placement, working condition and gain staging of all microphones.
2. Place the ALC into Bypass and set the RMS rate to 3 seconds (increase this time for noisy environments).
3. Verify the placement of all microphones.
4. Gain stage the elements in the circuit properly, so that the output volume in the zone is at what would be considered a maximum level. The speaker and sense input levels should be in the range of 0 to -20dB. This allows good resolution and sufficient headroom for ambient increases. During calibration, constant program material or pink noise should be used as a program source.
5. Allow the speaker and sense RMS level indicators to settle.
6. Enter Learn Mode by pressing the Learn Mode button in the Modes of Operation Block.
7. Decrease the level of the source material at the program input of the ALC device to the proper listening level for the current ambience.
8. Allow the RMS levels to settle at this new level.
9. Exit Learn Mode by again pressing, or deselecting, the Learn Mode button.
At this time, Return Gain will be updated and Ambient Level should begin to reflect the true ambient level. If for some reason, calibration fails, a message will be printed to the Terminal window. Upon a successful calibration, you may now set the dynamics controls for the gain cell (in the Gain Updates section) without having to recalibrate the device.
Calibration information will be saved with the view file as default information (for each instance of the ALC device). Presets or SubPresets may also be used to save calibration and device settings.