Ambient Level Compensation — Crown PIP-USP4

Introduction

Crown provides a solution where the USP4 can compensate for the ambient noise in the zone powered by that amplifier channel. The USP4 detects "gaps" in the program sources and mutes the amplifier. While the amplifier is muted, an analog relay on the USP4 routes the amplifier output signals to the USP4's DSP engine. It takes at least five seconds to make this measurement. During this time the loudspeaker is acting like a microphone. The ambient noise in the room moves the loudspeaker diaphragm which results in movement of the voice coil. The voice coil movements generate a proportional voltage back to the amplifier output. This property allows all the loudspeakers in a large room to work together like a microphone array and a more accurate picture of the overall ambient level can be obtained.

The ambient level is then used to add or subtract gain from the amplifier. The net result of all these efforts is that as the room gets louder gain will be added, likewise as the room gets quieter, gain will be reduced.

Control Overview

The controls for the Ambient Leveler are :

The exact controls displayed will vary by what mode the module is in.

Enable

Turns on/off the ambient leveler for that channel.

Status

The indicator turns on when the ambient leveler is sensing during a gap and adjusting the output level.

Average Output Voltage (Sense)

Meter shows the sensed ambient level measured through the speaker.

Ambient Sense Threshold

Sets the level of the sensed ambient level above which the leveler begins to adjust the program material.  This value is typically set to be equal to the "nominal" average output voltage meter while in testing mode.

Program Gap Threshold

This setting defines what constitutes a gap. When the Average Output Voltage is below this setting, the amplifier turns off and the ambient level is sensed.

Gain Meter

Identifies the amount of ambient level gain being added or subtracted.

Expansion Ratio

This control is used to determine how large of a change is made. It specifies how many dB of gain are added or subtracted for each dB the sense level is above or below, respectively, the sense threshold. Increasing above the default of 1.5 may improve program material intelligibility in the presence of high ambient levels. The larger the value, the faster the changes will take place.

Max/Min Gain

Use these to limit the amount of gain that may be added or subtracted based upon sensed ambient level.

Link

When enabled, the settings for Channel 1 are applied to Channel 2 of the USP4. The display of the available controls on Channel 2 will change.

Host/Client Modes

A host-client scheme has been implemented on the USP4. When the host wants to trigger  the clients to sense the ambient level, the host USP4 will broadcast a message. All devices on the network will receive this message. If the device is a USP4, has the ambient leveler enabled and it is in client mode, then it will cause the amplifier to mute off and begin to sense the ambient level. The purpose of the host/client mode is to tie multiple USP4 and amplifiers together so that all the USP4s will sense the ambient level at the same time.

The ambient leveler has 3 modes of operation.

  1. Standalone Mode: Host force messages are ignored. This would typically be used if the amplifier covers a "zone" all by itself.
  2. Host Mode: The host controls the "force" for itself and all clients. This is typically used when multiple amplifiers are required to cover one "zone". This allows all multiple amplifiers to mute and sense the ambient level together.
  3. Client Mode: The USP4 responds to the host force message as long as the ambient leveler is enabled. A client is not allowed to force a ambient level sense, it must wait for the host. Typically, the host and client devices receive the same audio input source material.

Notes:

Gaps

There are two different methods used to specify a gap when the USP4 will automatically sense the ambient level in the zone. During testing it can be helpful to force the module into measurement mode. The momentary Force button manually forces the module to make a measurement.  The Force button is disabled when in client mode. These methods apply  to the three different modes outlined above.

  1. By Time
    This approach is typically used when program gaps can not be detected reliably, such as a constant background music source with cross fades and no gaps between songs. The Gap Force Period is specified between 0 and 60 minutes. Using its internal timer, the USP4 module will create a gap by muting the audio at the interval specified. Setting it to 0 is the same as turning off the Gaps by Time feature.

  2. By Program Material
    The ambient level processing block will monitor the output level of the Input router. Whenever the signal falls below the Program Gap Threshold, the amplifier will be muted and the ambient level will be sensed. During this condition, the amplifier will remain muted the ambient level will continue to be measured as long as the level stays below the threshold.

Sense Delay

The amount of time required to accurately detect the ambient level of any space is highly dependent upon the reverberations present. The sense delay is used to compensate for the reverberation in the zone being monitored. It is specified in seconds and has a range of 1 second to 60 seconds. When a gap is forced or detected the amplifier is muted, at this point the Duration Timer starts running. The Duration Time is created by adding the sense delay and the five seconds of measurement. When the force duration timer expires, the sensing is disabled and the amplifier is unnmuted.

Additional Information

If you are connected to the Internet you can find additional information, including  video overviews at:
http://www.crownaudio.com/tutorials/