Bosch Appliances 3.5 Automatic volume control, 49.1Introduction, 49.2Sensing microphones, Type

Praesideo 3.5 Installation and User Instructions 9 System Configuration

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49 Automatic volume control

49.1Introduction

Automatic volume control (AVC) improves the intelligibility of calls and the audibility of BGM in noisy environments. It changes the call volume in a zone to compensate for ambient noise. The ambient noise is measured by sensing microphones that can be connected to any of the audio inputs on the power amplifiers. Although it is possible to connect the ambient sensing microphone to other inputs of the system, only the power amplifier inputs provide microphone supervision. Furthermore, the other inputs have a built-in audio limiter that will interact with the AVC, when the signal levels are relatively high, such as when coming from an electret microphone..

49.2Sensing microphones

49.2.1 Introduction

When AVC is enabled in a zone, a sensing microphone continuously measures the ambient noise. The system uses an averaging filter to derive the average ambient noise level from the signal of the sensing microphone.

Note

Between the start of a call and the end of a call, the AVC function does not process the signal of the sensing microphone.

49.2.2 Type

Use sensing microphones with high-quality cables, because sensing microphones are often installed in locations that are hard to reach. It is not required to use high-quality microphones. The choice for a directional or omni-directional microphone depends on the local situation.

49.2.3 Supervision

The connections between the system and the sensing microphones can be supervised (see section 43.3.3). When the sensing microphone of a zone fails, all calls in the zone are distributed at the maximum volume to comply with evacuation standards. When the sensing microphone of a zone fails, the attenuation of BGM is

set to AVC range (see figure 49.1). To prevent undesired supervision faults, make sure that:

•The supply current for condenser microphones is between 0.5 mA and 5 mA.

•The microphone impedance of dynamic microphones is between 120 Ω and 1300 Ω.

49.2.4 Installation

To make sure that the sensing microphone measures the correct ambient noise level, install it in a place which has a typical ambient noise level. Furthermore:

•Keep the cables of the sensing microphones and high-power cables (e.g. mains cables) separated. Interference can disturb the ambient noise measurement.

•Do not install the sensing microphone near an expected noise source. A loud and very local noise can disturb the ambient noise measurement.

•Do not install the sensing microphone near an air vent or air conditioning. Airflows can disturb the ambient noise measurement.

•Do not attach the sensing microphone to any part of a building. The vibrations of buildings can disturb the ambient noise measurement.

49.3Connection

Sensing microphones can be connected to the unit that contains the audio outputs of the AVC zone or to another amplifier. When the sensing microphone is connected to another amplifier and it is used in combination with an AVC zone, it permanently occupies a digital channel. As a result, the number of digital audio channels that are available for calls decreases.

Note

Spare amplifiers cannot take over the sensing microphone from a failing main amplifier when the signal of the sensing microphone is not routed over the optical network.

49.4Configuration

Before configuring AVC in a zone:

•Make sure that the ambient sensing microphone is correctly connected to the system.

Bosch Security Systems 2011-02