Distributed fire alarm systems which incorporate a plurality of ambient condition detectors, such as smoke, heat or gas detectors, are often installed in business or commercial buildings. Such systems often have a common control unit which can be in either unidirectional or bidirectional communication with multiple, spatially separated, ambient condition detectors.
One of the problems associated with transmission of information to or from such detectors is the presence of uncorrelated noise. Noise is uncorrelated wherein it is not related to a selected parameter or parameters which is/are being monitored.
In the event that the parameter being monitored is a level of ambient smoke, an ambient temperature, or a level of an ambient gas, the signals of interest are those which have a high correlation to the particular ambient condition being detected. Other signals, due to electrical or thermal noise which are not correlated to the ambient condition being detected, and which may in fact be random, are undesirable. Various techniques have been used in the past to minimize the effects of such uncorrelated noise signals.
One known type processing or filtering involves sampling the signals from at least one of the ambient condition detectors and calculating a running average based on a predetermined number of prior sample values, such as 6 or 8 or 10, along with the latest sample value. As each new sample value is received, the running average is updated. This technique provides a vehicle for minimizing or suppressing the effects of uncorrelated noise. This process can also be carried out continuously using analog circuits.
Filters can be implemented using analog or digital hardware. Alternately, they can be implemented digitally in software. One such system is described in U.S. Pat. No. 5,612,674 entitled High Sensitivity Apparatus and Method With Dynamic Adjustment for Noise assigned to the Assignee hereof and incorporated by reference herein.
While known approaches do provide a vehicle for suppressing or reducing uncorrelated noise in signals from ambient condition detectors, they also introduce delays. In the event that the parameter of interest, such as level of smoke or ambient temperature, does start to increase, the increases are attenuated and only appear in the output filtered signals after a delay interval which is characteristic of the type of averaging or filtering which is used.
As disclosed and claimed in the parent hereto, hereby incorporated by reference, a smoothing or filtering function can be altered in the presence of a fire indicating profile. It would be desirable to be able to start to adjust the degree of smoothing or filtering even before a fire indicating profile can be detected.