This invention relates to tone detection in general and, more particularly, to independent hysteresis apparatus for monitoring call progress tones utilized in telephone systems.
In certain telephone systems, there is employed a technique called Call Progress Tone Monitoring which is useful for applications such as outcalling and supervised transfers. Call Progress Tone monitoring is employed where the disposition of a call is used to determine the next course of action in the telephone system. Thus in such a system, a machine mimics the human's ability to discriminate between different call progress tones by detecting their distinctive tones and/or cadences. As one can understand, in telephone systems such tones are widely employed. Most familiar common signalling and control signals are dial tone, ring back and busy tones. These signals and their frequencies as well as the periods of on and off are well established although they can vary from system to system in regard to frequency, cadence and so on. As indicated in the above-noted copending application, to differentiate between different tones, one can provide a time representation of the call progress tone by transforming the analog signal into a frequency domain representation. Due to the nature of such tones, it becomes difficult to discriminate tones which are extremely close in frequency. As one will understand, tones employed in telephone systems all are accommodated within a bandwidth from about 0 to 4,000 Hz. It is within this bandwidth that such tones are employed and hence many tones are relatively close together in frequency. In this manner they become extremely difficult to reliably detect. Furthermore, when one desires to detect such tone signals on a telephone line or on a communication channel, there is a problem of identifying the tone or signal in the presence of noise. As indicated, the various tones which are desired for detection may be separated by as little as 30 Hz. One requires expensive and complicated filters to discriminate in regard to such close frequency differences and conventional filters can mistake one tone for the other tone during different time periods. In this manner the invention to be described herein employs a hysteresis technique to essentially enable one to accurately identify a tone by eliminating interference due to noise while also serving to discriminate between tones which are very close in frequency.
As is also well known in basic tone detection or signalling tones as employed in telephony, one may have tones which consist of two or three frequencies. For example, in certain PBX systems as well as in other telephone systems, dial tone may be represented as two frequencies, mainly 350 Hz plus 440 Hz. A signal referred to as "ring back" is a telephony signal which occurs for two seconds on and four seconds off and may be the combination of 440 Hz plus 480 Hz. As one can ascertain, such tone detection systems must discriminate between these frequencies which as above indicated may also be extremely close together. If one refers to the above-noted copending application, further examples of suitable tones are given. Thus prior art techniques which employed, for example, filters, and so on suffered in the sense that the tones could not be discriminated against accurately in the presence of noise and often a tone which was close to another tone in frequency would be confused with each other.
In this manner there is described a hysteresis apparatus which smooths out the frequency detection process to assist in the identification of a particular tone. The hysteresis apparatus as described enables one to adjust the sensitivity of the system to the noise associated with individual tones even in the same application as in monitoring the same telephone line or communications path.