Digital detector for a multi-frequency code in an analog signal

Claim 1

An apparatus for digitally detecting one or more of a plurality of fixed frequencies which are present in an incoming signal according to the correlation method, the fixed frequencies occurring in a first group of lower frequencies and in a second group of higher frequencies, said apparatus comprising: (a) means (2) for sampling the incoming analog signal of said fixed frequencies, (b) a reference memory means (5) having separate registers in which are stored sine reference code number pulses and a cosine reference code number pulses for a whole number of periods of each of the frequencies to be detected, the lower group of frequencies having six code number pulses per period which pulses have the values of +1, +1, 0, -1, -1, 0, and in the higher group of frequencies having two pulses per period, which pulses have the values of +1 and -1, (c) means (4) for the multiplication of the samples of said incoming signals with said reference code number pulses, (d) means (6) for adding the resulting multiplication products, (e) means (7) for comparing and selecting the highest valued multiplied product as corresponding to the frequency to be detected, and (f) a clock pulse and control circuit (3) means for controlling all of the above mentioned means including sampling the analog input signal at frequency the value of which is equal to a product of the number of code number pulses in said reference memory means and the frequency which is the highest common factor of the frequencies to be detected, and reading out said reference memory means said sine-cosine values at said sampling frequency.

Claim 2

An apparatus for digitally detecting one or more of a plurality of fixed frequencies which are present in an incoming signal according to the correlation method, the fixed frequencies occurring in a first group of lower frequencies and in a second group of higher frequencies, said apparatus comprising: (a) means (2) for sampling the incoming analog signal of said fixed frequencies, (b) a reference memory means (5) having separate registers in which are stored sine reference code number pulses and a cosine reference code number pulses for a whole number of periods of each of the frequencies to be detected, the lower group of frequencies having six code number pulses per period which pulses have the values of +1, +1, 0, -1, -1, 0, and in the higher group of frequencies having two pulses per period, which pulses have the values of +1 and -1, (c) means (4) for the multiplication of the samples of said incoming signals with said reference code number pulses, (d) means (6) for adding the resulting multiplication products, (e) means (7) for comparing and selecting the highest valued multiplied product as corresponding to the frequency to be detected, and (f) a clock pulse and control circuit (3) means for controlling all of the above mentioned means including sampling the analog input signal at frequency the value of which is equal to a product of the number of code number pulses in said reference memory means and the frequency which is the highest common factor of the frequencies to be detected, and reading out said reference memory means said sine-cosine values at said sampling frequency.