US 4,395,595 AGranted

Digital pushbutton signalling receiver

AssigneeNippon Electric Co., Ltd.

InventorsTakao Nishitani, Tadaharu Kato

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Abstract

A digital pushbutton (PB) signalling receiver having reduced sampling frequency for reducing the operations required by digital filters. The PB receiver is responsive to an in-band audio signal digitized at a conventional sampling frequency (e.g., 8 KHz) and detects two PB frequencies, one from a lower group and one from a higher group of frequencies. The input signal is successively digitally filtered and sampled, with each sampling frequency being reduced by 1/2 from the preceeding sampling frequency. This results in two digitized outputs, both of which are much lower sampled digitized signals than the input, and each of which contains information corresponding to said lower and higher groups of frequencies, respectively. The two outputs are then applied to two banks of frequency detectors, comprising digital band-pass filters and energy calculating circuits, for providing an indication of the presence of the lower and higher group of frequencies in the original input signal.

A digital pushbutton signalling receiver responsive to an input pushbutton (PB) telephone signal digitized at a first sampling frequency and containing two frequency components, one component consisting of one frequency signal selected out of a predetermined plurality of a lower group of frequencies and a second component consisting of one frequency signal selected out of a predetermined plurality of a higher group of frequencies, comprising:

(a) first means responsive to said input signal for successively digitally filtering and resampling said input signal at selected filtering bands and successively reducing sampling frequencies by 1/2 to produce a first means output digital signal containing all of said lower group of frequencies frequency transposed to first corresponding lower group of frequencies;
(b) second means responsive to said input signal for successively digitally filtering and resampling said input signal at selected filtering bands and successively reducing sampling frequencies by 1/2 to produce a second means output digital signal containing all of said higher group of frequencies frequency transposed to second corresponding lower group of frequencies;
(c) a first group of digital band pass filters, each centered at different ones of said frequencies of said first corresponding lower group of frequencies, connected to receive said first means output digital signal;
(d) first energy calculating means for calculating the energy content of said first means output digital signal and the outputs from each of said first group of digital band pass filters;
(e) first comparison means responsive to the outputs from said first energy calculating means for providing an output indicating the presence and identity of any of said predetermined plurality of lower frequencies in said input in-band audio frequency signal;
(f) a second group of digital band pass filters, each centered at different ones of said frequencies of said second corresponding lower group of frequencies, connected to receive said second means output digital signal;
(g) second energy calculating means for calculating the energy content of said second means output digital signal and the outputs from each of said second group of digital band pass filters, and
(h) second comparison means responsive to the outputs from said second energy calculating means for providing an output indicating the presence and identity of any of said predetermined plurality of higher frequencies in said input-in-band audio frequency signal.

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Abstract

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