US 6,608,896 B2 · Efficient digital ITU-compliant zero-buffering DTMF detection using the non-uniform discrete Fourier transform

Abstract

Computationally efficient DTMF detection methods and apparatus are presented that meet all of the ITU recommendations using the modified non-uniform DFT. The system of the present invention employs a high band filter block and two low band filter blocks to detect power at the 8 DTMF tones. The frame length of the high band filter blocks is one half the length of the low band filter blocks. The frame lengths are chosen to meet the ITU frequency selectivity requirements for all DTMF frequencies. The frames of the two low band filter blocks are staggered to produce outputs alternately, and are aligned with respect to the frame of the high band filter block to produce low band filter block outputs that coincided with the high band filter block outputs without the need for signal buffering. A system of power level tests are employed in conjunction with a system of timing tests to ensure that all ITU timing and frequency constraints are met. The present invention requires no buffering of input samples, and can perform DTMF decoding of 24 telephone channels of a T1 time-division multiplexed communication line, using a single fixed-point commercially available digital signal processor.

Claims19

Dependent claims

A method for detection of DTMF symbols in a signal, comprising the steps of:

determining the power in the signal at each high band DTMF frequency in each frame of a first succession of frames each of duration, L1;
determining the power in the signal at each low band DTMF frequency in each frame of a second and third succession of frames each of duration, L2, equals an integer multiple of L1;
wherein the end of each frame of the second succession of frames coincides with the end of a frame of the first succession of frames and is offset from the end of a frame of the third succession of frames by the duration, L1;
and
analyzing the power determined at each high band DTMF frequency and at each low band DTMF frequency to determine whether a DTMF symbol has been received.

2 more elements

A system, comprising:

a set of high band filters, wherein each high band filter operates in a frequency band encompassing a different one of the four high band DTMF frequencies;
each high band filter having a temporal frame length L1 selected to achieve sufficient frequency selectivity:
to accept signal energy in a frequency band that is within ±1.5% of the nominal high band DTMF frequency about which the filter operates;
and
to substantially reject signal energy outside a frequency band that is within ±3.5% of the nominal high band DTMF frequency about which the filter operates;
a first set of low band filters, wherein each low band filter in the first set operates in a frequency band encompassing a different one of the four low band DTMF frequencies;
each low band filter in the first set having a temporal frame length L2 selected to achieve sufficient frequency selectivity:
to accept signal energy in a frequency band that is within ±1.5% of the nominal high band DTMF frequency about which the filter operates;
and
to substantially reject signal energy outside a frequency band that is within ±3.5% of the nominal high band DTMF frequency about which the filter operates;
and
a second set of low band filters, wherein each low band filter in the second set operates in a frequency band encompassing a different one of the four low band DTMF frequencies;
each low band filter in the second set having a temporal frame length L2 selected to achieve sufficient frequency selectivity:
to accept signal energy in a frequency band that is within ±1.5% of the nominal high band DTMF frequency about which the filter operates;
and
to substantially reject signal energy outside a frequency band that is within ±3.5% of the nominal high band DTMF frequency about which the filter operates;
wherein L2 is an integer multiple of L1 and wherein the frames of duration L1 and L2 are temporally aligned such that an output of each low band filter will substantially coincide with the output of a high band filter;
and processing circuitry for:
determining the power in the signal at each high band DTMF frequency in each frame of a first succession of frames each of duration, L1;
determining the power in the signal at each high band DTMF frequency in each frame of a second and third succession of frames each of duration, L2, equals an integer multiple of L1;
and
analyzing if a DTMF symbol has been received.

19 more elements

Dependencies are inferred from the claim text.

Efficient digital ITU-compliant zero-buffering DTMF detection using the non-uniform discrete Fourier transform

Life of this patent

Efficient digital ITU-compliant zero-buffering DTMF detection using the non-uniform discrete Fourier transform

Life of this patent

Abstract

Claims19