Browse Prior Art Database

Improved MFR Detection

IP.com Disclosure Number: IPCOM000122373D
Original Publication Date: 1991-Dec-01
Included in the Prior Art Database: 2005-Apr-04
Document File: 2 page(s) / 42K

Publishing Venue

IBM

Related People

Arnaud, C: AUTHOR [+3]

Abstract

Dual-tone multi-frequency (DTMF) signalling is based on the simultaneous transmission of two frequencies picked up in two groups: . 697, 770, 852, 941 Hz (Group 1) . 1209, 1336, 1477, 1633 Hz (Group 2)

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Improved MFR Detection

      Dual-tone multi-frequency (DTMF) signalling is based on
the simultaneous transmission of two frequencies picked up in two
groups:
      .    697, 770, 852, 941 Hz (Group 1)
      .    1209, 1336, 1477, 1633 Hz (Group 2)

      Multi-frequency receiver (MFR) algorithms must operate within a
wide range of variations in the characteristics of the DTMF signals,
such as signal-to-noise (S/N) ratio as low as 20 dB, twist ratio
(relative amplitude of the two frequency components) -8 to +8 dB,
input level as low as -35 dBm0, frequency shift of -1.5% to +1.5%
from nominal frequencies, and tone duration as short as 45 ms.

      The device shown in the figure is aimed to improved the
detection in case of high twist factors. It makes an automatic
equalization of the input signal prior to DTMF detection, based on an
estimation of the twist ratio.

      The estimation process uses two band-pass filters BP1 and BP2
to filter the signal within each group band. The signal at the output
of each band-pass filter is squared and accumulated so as to measure
the energies E1 and E2 within each group band. Then, those energies
are compared to eventually select an equalization filter EQZ1 or
EQZ2:
     .    If E1 > 2.E2, then equalize using EQZ1: high-pass filter
with 6 dB emphasis on Group 2.
     .    If E1 > E2/2, then equalize using filter EQZ2: low-pass
filter with 6 dB emphasis on Group 1.
     .   Else, no equ...