Dual tone multi-frequency detector

DETAILED DESCRIPTION What is claimed is: 1.
An apparatus for identifying a first tone and a second tone present in a signal, the first tone being selected from a group of tones located within a low band and the second tone being selected from a second group of tones located within a high band, the low band and the high band each spanning a different frequency range and each having a width no greater then f.
sub.
0, the apparatus comprising: a first filter receiving said signal, said first filter for isolating and downconverting the portion of said signal falling within the low band so that it lies within a low frequency range extending form -f.
sub.
0 to f.
sub.
0 wherein the first filter comprises a first plurality of decimation filter stages arranged in cascade; and a second filter reseiving said signal, said second filter for isolating and dowconverting the portion of said signal falling within the high band so that it lies within the low frequency range, wherein the second filter comprises a second plurality of decimation filter stages arranged in cascade.
2.
The apparatus of claim 1 further comprising a tone detector stage for detecting the presence of the first tone by detecting the downconverted first tone from the first filter and for detecting the presence of the second tone by detecting the downconverted second tone from the second filter.
3.
The apparatus of claim 1 wherein each stage of the cascade of said first plurality of decimation filter stages decimates by a factor of two.
4.
The apparatus of claim 1 wherein each stage of the cascade of said second plurality of decimation filter stages share at least one decimation filter in common.
5.
The apparatus of claim 1 wherein the first and second plurality of decimation filter stages share at least one decimation in common.
6.
The apparatus of claim 1 wherein the decimation filters are recursive decimation filters.
7.
The apparatus of claim 1 wherein each of the decimation filters comprises at least one all-pass section.
8.
The apparatus of claim 7 wherein some of the decimation filters comprise two all-pass sections