Multifrequency tone detector

A signal recognition circuit (10) is disclosed which is capable of distinguishing between the digit tones used for dialing in multifrequency tone telephone sets, and spurious signals at the same frequencies, such as noise and voice. The discriminator comprises, in cascade, an AGC amplifier (11), an adjustable threshold comparator (12), and a signal validation circuit (13). The operation of the discriminator is based upon the fact that the true dialing tones have envelopes of constant magntiude whereas the spurious signals tend to have pronounced envelope variations. By the appropriate adjustment of the comparator threshold, these variations show up as gaps in the comparator output signal, which are readily detected by the validation circuit.

TECHNICAL FIELD 
This application relates to multifrequency tone receivers and, in 
particular, to a signal discriminator circuit for use in such receivers. 
BACKGROUND OF THE INVENTION 
The use of frequency encoded, decimal digit dialing in the telephone system 
requires a receiving circuit that is capable of detecting the tone signals 
originating at the subscriber's telephone. Such a receiver typically 
includes a tone detector capable of meeting two essential requirements. On 
one hand, it must be sufficiently sensitive to recognize the tone signals. 
On the other hand, it must be capable of discriminating between spurious 
signals, such as noise and speech, and valid dialing tones. These detector 
characteristics represent, to a large extent, conflicting requirements 
and, hence, any practical solution must involve a compromise between the 
ideal fulfillment of each. One approach, as described in an article by G. 
T. Kraemer and L. C. J. Roscoe, entitled "The Family of Touch-Tone 
Receivers," published in the 1965 July/August issue of the Bell 
Laboratories Record, Vol. 43, No. 7, pp. 282-287, employs the so-called 
"limiter guard action," whose operation is based upon the fact that an 
amplitude-limited signal contains a fixed amount of energy. Thus, an 
amplitude-limited spurious signal, containing many frequency components, 
will not contain sufficient energy at any one frequency to exceed a 
predetermined "critical threshold." A valid dial tone, on the other hand, 
has all its energy concentrated at the frequency of interest and, hence, 
readily exceeds the critical threshold. To increase the discriminator 
sensitivity, feedback to the limiter input is also employed. The net 
effect, however, is to limit the dynamic range of the receiver. 
In an alternate approach, a separate voice filter and detector are provided 
which serve to disable the receiver whenever the voice channel signal 
exceeds a predetermined level. If made too sensitive, however, receiver 
noise can cause the voice channel to disable the receiver in the presence 
of a valid digit tone signal. 
SUMMARY OF THE INVENTION 
The above-described limitations are avoided in a discriminator whose 
operation relies upon the fact that spurious digit-simulating signals 
typically have envelope variations that are not present in a valid digit 
tone signal. Accordingly, a multifrequency tone discriminator circuit, in 
accordance with the present invention, includes, in cascade, an AGC 
amplifier whose output signal envelope exceeds a predetermined level for a 
given dynamic range of input signals; a comparator circuit for comparing 
the amplifier output signal with a reference signal and for producing an 
output signal only when the envelope of the input signal to said 
comparator exceeds said predetermined level; and means for examining the 
comparator output signal for selected characteristics. Because of the 
envelope variations present in a spurious signal, the output from the tone 
discriminator circuit produced by a spurious signal is readily 
distinguishable from that of a valid digit tone signal.

DETAILED DESCRIPTION 
Referring to the drawings, FIG. 1 shows the basic elements of a digit tone 
discriminator in accordance with the present invention. Typically, the 
discriminator 10 comprises, in cascade, an AGC amplifier 11, a comparator 
12, and a frequency detector and validation circuit 13. 
The operation of the discriminator is based upon the recognition that the 
envelope of a true digit tone has a relatively constant amplitude. By 
contrast, voice and other spurious signals have pronounced envelope 
variations. Discriminator 10 is, accordingly, specifically designed to 
detect these variations. Consider, for example, a true digit tone. When 
received, it has an amplitude which falls within a specified range of 
amplitudes. AGC amplifier 11 is designed to produce an output signal which 
exceeds a predetermined level V for input signals within this dynamic 
range. FIG. 2 shows an amplifier output signal whose envelope, as can be 
seen, exceeds the predetermined level V over its entire duration. 
This signal is, in turn, coupled to comparator 12 along with a reference 
signal V, wherein they are compared, and an output signal produced 
whenever the input signal exceeds the reference signal. The resulting 
output signal from comparator 12 is shown in FIG. 3. It will be noted that 
inasmuch as the input signal to the comparator exceeds the reference level 
V once each cycle, the resulting output signal is a train of equally 
spaced, uniform pulses 30. The frequency and other characteristics of 
pulse train 30 are examined in the frequency detector and validation 
circuit 13, whose output indicates reception of a true digit tone. If, 
however, a spurious signal is applied to the discriminator circuit, a 
different result obtains. Consider, for example, the signal illustrated in 
FIG. 4. As can be seen, because of the envelope variations, the output 
signal from amplifier 11 does not exceed the threshold V each cycle. 
Consequently, the resulting output signal from the comparator, as 
illustrated in FIG. 5, is a pulse train 50 characterized by unequal spaces 
between pulses, and by pulses of unequal duration. These distinguishing 
features of such a pulse train are readily recognized by the detector and 
validation circuit as a spurious signal. Consequently, no output is 
produced by the validation circuit 13. An example of a frequency detector 
and validation circuit that can be employed to practice the invention is 
described in U.S. Pat. No. 3,537,001. 
FIG. 6 shows, in block diagram, a multifrequency receiver 60 incorporating 
the present invention. The receiver typically includes a dial tone 
rejection filter 61 for rejecting the dial tone and band-limiting any 
incoming noise. The receiver circuit then divides into two branches 50, 51 
for separately processing the high group digit tones and low group digit 
tones, respectively. In the illustrative embodiment, branch 50 includes, 
in cascade, a high group bandpass filter 62, an AGC amplifier 63, and a 
comparator 64. Similarly, branch 51 includes, in cascade, a low group 
bandpass filter 65, an AGC amplifier 66, and a comparator 67. The outputs 
from the two comparators are coupled to a frequency detector and 
validation circuit 68. 
The operation of each branch of receiver 60 is as explained hereinabove. 
It will be recognized that the receiver configuration shown in FIG. 6 is 
merely illustrative of such receivers. Depending upon the particular 
system, it may be adequate to employ the invention in only one of the 
receiver branches. Some systems may benefit by the inclusion of an 
additional AGC amplifier following the dial tone filter. Thus, it is clear 
that the discriminator circuit disclosed herein can just as readily be 
used in a variety of receiver configurations wherever signal 
discrimination between constant signal tones and variable amplitude tone 
is called for.