Note: Descriptions are shown in the official language in which they were submitted.
S848
This invention relates to the detection of predetermined
frequency components, or tones, in sampled signals.
It is commonly required in communication systems to detect
the presence of tones in received signals. For example, tone detection is
necessary in telephone communication systems using tone dialling and
multifrequency code (MFC) signalling. Frequently the signals in which
tones must be detected are sampled signals, for example they may be PCM
signals.
I~ is known to detect the presence of tones in sampled
signals using dfgital filters, one filter being provided for each tone
frequency which is to be detected. For example in "A Digital Signal
Processor for PCM/TDM Tone Decoding" by M.S. Buser, Proc. Euroncon 77,
Venice, Italy, May 1977, pages 2.5.5.1 to 2.5.5.6 there is described a
digital filter and discrete Fourier transform for MFC decoding, by means
of which 20 tone frequencies can be detected. In such a digital filter
PCM signal samples are delayed in a buffer line, and for each new sample
the variously delayed samples are read out from the buffer (for example
there may be 256 samples read out) and are supplied to arithmetic units
which serve to multiply the samples by sets of coefficients which are
stored in PROMs, the products then being summed and two resultant sums
being stored in a working memory. The arithmetic units also determine the
square root of the sum of the squares of these resultant sums. These
steps are performed for each frequency to be detected, for each PCM
channel, and for each sample. As a result, a relatively complicated,
large, and expensive arrangement is required in order to effect the
desired detection of tone frequencies.
,
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In order to provide a simpler method of tone detection than
is provided by digital filters, there has been proposed (A.D. Proudfoot,
"Simple Multifrequency-Tone Detector", Electronics Letters, Vol. 8 No. 21,
Oct. 19, 1972, pages 525 and 526) a digital correlator in which incoming
PCM signal samples are gated with two stored sequences of sign bits,
representing the sign of out-of-phase signals at the frequency to be
detected, and the resultant signals are accumulated in two accumulators
over a set correlation period. At the end of this period the larger
magnitude one of the accumulated signals is selected as an indication of
the presence or absence of the relevant frequency, and the accumulators
are reset. However, this arrangement suffers from a disadvantage in that
harmonics resulting from using the single bit sequences, which correspond
to square waves rather than preferred purely sinusoidal signals, can cause
problems because of aliasing. In addition, because of the accumulation
over the correlation period and subsequent resetting of the accumulators,
in order to detect tones which may occur at arbitrary times the
correlation period must be relatively short, for example 12 ms for
detecting tones of at least 40 ms duration. The selectivity of the
detector, which increases as the correlation period is increased, is
therefore relatively low.
In order to overcome the problem of aliasing, whilst still
using single bit correlation sequences, there is described in "A Digital
Receiver for Tone Detection Applications" by T.A.C.M. Claasen and J.B.H.
Peek, IEEE Transactions on Communications, Vol. COM-24, No. 12, December
1976, pages 1291 to 1300, a digital correlator in which signals to be
correlated are transformed into one-digit binary signals that have
approximately the same correlation function. The transformation is
114S84~3
effected by adding auxiliary signals to the signals to be correlated and
taking the sign of the sum signal. In order to enable detection of tones
occurring at arbitrary times, this article further describes that an
accumulator provided in the detector may be provided with some leakage
according to a predetermined factor, or may have its contents cyclically
reduced by a constant amount and be reset to zero if the result is
negative. Each of these proposals involves the disadvantage and
difficulty of selecting an appropriate factor or constant amount.
Accordingly, an object of this invention is to provide an
improved method of and apparatus for detecting the presence of a
predetermined frequency component in a sampled signal.
According to this invention there is provided a method of
detecting the presence of a predetermined frequency component in a signal
sampled with a sampling period t, comprising:- prcducing samples of said
signal delayed by a predetermined correlation period T; producing samples
~ with a sampling period t of first and second signal sequences representing
; respective signals at said predetermined frequency having a phase
difference other than 0 or ~; cumulatively producing for each signal
sample first and second accumulated signals, each of the first and second
; 20 accumulated signals being equal to the first or second, respectively,
accumulated signal produced for the previous signal sample modified in
dependence upon the product of the current sample of the first or second,
respectively, signal sequence with the difference between the current
signal sample and the current delayed signal sample; and deriving from the
first and second accumulated signals an output signal which represents the
presence or absence of the predetermined frequency component during the
preceding correlation period T. Preferably the first and second signal
il45~8
sequences represent signals in phase quadrature, i.e. sine and cosine
signals, at said predetermined frequency.
The cumulative production of the accumulated signals avoids
the need for periodic resetting of accumulators as in the prior art, and
provides for the production of the output signal at the same rate as the
sampled input signal so that tones occurring at arbitrary times can be
readily detected.
The invention also extends to apparatus for detecting a
predetermined frequency component in a sampled signal, comprising:- means
for delaying samples of the signal for a predetermined period; means for
producing samples of phase quadrature signals at the predetermined
frequency; means for producing, for each sample of the signal, the
products of a sample of each of the phase quadrature signals with the
difference between the current and delayed samples of the signal; means
for cumulatively summing each of said products; and means for deriving a
detection signal in respect of the predetermined frequency component from
the cumulative sums of said products.
The invention will be further understood from the following
description with reference to the accompanying drawings, in which:-
Fig. 1 illustrates a tone detector with reference to which
the principles of the invention are explained; and
Fig. 2 illustrates in greater detail a preferred form of
tone detector in accordance with the invention.
Referring to Fig. 1, there is shown therein a tone detector
in the form of a correlator comprising a delay unit 10, a subtracter 11,
sine and cosine coefficient generators 12 and 13, multipliers 14 and 15,
accumulators 16 and 17, and a root-sum-square circuit 18. The detector
~114S~
serves to detect the presence of a predetermined tone, or frequency
component, in a signal, sampled with a sampling period t, whose samples
are present on an input line 19, and to provide an output signal
representing the presence or absence of the tone on an output line 20.
The delay unit 10, which can comprise a RAM (random access
memory), serves to delay each signal sample on the line 19 by a period T
which is referred to as the correlation period. The period T is selected
to be a multiple of the sampling period t, for example T = 128 t so that
the correlation period extends over 128 signal samples. Preferably, for
selectivity, the period T is also such that ~T = n~/ where ~ is the
angular frequency of the tone to be detected and n is a positive integer,
so that the correlation period extends over a whole number of half-cycles
of a signal at the frequency to be detected. In order to avoid a
possibility of overflow of the accumulators 16 and 17 in the event that
the sampled input signal comprises a continuous sinusoid, preferably the
period T is such that ~T = 2n~, ~ and n having the same meanings as above.
The subtracter 11 is supplied with the input signal samples
on the line 19 and the delayed samples from the output of the delay unit
10, and produces the difference between these. This difference is
multiplied in the multipliers 14 and 15 by respective coefficients
supplied by the generators 12 and 13. Each of these generators 12 and 13
can comprise a ROM (read-only memory) in which the coefficients are stored
as digital signals. The coefficients produced by each generator are
samples of a sinusoidal signal at the frequency to be detected, the
samples having the sampling period t and extending over the correlation
period T. (Alternatively, the correlation period T can be an integral
multiple of the period over which the coefficient samples extend.) Thus
q l 5
~1~58~18
in the case where T = 128 t, each generator cyclically produces 128
coefficients during each period T. The signals represented by the
coefficients produced by the generators 12 and 13 differ in phase by other
than 0 or ~; preferably the signals are in phase quadrature and hence the
coefficients are referred to as sine and cosine coefficients.
The accumulators 16 and 17 cumulatively add the products
from the multipliers 14 and 15 to produce signals X and Y, and the
root-sum-square circuit 18 produces from these the output signal
proportional to ~x2 + y2~ It can be shown that this
signal is a representation of the presence or absence of the tone
frequency in the sampled signal on the line 19.
It can be seen that at any time the signal X is equal to the
sum of the products of the samples on the line 19 with the coefficients
from the generator 12 over the preceding correlation period T. By virtue
of the provision of the delay unit 10 and the subtractor 11, this signal X
is updated accurately in respect of each sample on the input line 19,
without requiring the accumulator 16 to be reset. Similar comments apply
in respect of the signal Y. Consequently, the output signal is a reliable
and accurate indication of the presence or absence of the relevant
frequency, and can be used to detect tones which occur at arbitrary times.
In practice, the signal samples on the line 19 will normally
be non-linearly (e.g. mu-law) encoded PCM signals. In order to
accommodate the non-linear encoding, a non-linear to linear decoder (not
shown) can be included in the input line 19, or the subtracter 11 may
incorporate means for producing the correct (not encoded) difference
signal from the non-linearly encoded signals with which it is supplied.
Alternatively, the components may be rearranged, as described below with
reference to Fig. 2, to accommodate appropriate decoding.
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Having described the principles of the present invention
with reference to Fig. 1, reference is now made to Fig. 2 which
illustrates a preferred form of tone detector. The tone detector of Fig.
2 comprises a delay RAM 21, sine and cosine coefficient PROMs
(programmable ROMs) 22 and 23, multiplier PROMs 24 to 27, subtracters 28
and 29, adders 30 and 31, latches 32 and 33, accumulator RAMs 34 and 35,
and further PROMs 36 to 38.
Eight-bit (sign bit and seven magnitude bits) mu law encoded
PCM input signal samples at a sampling frequency of 8 kHz (sampling period
t = 0.125 ms) are supplied via an input line 39 to the delay RAM 21, which
is cyclically addressed so that each sample is stored therein for a
correlation period of T = 16 ms (128 PCM frames) before being read out to
a line 40. The PROMs 22 and 23 store sine and cosine coefficients1 or
samples, of phase-quadrature signals at the tone frequency to be detected
over the period T = 16 ms, as already described with reference to Fig. 1,
and are likewise cyclically addressed to read out these coeffieients.
Each coefficient is in the form of a four-bit (sign bit and three
magnitude bits) digital signal.
The lines 39 and 40, and the outputs of the PROMs 22 and 23,
are connected to address inputs of the multiplier PROMs 24 to 27 as shown,
each of which is programmed to produce at its output the product,
converted into a linear code from the mu-law encoding of the PCM signal
samples, of the PCM sample and the coefficient supplied to its inputs.
Thus the current and delayed PCM signal samples are multiplied by the
current sine and cosine coefficients to produce four products. Each
product is in the form of an eight-bit (sign bit and seven magnitude bits)
digital signal.
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In the subtracter 28 ~he product of the sine coefficient
with the delayed PCM sample produced by the multiplier PROM 26 is
subtracted from the product of the sine coefficient with the current PCM
sample produced by the multiplier PROM 24, to produce a nine-bit (sign bit
and eight magnitude bits) digital signal which is equivalent to the signal
produced at the output of the multiplier 14 in Fig. 1. As in Fig. 1, this
signal is cumulatively added to produce the signal X. In the detector of
Fig. 2, the cumulative addition is effected by the adder 30 adding the
current signal supplied by the subtracter 28 to a sixteen bit (sign bit
and fifteen magnitude bits) previous sum which is read out from the
accumulator RAM 34, the new sum being latched in the latch 32 to
constitute the signal X and then being stored in the accumulator RAM 34 in
place of the previous sum. The signal Y is produced in an identical
manner from the outputs of the multiplier PROMs 25 and 27, using the
components 29, 31, 33, and 35.
The PROMs 36 to 38 correspond to the circuit 18 of Fig. 1.
Three individual PROMs are used in order to reduce the total memory
capacity required to handle the two signals X and Y, only the eight most
significant magnitude bits of each of which are used in producing the
output signal. The PROM 36 is programmed to producey from the eight
magnitude bits of the signal X with which it is addressed, a five-bit
digital signal A which is proportional to log X2. The PROM 37 is
similarly programmed to produce a five-bit digital signal B proportional
to log y2~ The PROM 38 is programmed to produce, from the two
five-bit signals A and B, the output signal proportional to log
(eA ~ eB), which signal is in fact dependent upon the sum
x2 + y2.
ll~S8~8
Although the tone detector shown in Fig. 2 has been
described in relation to the detection of a single tone frequency for a
single channel, it is in fact ideally suited to multiplexing for the
detection of a plurality of tone frequencies for a plurality of channels.
Thus for each of a plurality of tone frequencies, the PROMs 22 and 23 can
store a respective set of coefficients corresponding to the respective
frequencies, and the accumulator RAMs 34 and 35 can store respective
cumulative sums all over the same correlation period T. The output
signal, multiplexed in time for the different frequencies, can be
subsequently demultiplexed for detection of particular tone frequency
combinations which may be used for example in MFC signalling. The
arrangement may be further multiplexed for a plurality of PCM channels
whose signal samples are time division multiplexed on the input line 39.
In this case, the delay RAM 21 and the accumulator RA~,s 34 and 35 can
provide storage locations for the samples and cumulative sums,
respectively, of each of the channels. Again the output signal is
multiplexed for the various channels. A multiplexed tone detector as
described above with reference to Fig. 2 can for example be conveniently
used in MFC signalling for detection of 16 tones, or frequency components,
2C on each of 24 PCM channels.
As already described, it is preferred that ~T = n~, or more
preferably ~T = 2n~, where is the angular frequency to be detected and n
is an integer. For MFC signalling in which the tone detector is used in
the multiplexed manner described above for detection of a plurality of
tone frequencies, it is preferable to select the different tone
frequencies so that this relationship is satisfied for each of them.
However, it is not essential to the operation of the tone detector that
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1145848
this relationship be satisfied, especially when the correlation period T
is very large in relation to the period of the frequency to be detected.
As already described, the cumulative accumulation provided
in the tone detectors of the present invention avoids the need found in
the prior art to reset an accumulator periodically. However, in the
present tone detectors it is obviously necessary on initially commencing
operation to clear the accumulator. In addition, there is a risk that
the accumulated values X and Y may be affected by errors in the operation
of the various memories; this is particularly so if dynamic memories are
used. Accordingly, it is desirable to reset the accumulated values X and
Y to zero occasionally to ensure that any errors which may be present do
not accumulate. This is most conveniently achieved by arranging for the
values X and Y to be reset to zero a predetermined period after the end of
the detection of a tone frequency.
As decribed above, each of the coefficients produced by the
PROMs 22 and 23 has four bits, but a different number of bits may be used.
Preferably, however, each coefficient has from two to four bits.
Providing only one-bit coefficients can cause problems because of
aliasing, as already describ~d with reference to the prior art, and there
is no significant benefit in the overall tone detector performance from
using more than four bits.
Numerous other modifications, variations, and adaptations to
the described embodiments of the invention may be made without departing
from the scope of the invention as defined in the claims.
