GB2198568A

GB2198568A - Current tone detection

Info

Publication number: GB2198568A
Application number: GB08629695A
Authority: GB
Inventors: Michael Rottenberg
Original assignee: Motorola Israel Ltd
Current assignee: Motorola Solutions Israel Ltd
Priority date: 1986-12-12
Filing date: 1986-12-12
Publication date: 1988-06-15
Also published as: GB8629695D0

Abstract

A system for concurrently detecting tones periodically samples the amplitude of received tones, selectively accumulates the samples, and concurrently detects substantially monotonic accumulations of the selected samples, whereby substantially monotonic sample accumulations indicate tone detection. <IMAGE>

Description

Concurrent Digital Tone Detection Field of Invention This invention relates to detection of signalling tones. More particularly, this invention relates to concurrent digital detection and demodulation of one of a set of signalling tones.

Background of the Invention Tone signalling is a common method of coding information for transmission. Tone signalling is extensively used in the data modem industry for coding and transmitting binary, digital information. Tone signalling is also widely used in the two-way communications industry for coded signalling and control. Tone detectors have, however, proven troublesome. Concurrent detection of one in a set of tones has been suggested by using high speed multipliers to implement a fast fourier transform. However, general purpose microprocessors rarely facilitate the high speed multiplication required for real time concurrent tone detection. As more microprocessors are incorporated into more receiving apparatus, the desirability of efficient digital tone detection techniques increases.However, the speed of digital detection has, in the past, put a limit on the depth and breadth to which a digital signalling system could be economically employed.

A digital tone detector is described by Labedz in U.S.

Patent No.4,302,817 (S/N121,563), European Patent No.0045801 (W081/02353), assigned to Motorola. His digital tone detector is utilized to detect and demodulate tone coded squelch in two-way radio communications. In his detector, a single tone is detected when the number of properly combined sign-samples exceeds a predetermined threshold. The problem associated with the general applicability of this tone detector is that it only detects one tone at a time. His sign-sample accumulators must accumulate over a certain period of time before the detection of any single tone can be ascertained. Then, his accumulators must be cleared before they can attempt to detect the next in a set of expected tones. In other words, it performs sequential, rather than concurrent, digital tone detection.This sequential tone detection, which is intimately related to the microprocessor's speed, constrains the breadth of the set of tones that can be detected as well as constraining the rate at which symbols can be serially communicated. A binary set of two tones (mark and space) at relatively low baud rates is about all that can be economically decoded with these detectors.

It is, therefore, one object of the invention to provide concurrent digital tone detection.

Summary of the Invention According to the present invention, there is provided a system for concurrently detecting tones by periodically sampling the amplitude of received tones, selectively accumulating the samples, and concurrently detecting substanially monotonic accumulations of the selected samples, whereby substantially monotonic sample accumulations indicate tone detection.

Brief Description of the Drawings An exemplary tone detection system in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a concurrent digital tone detector according to the present invention.

Figure 2 is a representation of selective nonharmonic sample accumulations according to the present invention.

Figure 3 is a representation of selective harmonic sample accumulations according to the present invention.

Detailed Description of the Invention It can be shown that the cumulative absolute amplitude of a periodic wave coherently sampled is a monotonically increasing function. All other sample accumulations will be nonmonotonic functions. This property can be advantageously exploited for concurrent digital tone detection.

If a set of signalling tones is selected such that each tone (Fm) is an integral submultiple of the sampling frequency (Fm=Fs/m, m=integer) and no tone is a submultiple of any other (i.e., all tones are nonharmonic), then any tone in the set can be detected or demodulated by looking for that one having a monotonically increasing sum.

Figure 1 is a block diagram of a concurrent digital tone detector according to the present invention. The detector includes a switch (S) for sampling a received tone (U), an anolog-to-digital converter (A/D) for converting the sampled amplitude of the received tone (U) to a digital word (Unn), a memory (MEM) for storing the amplitude samples (Unn), and a microprocessor (uP) for storing the samples (Unn), selectively accumulating the samples (Fig. 2), detecting monotonic accumulations, and providing overall timing.

In operation, the received tone (U) is periodically sampled (S) at a frequency (Fs) that is an integral multiple of each of the tone frequencies (Fm) in the designated set (Fm=Fs/m, m=integer). The absolute amplitude of each sample (S) is converted (A/D) to a digital word (Unn) and sequentially stored in memory (MEM).

Once a sufficient number of samples (Unn) have been stored, the microprocessor (uP) can selectively accumulate the samples (Unn) to look for a monotonically accumulating sum.

Figure 2 is a representation of selective rionharmonic sample accumulations according to the present invention.

What has been shown are several illustrative selective accumulations for frequency F4 and F7 from the set of nonharmonic tones F4=Fs/4, F5=Fs/5, F6=Fs/6, and F7=Fs/7 (F8=Fs/8 would be disallowed as a harmonic of F4).

Every 4th, 5th, 6th, and 7th sample is accumulated (here, every 4th and 7th is shown) to determine whether samples are accumulating steadily (monotonically) over time. Various accumulations can be compared at various instants in time to ascertain whether samples are monotonically accumulating.

Alternatively, the accumulated samples over a fixed interval can be compared against a theoretically calculated reference standard. As additional confirmation of tone detection, the sampling instant can be altered (e.g., A70 to A72) by maintaining the spacing between memory locations (i.e., every 7th location) and merely indexing from a different memory location (i.e., U2). Altering the sampling instant makes the detection coherent, resulting in the fastest sample accumulation.

The instant invention, in the preferred embodiment, accomplishes concurrent detection of nonharmonic tones.

However, the invention can be extended to concurrently detect harmonic tones, as well.

In this extended embodiment, every other sample accumulation of F8 is subtracted to differentiate between harmonic frequencies F4 and F8. Thus, one harmonic accumulation (e.g., either A80 or A40) will be non-zero and increasing monotonically. That monotonic harmonic accumulation indicates a corresponding detected tone.

Thus, it has been demonstrated that when individual samples are stored discretely and when samples are nondestructively, selectively accumulated upon demand, tone detection of an entire set of tones can be performed concurrently on the same set of samples. There is no need, as in the prior art, to clear dedicated accumulators associated with a single frequency before detection of the next frequency may begin. All frequencies in a defined set can be detected concurrently, without multiplication.

This tone detector and demodulator is suitable for any form of coherent tone detection, including (M)FSK/PSK, two-way coded communications signalling and data modem tones. Because tone detection can be done concurrently, multi-level (q'nary) signalling can be economically demodulated in conventional microprocessors. While the preferred embodiment of the invention has been described and shown, it will be understood by those skilled in the art that other variations and modifications of this invention may be implemented.

Claims

1. A system for concurrently detecting tones characterized by: periodically sampling the amplitude of received tones, selectively accumulating the samples, and concurrently detecting substanially monotonic accumulations of the selected samples, whereby substantially monotonic sample accumulations indicate tone detection.

2. A system for concurrently detecting tones characterized by: means for periodically sampling the amplitude of received tones, means for selectively accumulating the samples, and means for concurrently detecting substanially monotonic accumulations of the selected samples, whereby substantially monotonic sample accumulations indicate tone detection.

3. A system as claimed in Claims 1 or 2 wherein the tones are from the the set of tones Fm=Fs/m, where m is an integer, and wherein the sampling is further characterized by: sampling the amplitude of received tones at frequency Fs.

4. A system as claimed in Claims 1 or 2 wherein the selective accumulation is further characterized by: predeterminely accumulating some samples and not others.

5. A system as claimed in Claims 1 or 2 wherein the accumulating is further characterized by: accumulating every m-th sample.

6. A system as claimed in Claims 1 or 2 wherein the detecting is further characterized by: concurrently comparing various accumulations over time to detect substanially monotonic accumulations of the samples between the various time instants.

7. A system as claimed in Claims 1 or 2 wherein the detecting is further characterized by: comparing accumulations against a predetermined reference to detect substanially monotonic accumulations of the samples.

8. A system as claimed in Claims 1 or 2 wherein the accumulating and detecting are further characterized by: accumulating, at various instants in time, every m-th sample, and concurrently comparing the various accumulations to detect substanially monotonic accumulations of the samples between the various time instants.

9. A system for concurrently detecting tones in the.

set of tones Fm=Fs/m, where m is an integer, characterized by: sampling the amplitude of received tones at frequency Fs, accumulating, at various instants in time, every m-th sample, and concurrently comparing the various accumulations to detect substanially monotonic accumulations of the samples between the various time instants, whereby substantially monotonic sample accumulations indicate tone detection.

10. A system for concurrently detecting tones in the set of tones Fm=Fs/m, where m is an integer, characterized by: means for sampling the amplitude of received tones at frequency Fs, means for accumulating, at various instants in time, every m-th sample, and means for concurrently comparing the various accumulations to detect substanially monotonic accumulations of the samples between the various time instants, whereby substantially monotonic sample accumulations indicate tone detection.

11. A system of concurrent digital tone detection substantially as shown and described herein.