MULTI-TONE SIGNAL GENERATOR

Abstract: A multi-tone signal generator and tone generating method for producing frequency combinations such as those used in dual-tone telephone dialing systems. A phase accumulator tone generator which in one embodiment comprises programmable divider (18) an accumulator (20), a ROM (22) and a latch (30) so interconnected as to generate samples of one of the tones to be encoded, at a rate much lower than would normally be required by the Nyquist criterion for the highest frequency desired. The output rate is specifically chosen so that a distortion product appears at the frequency of the second tone to be generated. The resulting samples are applied to a D/A converter (34) and then passed through a low pass filter (36).

Description: MULTI-TONE SIGNAL GENERATOR Background of the Invention 1. Field of the Invention This invention relates generally to the elec¬ tronic tone generating art and in particular to an improved method for generating a multi-tone signal. 2. Description of the Prior Art Multi-tone systems are presently in wide use in communication systems. For example, multi-tone telephone dialing systems employ a three by four switching matrix for selecting a composite audio signal comprised of one of four lower band frequencies and one of three upper band frequencies. Typically, these seven frequencies are substantially sinusoidal wave forms generated by hand wired conventional analog audio oscillators. These systems are subject to becoming detuned by shock or vibration and performance can vary with environmental factors such as temperature and humidity. In addition, these systems are relatively expensive to fabricate and require precision circuit elements. Some of these problems have been circumvented by digitally synthesiz¬ ing, either in discrete logic or in a microprocessor, the multi-tone signals and then combining them. This however is expensive because of the complexity and the processing load required for generating the tones individually. Although it has been recognized that reduction of the processing load for synthesis in these systems would permit reduced expense and the use of smaller processing units no effective means for such reduction has heretofore been devised. Summary of the Invention It is an object of this invention, therefore, to provide an improved multi-tone generating method that utilizes digital techniques with reduced processing requirements and reduced susceptibility to vibrations. It is another object of this invention to provide an improved multi-tone generator which is particularly adapted for implementation in a microproccessor and which greatly reduces processing requirements. It is yet another object of this invention to provide an improved multi-tone generator which is programmable. - Briefly, according to the invention, a multi-tone signal generator is provided for producing a signal composed of a plurality of desired frequencies. The generator comprises generating circuitry for generating a seσuence of digital words, each word corresponding to the magnitude of a sample of a substantially periodic wave form at one of the desired frequencies. The generating means is adapted to generate the sequence of digital words at a sample rate less than the Nyquist rate for the highest desired frequency such that the frequency components generated include all of the desired frequencies. A D/A converter is coupled to the generating means for converting the sequence of digital words to an analog waveform. In addition, a filter is coupled to the converter for filtering the analog OΪ ΓI waveform to eliminate the undesired frequency components generated. Brief Description of the Drawings The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description when taken in conjunction with the accompanying drawings. Figure 1 is a block diagram illustrating one embodiment of the inventive multi-tone generator according to the invention. Figure 2 is a block diagram illustrating a micro computer implementation of the preferred embodiment of the invention. Figure 3 is an overall program flow diagram of the computer program for the invention. Detailed Description of the Preferred Embodiment The principal of operation of the preferred embodiment of the invention is to generate two tones of desired frequencies f- j and f2 by generating digital samples of a sinusoidal waveform of one of the frequencies at a sample rate less than the Nyquist rate for the highest desired frequency. It is widely known in the art that to properly represent some band limited analog waveform digitally, the samples must occur at twice the highest frequency to be represented (i.e., the Nyquist rate). For the two tone generator, a phase accumulator frequency synthesizer is utilized to generate digital samples of the higher frequency, f2, but at a sample rate well under two times f2« If the sample rate is chosen to be f s = f2 + £ _ , then strong M spectral components are generated at the desired frequencies of f2 and f-j . In addition, many higher frequency components are generated, which are filtered out by means of a low pass filter. The result is generation of a signal composed of the two desired frequencies, while utilizing a lower sample rate and only a single synthesizer, thereby reducing speed requirements and complexity. Referring to Fig. 1, there is shown a block diagram of a discrete circuit embodiment of the inventive multi-tone generator. An input device 10 is provided to permit selection of the desired frequencies (designated f1 , and f2)« Many common input devices can be used, such as a set of binary switches, a multi-tone telephone key pad or even a voice input device. In the latter examples, conventional decoding circuitry would be required. The data from this input device determines the frequencies f-| and f2 generated, and is applied to the data bus 12. Part of the data is coupled to a latch 16, and the remainder of the data is coupled to a programmable divider 18, as shown. In addition, an enable pulse is coupled from the input device 10 to the enable input 14 of the latch 16 when new data is entered. The latch 16 and the divider 18 together with a 16 bit accumulator 20 and a read only memory (ROM) 22, configured as shown, form a phase accumulator sine wave synthesizer. The data is composed of a phase increment which is coupled to the latch 16 and stored in response to the enable pulse, and programming data which is coupled to the programmable divider 18. The phase increment is a value equal to f2/f≤ wn ich is stored in latch 16 when coupled from the input device 10. Clock pulses from a clock 24 are coupled, as shown, to the programmable divider 18. The divider 18 is programmed by the data from the input device 10 to produce output O ?l pulses at the terminal 26 having the desired sample frequency f s . These pulses are coupled, as shown, to the clock input 28 of the accumulator 20, and to the enable input 32 of a latch 30. Each pulse causes the phase increme ' nt stored in the latch 16 to be added to the contents of the accumulator 20. The eight most significant bits of the accumulator 20 are then coupled, as shown, to the address input of the ROM 22 which contains a table of amplitude samples of a sinusoidal waveform. The amplitude addressed by the contents of the accumulator 20 is coupled from the ROM 22 to the latch 30 where it is stored in response to the enable pulse coupled to the enable .input 32 from the divider 26, as shown. The sample stored in the latch 30 is coupled to a D/A converter 34 of conventional design and converted to analog form. The result is an analog waveform, generated at a sample rate f s = f-j + f2 which contains the frequency components f-j and f2/ as well as many higher frequency components. This analog waveform is coupled through a low pass filter 36 to remove the higher _frequency components, thereby resulting in a multi-tone signal of frequencies f-| and f2» Additional filtering may be used to provide pre-emphasiε, if desired. It should be rioted that signals of any number of tones can be generated using this method. For example, the next higher frequency generated by the generator of Fig. 1 is f s + f- j . Thus, if three frequencies are desired, the sample frequency f s and the frequencies f * and f2 would be chosen to generate the desired three frequencies f -_ , 2 and f s + f- j . Then the low pass filter would be chosen to filter out frequencies above f s + f-j, leaving the desired three tones. Figure 2 illustrates the preferred microprocessor implementation of the invention. .An input device 110, similar to that described hereinbefore, is utilized to select the desired tone frequencies f-] and f2- A data word and an input interrupt are coupled from the input device 110 to the CPU 120. A micro computer, such as a MOSTEK 3870, is preferred to implement the CPU 120. The CPU 120 determines and outputs the digital samples required to generate the desired multi-tone signal, as described in detail hereinafter. The output digital samples are then coupled, as shown, to a conventional D/A converter 130 which converts the digital samples to an analog waveform. The analog waveform is then coupled to an optional pre-emphasis filter 140 which high pass filters the waveform to provide pre-emphasis, if desired. The analog waveform is then low-pass filtered by a filter 150 to remove undesired high frequency components. The result is a multi-tone output signal of the desired frequencies. Referring to Fig. 3, there is shown a program flow diagram of the computer program for the microcomputer of Fig. 2 to implement the present invention as illustrated in Fig. 2. Operation of the program sequence begins by initializing values as indicated at 200 and by setting the counter variable CNTR1 to the number of input data words expected (i.e., 7 or 10 for a telephone system). The program then waits for an " input interrupt at 203 from the input device 110 and in response to an interrupt reads a data word from the input device 110 (see Fig. 2) and stores the data in memory, as shown at 204. The counter CNTR1 is then decremented, as shown at 206 and tested to determine if it is zero, as indicated at 208. If CNTR1 is not zero, the program flow proceeds to block 210, and waits for an input interrupt from the input device 110. In response to the interrupt the program flow returns to block 204, as shown, to read the next input data word. If, however, CNTR1 is equal to zero, this indicates that a full set of input data words has been read and the program proceeds to block 212 where the counter CNTR2 is set to the number of input data words stored. Program flow then proceeds to block 214 where, based on the first input data word, the required sample frequency f s , the phase increment f2/ f s an< -* the one duration variable time counter TMCNTR are obtained from a table stored in memory, and the sample frequency is then used to set the timer interupt to occur at a rate equal to 1/f s . The phase increment f2/fs -* s then added to the accumulator as shown at block 216, and the eight most significant bits of the sum in the accumulator are then used to get the proper amplitude sample from a table of sinusoidal waveform amplitude values, thus implementing a phase accumulator synthesizer, as indicated at 218. At this point, the sample is output to the D/A converter 130 (see Fig. 2), as shown at 220. Program flow proceeds to block 222 where TMCNTR is decremented and then to block 224 where TMCNTR is tested to determine if it is equal to zero. If it is not zero, indicating that the time duration of the tone is not over, program flow proceeds to block 226 and waits for the timer interrupt. When the timer interrupt occurs, the program proceeds to block 216 and generates the next sample. If TMCNTR is zero at 224, then the counter CNTR2 is decremented, as indicated at 228, and the program proceeds to test CNTR2 to determine if it is equal to zero at block 230. If CNTR2 is not equal to zero, indicating that all the multi-tone signals have not been generated " , program control returns to block 214, and the generation of the next tone pair begins. If CNTR2 is zero, then the program control returns to block 200 where the program is initialized to be ready for the next data input. In summary, a multi-tone generator suitable for use in a telephone dialing system has been described. The generator is particularly suitable for implementation in Oϊ?I a microcomputer and provides programability, reduced running time, and reduced complexity. While a preferred embodiment of the invention has been described and shown, it should be understood that other variations and modifications may be implemented. It is therefore contemplated to cover by the present application any and all modifications and variations that fall within the true spirit and scope of the basic underlined principles disclosed and claimed herein. What is claimed is: Q? ?I

Claims: Claims 1. A multi-tone signal generator for producing a signal composed of a plurality of desired frequencies, said generator comprising: generating means, for generating a sequence of digital words, each word corresponding to the magnitude of a sample of a substantially periodic waveform at one of the desired frequencies, said generating means adapted to generate the sequence of digital words at a sample rate less than the Nyquist rate for the highest desired frequency such that the frequency components generated include all the desired frequencies; converter means, coupled to the generating means, for converting the sequence of digital words to an analog waveform. 2. The multi-tone signal generator of claim 1, further comprising filter means, coupled to the converter means, for filtering the analog waveform to eliminate undesired frequency components. 3. The multi-tone signal generator of Claim 2 wherein the generating means comprises a phase accumulator sine wave synthesizer. 4. Multi-tone signal generator of Claim 2 or 3 for generating a dual frequency tone of frequencies f- | and f2, wherein the generating means is adapted to generate the sequence of digital words at a sample rate of approximately f-| + 2 - 5. The multi-tone signal generator of Claim 4 further comprising clock means, for generating sampling pulses at a rate of f-j + f2, and wherein the generating means is responsive to the clock pulses such that one digital word is generated for each clock pulse. 6. The multi-tone signal generator of Claim 5, wherein the generating means and the clock means are programmable and further comprising input means for programming the generating means and the clock means to the desired frequencies f-j and f2- 7. The multi-tone signal generator of Claim 6 further comprising pre-emphasis filter means, coupled from the converter means to the filter means, for providing pre-emphasis filtering of the multi-tone analog waveform. 8. Method of generating a multi-tone signal composed of a plurality of desired sinusoidal frequencies, comprising the steps of: a) generating digitized samples of one of the desired sinusoidal frequencies at a sample rate less than the Nyquist rate for the highest desired frequency such that the frequency components generated include all the desired frequencies; b) converting the digitized samples to an analog waveform. OMPI 9. The method of claim 8 further comprising the step of filtering the analog waveform to eliminate undesired frequency components. 10. The method of claim 9 for generating a dual frequency signal of frequencies f- j and f2 wherein the step of generating digitized samples is performed at a rate of f-j + f2-

Inventors: LABEDZ GERALD PAUL, LABEDZ, GERALD, PAUL
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1982
Main classification: H03B19
IPC codes: G06F1, H03B19, H04M1, H04Q1