GB2066005A - A tone generator for producing a tone signal from a digital signal - Google Patents

Abstract

A keyboard KS programmes dividers PD1, PD2 to select two tone frequencies to be generated, counters IC1, IC2 each addressing a ROM to provide a bit fob each count to an integrator I which effects delta modulation decoding of each of the two bit streams to produce the two tones. Each bit stream has a respective resistive input R1 or R2 into the integrator so that the integrator time constant for each stream is different. As a result, the low pass filter effect of the integrator does not favour the lower of the two tone signal frequencies to give different amplitudes to the tone signals: instead, these tone signals have substantially the same amplitude at an output terminal OT. <IMAGE>

Description

SPECIFICATION Signal tone generators This invention relates to signal tone generators.

Traditional methods of signal tone generation employ techniques which are essentially analogue in nature, the frequency of oscillation being determined by electrical networks involving resistance, capacitance and inductance. The level of oscillation is determined either by the limits of performance of an active device of an oscillator, or by a level dependent gain control feedback loop.

The distortion produced by such means is not always small and low pass filtering may be necessary to achieve a sufficiently low harmonic content for signalling purposes.

Thus, an analogue approach to signal tone generation may suffer from at least some of the following drawbacks, viz: high cost and volume of frequency determining components and components of a low pass filter; difficulty of assembling all active components into a single integrated circuit in order to reduce cost; and difficulty of predicting long term stability of frequency and amplitude of the signal tones, and long term effects of spurious signals.

More recently, the exansion of techniques which enable digital signals to be processed with relative ease, low cost and precision, has led to proposals of signal tone generation involving a combination of digital and analogue techniques.

In a known type of signal tone generator involving such a combination, the period of a signal tone waveform to be generated is divided into a number of equal increments of time, and the voltage level of each increment is assumed constant throughout the increment and is approximated by a fixed level which is chosen for the increment in accordance with its position along the waveform.This is illustrated in the accompanying figure 1, which shows for a sine wave SW on a graph of voltage V against time T, an approximation sine wavu ASW composed of increments al, a2 ... a18 which are produced in respect of successive 200 portions of the approximation sine wave ASW and have respective voltage levels V between 1 .OV to O to -1 .0V in dependence on the slope and polarity of the corresponding portion of the sine wave SW.

The difficulty with this known type of signal tone generator is that is order to obtain a low spurious signal content in the generated waveform, the number of increments required is large, and the precision with which each level is produced has to be high. This presents a sever problem for an integrated circuit implementation of the signal tone generator since the achievement of such accuracy "on chip" is difficult and "off chip" production utilises a large number of reference resistors which require a large number of pin connections.

It is an object of the present invention to provide an improved signal tone generator involving a combination of digital and analogue techniques.

According to the present invention there is provided a signal tone generator comprising, means for determining the period of a signal tone waveform to be generated, counter means responsive to the output from said means for determining the position of each of a number of increments of time along the waveform, coding means responsive to the output from the counter means to produce a digital signal having in respect of each increment either a binary value '1' or a binary value '0' in accordance with a predetermined pattern, and integrator means responsive to the digital signal to produce an output signal which is an approximation of the required signal tone waveform.

In a signal tone generator according to the invention the application of the digital signal to the integrator means results in what is, in effect, delta modulation decoding of this digital signal to produce the output signal. This affords the advantage that a more accurate approximation of the required signal tone waveform is produced compared with that produced by the known type of signal tone generator previously referred to.

In carrying out the invention the number of delta modulation slopes can be increased beyond the two (positive and negative) slopes which the binary '1' and binary '0' values of the digital signal provide. Thus, the coding means can be arranged to produce a tri-state output by open-circulating its output terminal, at which the digital signal appears, in respect of selected increments, thereby providing positive, negative and zero delta modulation slopes.

Alternatively, or additionally, the coding means can be arranged to have more than one resistive input, of different values, into the integrator means so that the time constant of the latter and thus the delta modulation slope, is different for each resistive input. The use of a zero delta modulation slope affords the advantage that a better approximation of the required signal time waveform is produced. The use of more than one integrator slope to increase the number of delta modulation slopes affords the advantage that when signal tones of different frequencies are to be produced, the low pass filter effect of the integrator means can be compensated for so that tones of different frequencies have output levels within permissable limits.

It has also been found that with a signal tone generator according to the invention the levels of spurious output signals are within acceptable limits.

In order that the invention may be more fully understood reference will now be made by way of example to the accompanying drawings, of which: Figure 1 shows, as aforesaid, a graph of an approximation sine wave as produced by a known type of signal tone generator; Figure 2 shows a graph of an approximation sine wave as produced by a signal tone generator according to the invention; Figure 3 shows a block diagram of a signal tone generator according to the invention; Figure 4 illustrates how the digital signal pattern for the coding means of the signal tone generator of Figure 3 can be derived; Figure 5 shows a basic circuit for the integrator means of the signal tone generator of Figure 3; and Figure 6 shows a basic output circuit for the coding means of the signal tone generator of Figure 3.

Referring to the drawings, in Figure 2a there is shown a sine wave SW on a graph of voltage V against time T, as is also shown in Figure 1. An approximation sine wave ASW' is superimposed on the sine wave SW. Uniike the approximation sine wave ASW in Figure 1 which is composed entirely of stepped increment levels, the approximation sine wave ASW' more nearly follows the shape of the sine wave SW. As will be described, this approximation sine wave ASW' is generated as the output voltage from a simple analogue integrator in response to a digital signal applied to the integrator, this digital signal having a predetermined pattern of binary '1' and binary '0' values and being represented by the digital signal DS shown in Figure 2b.If thr zero axis 0 of the sine wave SW corresponds to half the binary '1' 'level of the digital signal DS, then upon application of this digital signal DS to an integrator which has a suitable time constant relative to the bit rate of the digital signal DS, it can be seen that the approximation sine wave ASW' will be generated as the output voltage from the integrator by what is, in effect, delta modulation decoding of the digital signal DS. Thus, a succession of binary '1' values produce the rising slope of the sine wave, and a succession of binary '0' values produce the falling slope of the sine wave. The peaks of the sine wave are approximated by alternate binary '1' and binary '0' values.

The signal tone generator shown in Figure 3 as a block diagram is an example of a type of multitone generator for generating simultaneously mout-of-n tones, where rn > 1 and n > m. In the present instance the generator is a 2 out of 8 tone generator, and an envisaged application is as a dialling tone generator employed in telephone signalling. This signal tone generator comprises a master oscillator (or clock source) MO which feeds two programmable frequency dividers PDl and PD2.The frequency divider PD1 is provided in respect of a low frequency group comprising four possible tone frequencies f1 to f4, and the frequency divider PD2 is provided in respect of a high frequency group comprising four possible tone frequencies f5 to 8. Under the control ofa keyboard selector KS, control logic CL determines for each of the frequency dividers PD1 and PD2 which one of the particular four tone frequencies in the relevant frequency groups (low or high) it is to select. The two frequency dividers PD1 and PD2 feed respective increment counters IC1 and IC2.For each cycle of the particular tone frequency applied to it, each of the increment counters IC1 and IC2 provides a number of increment signals pertaining to successive increments of time along the period of the cycle.

The number of increments per cycle which is chosen is governed inter-alia by the tone frequency to be produced. For example, for a tone frequency of 1 KHz, 168 increments per cycle have been found to be suitable. The increment signals from the two increment counters IC1 and IC2 are applied to respective digital coders DC1 and DC2 which comprises respective read-only memories ROM 1 and ROM 2 and respective output circuits OC1 and OC2. In response to the successive increment signals applied to it, each of the read oniy memories ROM 1 amd ROM2 produces a digital signal output in accordance with a predetermined pattern of binary '1' and binary 'O' values such as represented by the digital signal DS in Figure 2b.

Figure 4 illustrates how the predetermined pattern of binary '1' and binary '0' values which each read-only memory is to produce may be derived. If Y1 is the approximated value of a wave function F at time tri, then at time t2 the approximated value may be either Y2 or Y3. If the actual value of the wave function F at time t2 is A, then if A lies above Y 1, Y2 is the closer approximation: conversely, if A lies below Y1, Y3 is the closer approximation. An approximate binary value '1' or 'O' is entered into the read-only memory for time t2 and the approximated waveform is updated to Y2 ore3, as the case may be and the procsss repeated for the increment between time t2 and time t3, and so on.Since both tone frequencies to be produced are sine waves, the tone read-only memories ROM 1 and ROM2 can be identical, provided the same number of increments per cycle is chosen for each group of tone frequencies.

In response to the digital signal outputs from the respective read-only memories ROM 1 and ROM2, the two outputs from the respective readonly memories ROM 1 and ROM2, the two output circuits OC1 and OC2 feed an integrator I. The action of the integrator I is, in effect delta modulation decoding of the two digital signals applied to it, with the result that its output voltage at an output terminal OT contains two approximation sine waves, each such as represented by ASW' in Figure 2a, which form two generated signal tones.

For the aforementioned example of a tone frequency of 1 KHz using 168 increments per cycle, the integrator I may comprise a simple R--C combination comprising an 1 8Kw resistor and a 47nF capacitor. However, the voltage output level of such an integrator I will fall by several db per octave. Therefore, RC values which are chosen for a tone frequency of 1 KHz, which may be one of the tone frequencies of the high frequency group, may also be suitable for other tone frequencies of the same group, but not for tone frequencies of the low frequency group. In other words, the low pass filter action of the integrator will exert less attenuation on the lower frequencies than on the higher frequencies.

In order to make the voltage output level of the integrator I more nearly the same for all the tone frequencies in both frequency groups, each of the output circuits OC1 and OC2 has its own resistive input into the integrator I, thereby providing separate different delta modulation slopes for the two frequency groups. Thus, as illustrated by the waveform ASW'/2 in Figure 2a, a reduced delta modulation slope for approximating a sine wave SW/2 which is half the frequency of the sine wave SW gives the waveform ASW'/2 and an amplitude level commensurate with that of the approximation sine wave ASW'. If the same delta modulation slope were used for ASW'/2 as for ASW', then the former wave would have much greater amplitude than the latter wave.Figure 5 which shows a basic circuit for the integrator I, comprising a first resistive input having a resistor RI to which the output circuit OC1 is connected and a second resistive input having a resistor R2 to which the output circuit OC2 is connected. The other circuit components are a capacitor C1 and a further resistor R3. The values of the resistors Ri and R2 can be chosen to give a difference in output voltage level of, say, 2db for two frequencies in the high and low frequency groups. The value of the combination Cl, R3 is suitably chosen to give a flat frequency response to the high frequency edge of the high frequency group. This requires that R3 R1 or R2.In order to reduce the level of spurious signal products at the output of the integrator I, a simple R-C filter comprised by a resistor R4 and a capacitor C2, as shown, may be provided at the output side of the integrator I.

The level of spurious signal products at the output of the integrator I can be further reduced by increasing the number of delta modulation slopes used. In this respect, each of the output circuits OC1 and OC2 could be provided with a separate resistive input into the integrator I for each tone frequency of the relevant frequency group.

Additionally, or alternatively, each of the output circuits OC1 and OC2 can be arranged to have tristate outputs so as to provide a zero delta modulation slope as well as positive and negative delta modulation slopes. This is illustrated in figure 6 which shows an output circuit OC having a 3position selector switch SW which is shown as a mechanical switch in the drawing but which, in practice, would be implemented as an electronic switch. In response to the output from the relevant read-only memory at an input l/OC, which can be a multi-lead input, the switch SW assumes one of its three possible positions (+ve, earth or opencircuit) in accordance with the sine wave increment being dealt with.The result is that at an output O/OC of the output circuit OC, the digital signal (DS) which is produced now has opencircuit interruptions in it in selected bit periods.

The effect of this on the approximated sine wave which is produced at the output of the integrator is to flatten this wave at its peaks, as indicated in dotted lines on wave ASW' in Figure 2a, and thereby improve its shape by rendering it a closer approximation to a sine wave.

For an integrated circuit implementation of the tone generator the use of a tri-state output circuit has the advantage that the neither the number of components of the external circuit nor the number of pins required on the i.c. package is increased.

Other modifications are also possible. For instance, the number of increments of time produced by the counter means need not be equal, but instead can be made of variable length, for example by continuously programming the frequency dividers in accordance with a predetermined algorithm to change the frequency and thus the period of the waveform to which the counter means is responsive. Also, complex predetermined patterns of binary values '1's and 'O's can be produced by the coding means in response to a succession of increment signals to build-up the required analogue waveform. For example, in this latter respect sufficient increments may be produced along the period of the waveform for the digital signal to cause the production of an approximation sine wave having both rising and falling slopes, as well as its peaks, made up from a succession of small triangular portions.

Claims (5)

1. A signal tone generator comprising, means for determining the period of a signal tone waveform to be generated, counter means responsive to the output from said means for determining the position of each of a number of increments of time, along the waveform, coding means responsive to the output from the counter means to produce a digital signal having in respect of each increment either a binary value '1' or a binary value '0' in accordance with a predetermined pattern, and integrator means responsive to the digital signal to produce by delta modulator decoding an output signal which is an approximation of the required signal tone waveform.

2. A signal tone generator as claimed in Claim 1, wherein the coding means produces a tri-state output by open-circuiting its output terminal, at which the digital signal appears, in respect of selected increments, thereby providing positive, negative and zero delta modulation slopes.

3. A signal tone generator as claimed in Claim 1 or Claim 2, wherein the coding means has more than one resistive input, of different values, into the integrator means so that the time constant of the latter, and thus the delta modulation slope, is different for each resistive input.

4. A signal tone generator as claimed in Claim 3, arranged as a multi-tone generator for generating simultaneously m-out-of-n tones where m > 1 and n > m, the coding means having m resistive inputs, one in respect of each of the m tones, into the integrator means, each input being of a respective different resistive value appropriate for the appertaining tone such that in response to respective digital signals from the coding means the tones of different frequencies in the output signal have respective levels within given limits with respect to each other.

5. A signal tone generator substantially as herein before described with reference to the accompanying drawings.