JPH10327222A - Dtmf signal generating circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make distortion in a dual tone multi-frequency DTMF signal low and to reduce the cost of the generating circuit. SOLUTION: Each digital sine wave signal generating circuit generates digital sine wave signals obtained by quantizing a sine wave signal by each bit value for each sampling period respectively corresponding to two low/high group frequencies. In this case, a frequency division counter 21 sets a sampling frequency where a change in each sampling period of the digital sine wave signal is selected to be unit bits or below, the changes in the unit bits are recorded on a table in a table storage means 23 in advance, the change is read for each sampling period, an absolute value of the digital sine wave signal is decoded by an up-down counter 24 and a phase detection circuit 25 and the result is outputted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DTMF signal generation circuit, and more particularly to a DTMF signal generation circuit for digitally generating and synthesizing a sine wave signal having two frequencies of a high frequency group and a low frequency group from a reference clock signal. .

[0002]

2. Description of the Related Art Conventionally, a DTMF signal generation circuit of this type uses two frequencies of a high frequency group and a low frequency group of a voice frequency band as a selectable signal that can be recognized from a terminal to an exchange in a voice transmission line of a telephone system. Combined DTMF (du
It is widely used to generate and transmit al tone multi-frequency signals.

FIG. 4 is a block diagram showing a sine-wave composite signal generating circuit disclosed in Japanese Patent Laid-Open No. 61-79308, for example, as a configuration example of this conventional DTMF signal generating circuit. This conventional DTMF signal generation circuit includes counters 41a and 41b which divide the frequency of each clock signal into division ratios 1 / M and 1 / N and output 1 / M frequency signals and 1 / N frequency signals, respectively. , An address pointer 42a and a ROM 43a for converting a 1 / M frequency signal into a 1 / M digital sine wave signal, and
Pointer 4 for converting to / N digital sine wave signal
2b and ROM 43b, an addition circuit 4 for adding the 1 / M digital sine wave signal and the 1 / N digital sine wave signal
4 and D for converting the added digital value into an analog signal
/ A converter 45.

First, each of the clock signals ck1 and ck2 is
The signals are input to counters 41a and 41b having frequency division ratios of 1 / M and 1 / N, respectively, and are converted into 1 / M frequency signals and 1 / N frequency signals respectively corresponding to two frequencies of a high group and a low group of an audio frequency band. Is done. Converted 1 / N frequency signal, 1
/ M frequency signals are respectively assigned to address pointers 42a,
It is output to 42b and counts up the count value. The count outputs of the address pointers 42a and 42b are output to the ROMs 43a and 43b, and are stored in advance by dividing the sine wave signal into 16 to 32 equal parts with respect to the time axis.
Indicates the addresses of the OMs 43a and 43b. ROM 43a, 43
b successively outputs the data stored at the addresses indicated by the address pointers 42a and 42b.

In this manner, the two ROMs 43a, 4
The digital sine wave signal output from 3b is
The signal is added at 4 and synthesized into a signal combining two frequencies. The synthesized signal is converted into an analog signal by the D / A converter 45, and is transmitted as a selection signal from the terminal to the exchange as a DTMF signal combining two frequencies of a high group and a low group of a voice frequency band.

In another conventional DTMF signal generation circuit, when a DTMF signal is transmitted to an audio transmission line, the DTMF signal is transmitted through a low-pass filter to sufficiently reduce aliasing noise and harmonic components generated during signal synthesis. To meet the standards.

[0007]

However, in these conventional DTMF signal generation circuits, a sine wave having a different amplitude change rate depending on the phase is uniformly divided from 16 to 32 with respect to the periodic axis, and a quantized value is recorded. Since the table is used, the fidelity to the amplitude of the sine wave and the fidelity to the frequency of the sine wave are extremely low, and the distortion rate of the DTMF signal is large.

In general, a D / A converter having a resolution of 5 bits or more is used as a D / A converter provided at a subsequent stage in the DTMF signal generation circuit. Since all the values up to the full scale are not input, the resolution of the D / A converter cannot be effectively used.

In order to effectively use the resolution of the D / A converter, means for simply increasing the number of divisions with respect to the periodic axis may be considered. However, a table is set in proportion to the product of the increased number of divisions and the number of D / A bits. This is difficult because the number of circuits that store the data increases.

When transmitting a DTMF signal to a voice transmission path, it is necessary to sufficiently remove aliasing noise and harmonic components generated during signal synthesis according to the standard. However, distortion is reduced by using a table with low fidelity. Therefore, it is common to prepare a high-order low-pass filter having a steep decay rate. As a result, the area of the low-pass filter of the integrated circuit increases, and the effort required to keep the frequency shift and the amplitude change due to the variation in the filter internal elements incalculable.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to achieve both a low distortion factor and a low cost of a DTMF signal.

[0012]

Therefore, according to the present invention, a digital sine wave signal obtained by quantizing a sine wave signal with a bit value for each sampling period corresponds to two frequencies of a high group and a low group of a voice frequency band. Two digital sine wave signal generating circuits, respectively, an adding circuit for adding the digital sine wave signal outputs of these two digital sine wave signal generating circuits, and a D / A for D / A converting the output of the adding circuit. And a DT that combines the two frequencies.
In the DTMF signal generation circuit for generating and transmitting the MF signal, each of the digital sine wave signal generation circuits sets the amount of change in the sampling period of the digital sine wave signal to a value equal to or less than a unit bit by setting the sampling frequency. The change amount of the unit bit is recorded in a table in advance, and is read out at each sampling period to restore and output the absolute amount of the digital sine wave signal.

Each of the digital sine wave signal generating circuits is a frequency dividing counter for dividing an input reference clock signal to the sampling frequency, and counts an output of the frequency dividing counter to be initialized by an initialization signal. An address pointer that outputs an address signal corresponding to each of the sampling periods; a table storage unit that records the amount of change in unit bits in advance in a table corresponding to the address signal and reads out the change amount in each of the sampling periods; An up / down counter that counts up or down in response to an up / down control signal using an output as a count input and outputs the digital sine wave signal to the adder circuit; and a digital controller corresponding to the count value of the up / down counter. The phase of the sine wave signal is detected and the up / down And output switching control signals and a phase detection circuit that outputs the initialization signal to the address pointer.

Alternatively, in order to further reduce the distortion, the above D
And a low-pass filter for low-pass filtering the output of the / A conversion circuit and outputting the DTMF signal.

[0015]

Next, the present invention will be described with reference to the drawings. 1 and 2 are a block diagram showing an embodiment of a DTMF signal generation circuit according to the present invention, and a partial detailed block diagram thereof. The DTMF signal generation circuit according to the present embodiment includes:
The circuit includes first and second digital sine wave signal generation circuits 1 and 2, an addition circuit 3, a D / A converter 4, and a low-pass filter 5. In each of these blocks, an addition circuit 3, D / A
Converter 4 is the same as each block of the conventional DTMF signal generation circuit shown in FIG.

The first and second digital sine wave signal generation circuits 1 and 2 convert digital sine wave signals obtained by quantizing the sine wave signals with bit values for each sampling period into two groups of high and low groups of audio frequency bands. Occurs for each of the three frequencies. At this time, the change amount of the digital sine wave signal at each sampling period is set to a value equal to or less than a unit bit by setting the sampling frequency, and the change amount of the unit bit is recorded in a table in advance and read at each sampling period, and the absolute amount of the digital sine wave signal is read. Is restored and output. The low-pass filter 5 is a low-order low-pass filter that low-pass filters the output of the D / A converter 4.

Further, as shown in FIG. 2, the first and second digital sine wave signal generation circuits 1 and 2 include a frequency division counter 2
1, an address pointer 22, a table storage means 23, an up / down counter 24, and a phase detection circuit 25.

The frequency dividing counter 21 frequency-divides the input reference clock signal to a sampling frequency and outputs it to the address pointer 22. At this time, the divided sampling periods are the first and the first so that the amount of change in each sampling period of the digital sine wave signal becomes a value equal to or less than a unit bit.
These are set in the second digital sine wave signal generation circuits 1 and 2, respectively.

The address pointer 22 is a frequency dividing counter 2
The output of 1 is counted, and an address signal corresponding to each sampling cycle initialized by the initialization signal is output to the table storage means 23.

The table storage means 23 comprises a ROM or a gate circuit for inputting the output of the address pointer 22 as an address signal. The change amount of the digital sine wave signal at each sampling cycle is recorded in advance as a logical level “1” or “0” of the unit bit signal at each address of the address signal corresponding to each sampling cycle, and is read at each sampling cycle. And output to the up / down counter 24.

FIG. 3 shows a change amount and an address of a unit bit previously recorded in a table in the table storage means 23,
FIG. 4 is an explanatory diagram showing a relationship between a digital sine wave signal and a quantization value for each sampling period. Here, the frequency dividing counters 2 are set in advance so that each sampling cycle 32 has a quantization value difference of 1 LSB or less for each sampling cycle.
1 is set to sufficiently high speed, 0 is set to the address 0 of the address signal corresponding to the sampling period, and the ideal sine wave signal 31 is sampled and quantized to the Kth and K-1st digital sine wave signal data. This shows that the difference is stored in the address K as the change amount of the unit bit. In the case of the present embodiment, the sampling interval of the sine wave to be output is not one cycle from the phase 0 to the phase 2π, but the sampling and quantizing data from the phase having the lowest value to the phase having the next maximum value. This is a half cycle, and this is repeatedly used.

The up / down counter 24 receives the output of the table storage means 23 as a count input, counts up or down in response to an up / down control signal, and outputs a digital sine wave signal to the addition circuit 3.

The phase detection circuit 25 detects the phase of the digital sine wave signal corresponding to the count value of the up / down counter 24, switches and outputs the up / down control signal of the up / down counter 24, And output the conversion signal.

Next, the operation of the DTMF signal generation circuit of the present embodiment will be described with reference to FIGS.

First, in the first and second digital sine wave signal generating circuits 1 and 2, the frequency dividing counter 21 counts the number of clock pulses supplied from the reference clock signal, and calculates 1 / n cycle of the reference clock signal. For each time, the logic level "1" is output as a sampling signal for the same time as the cycle of the reference clock signal. When the address pointer 22 detects the logic level "1" of the frequency division counter 21, the address pointer 22 synchronizes with the reference clock signal. The address pointer value is sequentially incremented. Further, the table storage means 23 uses the output of the address pointer 22 as an address signal and outputs a logical level “0”.
Alternatively, a pulse train of “1” is output as a change amount between successive sampling periods.

Next, if the up / down control signal from the phase detection circuit 25 is "1", the up / down counter 24 detects the logic level output "1" from the table storage means 23 and counts up. The value is sequentially incremented in synchronization with the reference clock signal. Next, when the digital sine wave signal output from the up / down counter 24 matches the maximum value of the quantized value,
An initialization signal is output to the address pointer 22, and an up / down control signal is switched to “0” and output to the up / down counter 24.

Next, the address pointer 22 clears the address pointer value by the initialization signal, and sequentially increments again from 0 by the sampling signal. Also, the up / down control signal is switched to “0”,
The up / down counter 24 is set to the down count state, and starts decrementing the count value. next,
When the digital sine wave signal output from the up / down counter 24 matches the minimum value of the quantized value, the phase detection circuit 25 outputs an initialization signal to the address pointer 22 and an up / down control signal to the up / down counter 24. Is switched to “1” and output. Thereafter, digital sine wave signals are output one after another by these repetitions.

As described above, the digital sine wave signals output from the first and second digital sine wave signal generation circuits 1 and 2 are added by the addition circuit 3, and the added signal is
After being converted to an analog signal by the D / A converter 4 and removing unnecessary aliasing noise by the low-pass filter 5, the signal is output as a DTMF signal obtained by synthesizing two frequencies of a high group and a low group of a voice frequency band.

The DTMF signal generating circuit having the above-described configuration is different from the conventional method of recording the quantized value of each sine wave for each sampling as it is, of recording the amount of change for each sampling. Therefore, for a fixed capacity of the table storage means, the sampling period can be made faster and the data amount of the change amount for each sampling can be increased.

Further, while the conventional DTMF signal generation circuit stores one cycle of the digital sine wave signal, the DTMF signal generation circuit of the present embodiment requires only one half cycle, so that This is useful for increasing the data amount of the change amount for each sampling. Due to this increase in data amount, the number of bits of the digital sine wave signal can be increased, and D
The / A converter 4 can use its resolution from 0 to full scale, and realizes effective use of the resolution of the D / A converter 4. Therefore, the analog signal output from the D / A converter 4 is a DTMF signal having a lower distortion than the conventional one at that time.

Further, the DTMF signal generation circuit of the present embodiment incorporates a low-order low-pass filter in order to further reduce the distortion, but the high-order low-pass filter required in the conventional DTMF signal generation circuit is provided. The circuit area of the analog circuit is smaller than that of the filter, which also has the effect of reducing the number of adjustment steps after manufacturing.

[0032]

As described above, the DT according to the present invention is used.
The MF signal generation circuit records the change amount of the digital sine wave signal for each sampling period in the table storage means, thereby increasing the sampling frequency and increasing the data amount of the change amount for each sampling period for the storage capacity. And the distortion rate of the digital sine wave is greatly improved.

Further, for example, the digital sine wave value quantized by R bits does not take discrete values between 0 and 2 ^R -1 as in the prior art, but instead takes values between 0 and 2 ^R -1. Can be replaced with a large number of continuous data that changes only by 1 LSB, so that the resolution of the D / A converter can be used to the maximum and a DTMF tone signal with lower distortion than before can be output at the stage of output from the D / A converter. it can. DTMF
Even when the signal is further reduced in distortion, it can be realized only by a low-order low-pass filter.

In general, the analog circuit constituting a low-pass filter is greatly affected by the characteristics of the circuit due to changes in design rules and manufacturing processes. However, the number of elements of the analog circuit can be reduced by reducing the order of the filter. The subsequent man-hours for adjustment can be reduced, and the reliability can be improved.

At present, digital circuits of semiconductor integrated circuits are mainly designed automatically, whereas analog circuits still rely on human labor in many cases. The DTMF signal generation circuit according to the present invention can reduce the ratio of analog circuits in the whole circuit, and can shorten the development time.

Further, even if the design rule is miniaturized, the number of parts of the analog circuit which does not become smaller in absolute area can be reduced, and a low distortion factor and cost reduction of the DTMF signal can be achieved in the future. With such effects.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a DTMF signal generation circuit of the present invention.

FIG. 2 is a block diagram showing a digital sine wave signal generation circuit in the DTMF signal generation circuit of FIG. 1;

FIG. 3 is an explanatory diagram for explaining an operation of the digital sine wave signal generation circuit of FIG. 2;

FIG. 4 is a block diagram illustrating a configuration example of a conventional DTMF signal generation circuit.

[Explanation of symbols]

 1, 2nd and 1st digital sine wave signal generation circuit 3 addition circuit 4 D / A converter 5 low-pass filter 21 division counter 22 address pointer 23 table storage means 24 up / down counter 25 phase detection circuit 31 ideal sine wave signal 32 Sampling period 33 Digital sine wave signal 41a, 41b Counter 42a, 42b Address pointer 43a, 43b ROM 44 Adder circuit 45 D / A converter

Claims (3)

[Claims] 1. A digital sine wave signal generated by quantizing a sine wave signal with a bit value for each sampling period, corresponding to two frequencies of a high group and a low group of an audio frequency band. A generation circuit, an addition circuit for adding the digital sine wave signal outputs of these two digital sine wave signal generation circuits, and a D / A
And a D / A conversion circuit for converting and generating a DTMF signal combining the two frequencies.
In the F signal generation circuit, each of the digital sine wave signal generation circuits sets a change amount of the digital sine wave signal for each sampling cycle to a value equal to or less than a unit bit by setting a sampling frequency, and sets the change amount of the unit bit in advance. A DTMF signal generation circuit, which records a table, reads out the digital sine wave signal at each sampling period, and restores and outputs the absolute amount of the digital sine wave signal. 2. The digital sine wave signal generation circuit according to claim 2,
A frequency division counter for dividing the frequency of the input reference clock signal to the sampling frequency, and an address for counting the output of the frequency division counter and initializing the initialization signal with an initialization signal and outputting an address signal corresponding to each sampling period A pointer, table storage means for pre-recording the change amount of the unit bit corresponding to the address signal in a table, and reading out the change amount for each sampling period; and using the output of the table storage means as a count input corresponding to an up / down control signal. An up / down counter that counts up or down to output the digital sine wave signal, and detects the phase of the digital sine wave signal corresponding to the count value of the up / down counter to switch the up / down control signal and output The initialization signal is stored in the address pointer. And a phase detection circuit that outputs a, DTMF signal generating circuit according to claim 1, wherein. 3. The DTMF signal generation circuit according to claim 1, further comprising a low-pass filter that low-pass filters an output of the D / A conversion circuit and outputs the DTMF signal.