JPS6345056Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a digitally controlled attenuator comprising a multiplier D/A converter and an operational amplifier.

FIG. 1 is a block diagram of an attenuator consisting of a general multiplication type DA converter and an operational amplifier.
1 is an input terminal, 2 is a multiplication type DA converter, 3 is an operational amplifier, 4 is an attenuation amount indicating circuit, and 5 is its digital control signal. The amount of attenuation of the input signal is determined by the digital control signal 5 and is output. However, this configuration generates noise when the amount of attenuation changes. FIG. 2 shows a waveform diagram when the input signal is a DC signal. Figure 2 shows that the digital control signal is at time T.
The case where the value changes from 011...1 to 100...0 is shown. Figure 2-a shows how the MSB changes from 0 to 1, and Figure 2-b shows how the lower bits other than the MSB change to 1.
This shows how it becomes 0. In this case, as shown in the figure, a time difference of Δt occurs at the time of switching due to various factors of the digital control signal 5 and the multiplication type DA converter 2 (for example, variations in elements, stray capacitance, variations in switching time, etc.), and during this period, the digital control signal becomes 111...1. As a result, a noise called a glitch as shown in FIG. 2c is generated in the output, making the configuration of FIG. 1 unsuitable as a conventional attenuator for musical tone signals. The object of the present invention is to provide an attenuator that reduces such noise.

The present invention will be explained based on one embodiment. FIG. 3 is a block diagram of one embodiment of the present invention, and FIG. 4 is a waveform diagram of each part. Blocks having the same functions as those in FIG. 1 are labeled with the same numbers. The output of an attenuator consisting of a multiplier DA converter 2 and an operational amplifier 3 is branched into two,
One output passes through a low-pass filter 13 and a multiplier 16, the other passes through a multiplier 17, and the respective outputs are sent to an adder 2.
Add to 0. Digital signal 8 from change instruction circuit 4' is applied to multiplication type DA converter 2 via latch circuit 7. A change instruction signal 9 from a change instruction circuit 4' is applied to monomultis 10 and 11. The output of monomulti 10 is applied to latch circuit 7, the Q and output of monomulti 11 are applied to integration circuits 18 and 19, respectively, and their outputs are applied to multipliers 16 and 17, respectively.

In the above configuration, the content of the digital control signal 8 is changed at the same time as the change instruction signal 9 shown in FIG. 4A is generated. The digital control signal 8 is first latched by the latch circuit 7 with a delay of a certain time W by a control pulse having a width W shown in FIG. 5 controls the multiplication type DA converter 2. This W may be sufficiently short. In synchronization with the rise of this pulse, the latch circuit 7 updates and latches the digital control signal 8. This low-pass filter 13 sets its cutoff frequency sufficiently low to remove the harmonic components of the glitch noise and reduce the noise components. The signal passed through the low-pass filter is added to the adder 20 only when the signal changes. Control signals 12 and 15 for performing such switching are transmitted from the change instruction signal 9 from the attenuation amount instruction circuit 4' to the monomulti circuit 1.
Made by 1. That is, the control signals 12 and 15 in FIG. 4 are generated simultaneously with the change instruction signal 9, and are integrated by the integrating circuits 18 and 19 as shown by dotted lines. The output signal and input signal of the low-pass filter 13 are multiplied by a control signal having a time constant at the rise and fall, respectively, and then added by an adder 20. In this way, the low-pass filter 13 is used only during the period when grit noise occurs.
A signal with reduced glitches that has passed through appears at the output terminal 6, and during other periods, a normal signal that has not passed through the low-pass filter appears at the output terminal. Therefore, the pulse width X of the control signals 12 and 16 is set to a value larger than W and up to about 250 ms, which is the time required for the filter 13 to sufficiently reduce the glitch noise component. In this way, the output of the low-pass filter 13 is connected together with the change instruction signal 9, and after a delay of W, the digital control signal changes and is set to a new attenuation amount. It is reduced and returns to its original state after X has elapsed. In this way, the band of the musical sound signal is limited only during the period of No problem arises in the case of musical tone signals.

In order to reduce the difference when the signal switches to the signal passed through the low-pass filter, by integrating the control signal as described above, the two types of signals crossfade smoothly and the signal switches. The effects caused by this are reduced, resulting in a more natural attenuated signal with reduced glitches.

If it is not necessary to crossfade smoothly like this, the multipliers 16 and 17 may not be used, and a switch may be used to switch between the signal that has passed through the low-pass filter 13 and the normal signal that does not pass through. This is a moot point.

As explained above, according to the present invention, a low-pass filter for removing glitch noise components is inserted into the output of a digitally controlled attenuator consisting of a multiplier type DA converter and an operational amplifier, and the low-pass filter is inserted only before and after the point at which the amount of attenuation changes. A good attenuator can be obtained by outputting the output and directly outputting it in steady state.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional example, FIG. 2 is a waveform diagram of each part thereof, FIG. 3 is a circuit diagram showing an embodiment of the present invention, and FIG. 4 is a waveform diagram of each part thereof. In the figure, 2 is a multiplication type DA converter, 4' is an attenuation amount indicating circuit, 13 is a low-pass filter, 16 and 17
is a multiplier.

Claims (1)

[Scope of utility model registration request] In an attenuator that controls the amount of attenuation of an attenuator consisting of a multiplier DA converter and an operational amplifier using a digital control signal, a low-pass filter for removing a band containing glitchy noise is connected to the output terminal of the attenuator. In addition, selection means is provided for selecting either the output of the attenuator or the output of the low-pass filter, and from before the digital control signal is changed, the selection means is controlled to select the output of the low-pass filter during the period in which the glitch noise occurs. A digitally controlled attenuator characterized in that the output of a pass filter is selected.