The invention relates to an apparatus for controlling or regulating the quality of sound generated by an audio signal by regulating the individual amplitudes of signals in certain predetermined frequency bands.
Conventional audio apparatuses such as automobile audio systems, TV units, and audio micro-components are equipped with sound control filter circuits in a signal channel, which is provided for a user to adjust some parameters to enhance, attenuate, or remove certain high and/or low frequency bands of the sound. In these apparatuses, resistors and condensers of the filter circuits occupy large spaces, which hinders integration of the filter circuits into an IC circuit, so that the filter circuits are designed as external components.
An audio system has been disclosed in Japanese Patent Application JP-A-10-233653 for example, which uses switched capacitor (SC) filters substituting for the external sound control filters.
FIG. 1 illustrates a conventional multi-stage sound quality regulation apparatus utilizing SC filters. This sound quality regulation apparatus includes in the audio signal channel thereof a first filter section 1, which comprises: an amplifier 1-1 having an operation amplifier OP, an antialising filter formed of a low-path filter 1-2, a SC filter section 1-3 realized by SC technology, and a voltage setting section (voltage divider) 1-4.
The antialiasing filter 1-2 can be a filter provided in the pre-SC filter stage, adapted to sufficiently and selectively attenuate higher frequency bands exceeding the maximum frequency realized by SC technology. Similarly, a second filter section 2 of the apparatus, provided in the audio signal channel, comprises: an amplifier 2-1 having an operation amplifier OP; an antialising filter 2-2 formed of a low-path filter; an SC filter section 2-3 in the form of a resistor realized by SC technology, and a voltage setting section (voltage divider) 2-4. An appropriate set of such filter sections 1 and 2 is provided in each of the frequency bands to be regulated.
Performance of the sound quality regulation apparatus depends on the characteristics of the filters. Take the first filter section 1 of FIG. 1 for example. The characteristic of the filter is determined by the position of the tap on a voltage divider 1-4. When the tap of the voltage divider 1-4 is set to be short circuited to the output of the amplifier 1-1, the first filter section operates as a voltage follower having a constant amplification factor. In this case, the first filter stays neutral, disconnecting the antialiasing filter 1-2 and the SC filter section 1-3 from the signal channel, so that no noise is supplied from the antialiasing filter 1-2 and the SC filter section 1-3 to the signal channel.
On the other hand, when the tap of the voltage divider 1-4 is positioned so as to divide the output of the amplifier 1-1, the antialiasing filter 1-2 and the SC filter section 1-3 are plugged in the signal channel to change the frequency characteristic of the first filter section. That is, the signal output from the amplifier 1-1 is negatively fed back to the amplifier 1-1 through the antialiasing filter 1-2, SC filter section 1-3, and the voltage divider 1-4.
Thus, the sound quality regulation apparatus as shown in FIG. 1 in the amplification or attenuation modes can amplify or attenuate the audio signal in a given frequency band, or, in the neutral mode, pass the audio signal as it is without adding any noise of the antialiasing filter 1-2 and the SC filter section 1-3.
In the conventional sound quality regulation apparatus, however, each of the frequency bands requires a filter section, so that a series of filter sections are placed in the audio signal channel.
Then, since each stage associated with one frequency band requires an operational amplifier and an antialiasing filter such as a low-pass filter, it is difficult to construct a compact multi-stage sound quality regulation apparatus.
Furthermore, since the multi-stage sound quality regulation apparatus involves as many operational amplifiers OP and filters as the stages in series with the audio signal channel, they add their noises to the signal, thereby disadvantageously decreasing the singnal-noise (S/N) ratio of the signal. Therefore, miniaturization of an audio system equipped with a graphic equalizer for example utilizing as many as 5 to 12 split frequency bands suffers the above mentioned problems if the sound quality must not be sacrificed.
It is therefore an object of the present invention to provide a compact sound quality regulation apparatus, i.e., a compact tone control apparatus capable of regulating individual split frequency bands of an input audio signal using SC filters and only a few operational amplifiers in the signal channel irrespective of the number of the split frequency bands, thereby increasing the S/N ratio of the apparatus.
In accordance with one embodiment of the invention, a sound quality regulation apparatus has at least one regulation device for regulating the amplitudes of a predetermined number of split frequency bands of an audio input signal, the regulation device comprises:
an attenuation unit which is capable of receiving an input signal and a regulation signal associated with at least one predetermined frequency band, and is adapted to generate an attenuated signal of the input signal which is attenuated in the frequency band based on the regulation signal;
an amplification unit which is capable of receiving the attenuated signal from the attenuation unit and a regulation signal associated with at least one frequency band, and is adapted to generate an output signal which is amplified in the frequency band based on the regulating signal; and
at least one sound regulator associated with one frequency band of the input signal, the sound regulator including
a SC filter receiving the attenuated signal from the attenuation unit and permitting the frequency band associated with the sound regulator to pass therethrough as a regulation signal, and
a selection device connected with the SC filter and adapted to selectively feeding the regulation signal to either the attenuation unit, the amplification unit, or none of the units.
The sound quality regulation apparatus can perform attenuation, amplification, or do nothing on the individual frequency bands using the one attenuation unit, one amplification unit, and one or more SC filter(s) associated with the split frequency band(s), or leave the input signal unchanged. Use of such common attenuation unit and amplification unit for the entire split frequency bands along with SC filters enables construction of a very compact sound quality regulation apparatus, irrespective of the number of split frequency bands. Moreover, since the signal channel includes only one common attenuation unit and one common amplification unit, irrespective of the number of split frequency bands, noise sources do not increase if the number of the split frequency bands are increased.
In the sound quality regulation apparatus, the attenuation unit can be formed of:
an adder for adding the input signal and the regulation signal; and
an inverting amplifier for inverting and amplifying the output of the adder.
The amplification unit can be formed of:
an adder for adding the attenuated signal and the regulation signal; and
an inverting amplifier for inverting and amplifying the output of the adder.
In this way individual attenuation and amplification of the respective frequency bands of an input signal can be performed in a simple manner through addition operations and inversion operations in the two units.
Each of the sound regulators includes, in a pre-SC filter stage thereof an antialiasing filter, and in the post-SC filter stage thereof a smoothing filter and a gain regulator, all connected in series to perform necessary filtering and gain regulation.
The antialiasing filters in the respective sound regulators can be replaced by a single common antialiasing filter providing the attenuated signal to the respective sound regulators.
The attenuation unit of the invention can be formed of: an operational amplifier; a feedback resistor connected between the input and output terminals of the operational amplifier; a first input resistor connected with the inverting terminal of the operational amplifier to receive the input signal; and a second input resistor connected with the inverting terminal of the operational amplifier to receive the regulation signal.
The amplification unit can be formed of: an operational amplifier; a feedback resistor connected between the input and output terminals of the operational amplifier; a first input resistor connected with the inverting terminal of the operational amplifier to receive the attenuated signal; and a second input resistor connected with the inverting terminal of the operational amplifier to receive the regulation signal.
In this arrangement, additions and inversions of signals required in the attenuation and amplification of an audio signal can be performed by a pair of such attenuation unit and an amplification unit, making the sound quality regulation apparatus quite simple in structure.
Each of the sound regulators has in the pre-SC filter stage thereof an antialiasing filter and in the post-SC filter stage thereof a smoothing filter to perform necessary filtering of the signal.
Each of the input resistors receiving a regulation signal can be a variable resistor, which can regulate the gain of the sound regulator without any dedicated gain regulator.
A condenser may be connected in parallel with the feedback resistor of the amplification unit. The condenser serves as a smoothing filter in collaboration with resistors connected with the amplification unit, thereby eliminating need of a dedicated smoothing filter.