Audio signal bass frequency enhancement device

Abstract: A device for enhancing the bass frequency components in an audio signal while avoiding overload problems associated with fixed levels of bass frequency enhancement. The device divides the input signal into two parts, the first of which passes through to the output of the device. The second part of the input signal is input into a low pass filter (2) which passes only the bass components of the input signal and imparts a frequency dependent phase shift to the bass components. The bass components then pass through a variable gain amplifier (3) which may or may not invert the polarity of the bass components and then are recombined with the first part of the audio signal. The gain of the variable gain amplifier (3) is varied in accord with the level of the bass components so as to provide substantially the same output level over a wide range of input levels. As a consequence the device provides greater enhancement of the bass frequencies when they are at lower levels and less enhancement when the bass frequencies are at higher levels, thereby reducing the likelihood of overloading any subsequent audio signal amplifiers. The enhancement is the result of a dynamically changing frequency response and increased duration of the bass frequencies.

Description: 1. Background of the Invention a. Field of the Invention This invention pertains to audio signal processing devices. More particularly this invention pertains to devices used to increase the amount of energy contained in the lower or bass frequencies of an audio signal relative to the higher frequency components without overloading any subsequent audio amplifiers. b. Description of the Prior Art Several methods have been used in the prior art to increase the energy contained in the lower or bass frequencies in an audio signal relative to the higher frequency components. For instance, a set of band-pass filters, each having an adjustable attenuation, has been used to emphasize or de-emphasize the frequency components within each band of frequencies relative to the components within the other bands. If the attenuation of the low frequency band-pass filter is adjusted to be less than the attenuation of the higher frequency band-pass filters, the low frequency components are enhanced relative to the higher frequency components. A single filter that exhibits a lower attenuation for low frequency components relative to its attenuation for higher frequency components may also be used for this purpose. However, because the peak level of the audio input signal is typically primarily determined by the peak level of the low frequency components, a simple "preemphasis" of the low frequency components often results in the overloading of any subsequent audio amplifiers. Multiband compression has also been used to emphasize the bass components. A multiband compression system splits the audio signal into separate bands of frequency components and then separately compresses the amplitude range of the signals within each of these bands. The energy in the bass components relative to the higher frequencies has also been increased by combining the input audio signal with subharmonics of the input audio signal. This method, however, very often substantially increases the peak level of the signal. US-A-4 249 042 discloses a multiband compressor which exhibits a circuit structure, including a filter, variable gain amplifier, and a signal level detector, in the low and high frequency signal paths, similar as in the bass signal path of the present application. 2. Summary of the Invention The device according to the invention as defined in Claim 1 divides the input signal into two parts, the first of which passes directly to the output of the device. The second part passes through a low pass filter which passes only the bass components of the input signal and imparts a frequency dependent phase shift to those bass components. The bass components then pass through a variable gain amplifier and then are recombined with the first part of the audio signal. The gain of the variable gain amplifier is varied in accord with the level of the bass components so as to provide the same output level over a wide range of input levels. As a consequence the device provides greater enhancement of the bass signals when the bass signals are at a lower level and less enhancement when the bass signals are at a higher level, thus reducing the likelihood of overloading any subsequent audio signal amplifiers. Due to the amplitude and time relationships between the audio signals in the two paths, the recombination of the two parts of the input signal produces a dynamically changing frequency response characteristic and a longer duration of bass frequencies. 3. Brief Description of the Drawings Figure 1 is a block diagram of the invention. Figure 2 is a schematic diagram of the invention. 4. Detailed Description Referring to Figure 1, the audio signal is input to divider 1, which divides the audio signal into first and second parts. The first part of the input signal is input to combiner 4 where it is "mixed" or combined with the audio signal output from variable gain amplifier 3. The second part of the input signal is input to lowpass filter 2 which passes only the low frequency components, i.e. the bass frequencies that are contained within the second part of the input signal. The bass frequencies output from lowpass filter 2 are input to variable gain amplifier 3, which amplifies the bass frequencies. Circuitry within signal level detector 5 senses the amplitude level of the signal output from lowpass filter 2 and outputs a control voltage to variable gain amplifier 3 so as to provide substantially the same output level over a wide range of input levels so that maximum level of the bass signal that is output from variable gain amplifier 3 does not exceed a preselected level. The signal output from variable gain amplifier 3 is then combined or mixed in combiner 4 with the first part of the input audio signal to produce the enhanced audio signal that is output from the device. By preselection, the polarity of signal output from variable gain amplifier 3 may or may not be inverted relative to the signal input to variable gain amplifier 3. Referring to figure 2, a second order Sallen-Key low pass filter having a cut-off frequency of 80 hertz operates as low pass filter 2. Variable attenuator 10 in variable gain amplifier 3 adjusts the level of amplification of bass signals having a low signal level. Operational transconductance amplifier 11 functions as the variable gain amplifier. Potentiometer 12 is adjusted to minimize the shift in D.C. voltage at the output of amplifier 11 with varying control voltage signal levels. Darlington transistor 13 buffers the output of amplifier 11 and potentiometer 14 is used to set the upper limit of the bass signal level that is output to combiner 4. Resistor 15 provides a d.c. voltage reference level for the device. In the preferred embodiment, the amplified bass signal that is input to combiner 4 is inverted relative to the bass signal that is input to variable gain amplifier 3. However, where desirable, an inverting amplifier may be substituted for one of the non-inverting amplifiers within variable gain amplifier 3 so as to supply a non-inverted bass signal to combiner 4. Full wave rectifier 16 is used in combination with operational transconductance amplifier 17 followed by transistors 18 and 19 and Darlington transistor 20 to generate a voltage responsive to the level of the bass signal. The voltage output from Darlington transistor 20, in turn, operates through resistor 25 to control the current amplification provided by operational transconductance amplifier 11. A bass signal input to full wave rectifier 16 causes the voltage at the output of operational transconductance amplifier 17 to move in a negative direction, which causes transistor 18 to begin to conduct current which, in turn, causes transistor 19 to begin to conduct, causing capacitor 21 to begin to discharge through timing resistor 22. Darlington transistor 20 buffers the voltage across capacitor 21, which voltage is converter to a current through resistor 25, and used to control the amplification provided by operational transconductance amplifier 11. The LED and the other components within dashed lines 9 respond to the variations in the voltage across capacitor 21 to provide a visual indication of the operation of variable gain amplifier 3.

Claims: An audio signal processing device for enhancing the bass frequency components of an input audio signal comprising: a signal divider (1) for dividing the input audio signal into first and second parts, the first part of which contains all frequency components of the input audio signal, a low pass filter (2), the second part of the input audio signal being input to the low pass filter, the low pass filter outputting the bass frequency components of the second part of the input audio signal and imparting a frequency dependent phase shift to said bass frequency components, a variable gain amplifier (3), the bass frequency components output by the low pass filter (2) being input to the variable gain amplifier, the variable gain amplifier amplifying the input signal and outputting amplified bass frequency components, a signal level detector (5) providing an output signal responsive to the amplitude level of the bass frequency components only, the gain of the variable gain amplifier (3) being responsive to the output of the signal level detector so as to maintain the signal level of the bass frequency components output by the variable gain amplifier (3) at substantially the same level for a wide range of signal levels input to the variable gain amplifier (5), a signal combiner (4), the signal combiner combining the unaffected first part of the input audio signal being output by said signal divider (1) with the amplified bass frequency components, the combined signals being output by the signal combiner (4) as an audio signal having enhanced bass frequency components. The device of claim 1 in which the signal output from the variable gain amplifier (3) is inverted relative to the signal input to the variable gain amplifier. The device of claim 1 in which the signal output from the variable gain amplifier (3) is not inverted relative to the signal input to the variable gain amplifier.

Inventors: WERRBACH DONN R, WERRBACH, DONN R.
Applicants: APHEX SYSTEMS LTD, APHEX SYSTEMS, LTD.
Application year: 1992
Main classification: H03G9
IPC codes: H03G9, H03G3, H03G5