Patent Application
20080152168
The present invention relates to signal processing circuits for boosting a desired range of frequencies in an audio signal and particularly, but not exclusively, to bass-boost audio circuits which mitigate output signal distortion.
Bass-boost circuits are used to increase the level of bass, often to compensate for poor bass response in low-cost headphones and loudspeakers. A typical application of where a bass-boost circuit may be used is when a headphone is coupled to the output of an amplifier, which is powered by a single ended (i.e. a unipolar) power supply, that has a DC-blocking capacitor C in its output signal path. The blocking capacitor C and the headphone, that is, the load, resistance RL act as an R-C high-pass filter with a cut-off frequency given by:
As an example, for a 32Ω load resistance RL, a blocking capacitor C with a capacitance of approximately 250 μF would be required to achieve a cut-off frequency FC of approximately 20 Hz, which is typically the lowest audible audio frequency.
In portable audio applications such as MP3 players, mobile communications and the like, the capacitor C should be as physically small as possible so as to maintain a small overall form-factor for the portable application. This equates to using a capacitor C with a smaller value, which consequently raises the cut-off frequency FC, which in turn reduces the bass level. Other non-portable audio applications such as Hi-Fi systems, In-Car Entertainment systems and the like, may also benefit, for reasons of cost for example, from using a capacitor with a smaller physical and capacitive value. One method of compensating for this type of reduced bass level is to use a bass-boost circuit.
A typical audio amplifier circuit 10 incorporating a bass-boost circuit having: a digital input signal is shown in
Referring to
A known bass-boost circuit 22 is shown in
gain(dB)=20 log 10(K)
A known alternative bass-boost circuit 32 is shown in
gain(dB)=20 log 10(K+1)
If either of these bass-boost circuits 22, 32 is used to boost bass frequencies, the signal SOUTA at the output of the amplifier circuit 20 will begin to reach a level which exceeds the maximum signal headroom. That is to say, the total gain (given by the two respective gain equations above) plus input signal level, at a particular frequency, cannot be amplified any further due to the limits of the amplifier circuit 10. Such a scenario is known as signal “clipping”. Whenever the output signal SOUTA clips, audible distortion will be introduced into the output signal SOUTA, resulting in poor quality audio signals being output from the headphone or loudspeaker.
In applications where headphones are used, the output signal SOUTA voltage level at typical listening levels will be below the “clipping level” voltage. However, there will be occasional peaks in output signal SOUTA where the maximum signal level is exceeded, resulting in clipping and distortion. This is particularly the case when the bass signals are boosted to compensate for the use of a smaller AC coupling capacitor.
In a digital implementation of the circuits shown in
The invention aims to provide boosting of particular frequency ranges of an audio signal whilst mitigating noise caused by “clipping” of the signals.
According to a first aspect of the present invention there is provided a signal processing circuit for boosting a desired range of frequencies in an audio signal, the circuit comprising:
Preferably, the first filter is a bandpass filter, the bandpass filter having a pre-determined centre frequency.
Preferably, the bandpass filter attenuates frequencies of substantially three times the centre frequency, such that those attenuated frequencies are substantially inaudible during audio playback.
Preferably, the bandpass filter has a centre frequency between 50 Hz and 100 Hz and a bandwidth between 50 Hz and 100 Hz.
Alternatively, the filter is a low-pass filter.
Preferably, a second filter is coupled between the audio input and the signal adder in parallel to the amplifier, limiter and first filter.
Preferably, wherein the second filter is a notch filter.
Alternatively, wherein the second filter is a high-pass filter.
Preferably, the limiting function limits the amplitude of the amplified signal to within a threshold in the range 0.6 to 0.95 of full scale.
Preferably, the circuit further comprises a pre-filter coupled between the audio input and the amplifier.
Preferably, the pre-filter has substantially the same bandwidth and centre frequency as that of the first filter.
Preferably, said amplifier comprises a static gain stage.
Preferably, said amplifier comprises at least one variable gain stage.
Preferably, the circuit further comprises a control circuit for varying the gain of the variable gain stage automatically in response to actual signal levels.
Preferably, the control circuit comprises a detector for detecting a signal level at the amplifier output for comparison with at least one pre-determined threshold.
Preferably, the control circuit is arranged to reduce the gain of the variable gain stage, if the signal level detected by the detector is above the at least one pre-determined threshold.
Preferably, the control circuit is arranged to increase the gain of the variable gain stage, if the signal level detected by the detector is below the at least one pre-determined threshold.
Preferably, the control circuit further comprises a ramp means enabled to vary the gain of the variable gain stage at a pre-determined rate in response to the comparison of the signal level and the pre-determined threshold.
Preferably, the gain of the variable gain stage is reduced by the ramp means, when required, at a pre-determined attack rate.
Preferably, the pre-determined attack rate is between 10 μs/dB and 500 ms/dB.
Preferably, the pre-determined attack rate is in the range 100 to 400 ms/dB
Preferably, the gain of the variable gain stage is increased by the ramp means, when required, at a pre-determined decay rate.
Preferably, the pre-determined decay rate is between 10 ms/dB and 5 s/dB.
Preferably, the pre-determined decay rate is in the range 500 ms/dB-2 s/dB.
Alternatively, the control circuit varies the gain of the variable gain stage using pre-defined values derived from a plurality of gain curves, the control circuit arranged to compare the signal level detected by the detector and the at least one pre-determined threshold and select one of the plurality of gain curves, dependent on the static gain, and vary the gain of the variable gain stage accordingly.
Preferably, the detector is a peak signal detector.
Alternatively, the detector is a peak RMS signal detector.
Preferably, if the signal is below a threshold set by the limiting function, the gain of the variable gain stage is automatically switched into a second decay rate, which is faster than the first decay rate, by the control circuit.
Preferably, the control circuit maintains the second decay rate until the signal level reaches the threshold set by the limiting function.
Preferably, if the gain of the variable gain stage reaches the static gain, the first decay rate is again selected.
According to a second aspect of the invention there is provided a signal processing means comprising: audio input means for receiving an input signal; amplification means for amplifying the input signal and providing an amplified signal; limiting means for limiting the amplified signal by applying a limiting function and providing a limited signal; first filtering means for filtering the limited signal; and adding means coupled to the first filtering means and audio input means and for adding received signals and providing a signal output.
According to a third aspect of the invention there is provided a method of processing signals for boosting a desired range of frequencies in an audio signal, comprising: amplifying the, or a derivative of, the input signal and providing an amplified signal; limiting the amplified signal by applying a limiting function and providing a limited signal; filtering the limited signal providing a first filtered signal; and adding the first filtered signal to, or a derivative of, the input signal providing a signal output.
Preferably, the step of filtering the limited signal comprises applying a bandpass filter to the limited signal, the bandpass filter having a pre-determined centre frequency.
Preferably, the bandpass filter attenuates frequencies of substantially three times the centre frequency, such that those attenuated frequencies are substantially inaudible during audio playback.
Preferably, the bandpass filter has a centre frequency between 50 Hz and 100 Hz and a bandwidth between 50 Hz and 100 Hz.
Alternatively, the step of filtering the limited signal comprises applying a low-pass filter to the limited signal.
Preferably, the step of filtering the input signal providing a second filtered signal and the step of adding comprises adding the second filtered signal, which is a derivative of the input signal to the first filtered signal.
Preferably, the second filter is a notch filter.
Alternatively, the second filter is a high-pass filter.
Preferably, the limiting function limits the amplitude of the amplified signal to within the range of 0.6 to 0.95 of full scale.
Preferably, the method further comprises the step of pre-filtering the input signal, providing a pre-filtered signal for the step of amplifying.
Preferably, the step of filtering the input signal to provide a pre-filtered signal utilises a filter response having substantially the same bandwidth and centre frequency as that of the step of filtering the limited signal to provide a first filtered signal.
Preferably, wherein the step of amplifying comprises providing variable gain amplification.
Preferably, the step of amplifying further comprises controlling the variable gain amplification automatically in response to actual signal levels.
Preferably, the step of controlling further comprises detecting a signal level at the output of the amplifying step for comparison with at least one pre-determined threshold.
Preferably, if the signal level detected is above the at least one pre-determined threshold, the gain of the variable gain amplification is reduced.
Preferably, if the signal level detected is below the at least one pre-determined threshold, the gain of the variable gain amplification is increased.
Preferably, the gain of the variable gain amplification is varied by ramping through a set of gain settings, the gain being varied at a pre-determined rate in response to the comparison of the signal level and the pre-determined threshold.
Preferably, the gain of the variable gain amplification is reduced, when required, at a pre-determined attack rate.
Preferably, the pre-determined attack rate is between 10 μs/dB and 500 ms/dB.
Preferably, the pre-determined attack rate is 100 to 400 ms/dB
Preferably, the gain of the variable gain amplification is increased, when required, at a pre-determined decay rate.
Preferably, the pre-determined decay rate is between 100 ms/dB and 5 s/dB.
Preferably, the pre-determined decay rate is 500 ms/dB-2 s/dB.
Alternatively, the gain of the variable gain amplification is varied using pre-defined values derived from a plurality of gain curves, the gain being varied in accordance with comparison of the signal level and the at least one pre-determined threshold and selecting one of the pre-defined gain curves, dependent on the static gain, and varying the gain of the variable gain amplification accordingly.
Preferably, the step of detecting the signal level comprises detecting a peak signal.
Alternatively, the step of detecting the signal level comprises detecting a peak RMS signal.
The invention for example also provides audio apparatus including a signal processing circuit according to the invention set forth above.
The audio apparatus may be portable.
The audio apparatus may be an in-car audio apparatus, a headphone or a stereo headphone apparatus or a communications apparatus such as a mobile phone or PDA.
The audio apparatus may further include an audio output transducer, such as a speaker, connected as a load connected to an output terminal of the signal processing circuit.
These and other features and advantages of the invention in its various embodiments will be understood from a consideration of the detailed description which follows.
Embodiments of the invention will now be described, by way of example only, by reference to the accompanying drawings, in which:
The present invention allows an increase in the level of certain frequency ranges of audio signals to be produced for typical listening levels, whilst mitigating the audible distortion due to clipping at high output signal levels. This is particularly applicable to bass frequencies and the remaining description describes, by way of example, bass-boost circuits.
Referring to
An audio signal SIN received at the audio signal input terminal 36 of the bass-boost circuit 34 is amplified by the amplifier 38 before being passed to the limiter 40. It should be noted that, although amplifier 38 is shown as a variable gain amplifier 38, its gain will typically be set by a user of the system and, as such, is effectively a static gain. If the level, or amplitude, of the amplified audio signal S1 exceeds a pre-determined limiter threshold, in either the positive or negative direction, the signal S2 at the output of the limiter 40 will “clip”, which means that it is prevented from exceeding the pre-determined limiter threshold. While the following description refer to signals varying in both positive and negative directions about a ground reference, the skilled reader will appreciate that in practice the ground reference may be zero volts or a different voltage. Where the circuit operates from a single-ended supply, for example, the ground reference will usually be the mid-level between zero volts and a unipolar supply voltage.
When the signal S2 is clipped, odd-order harmonics of the frequency of the input audio signal S1 will be generated and appear in the resultant output signal S2 of the limiter 40. In this embodiment, the filter 42 is a band-pass filter, which is selected such that odd-order harmonics within the signal S2 generated by the limiter 40 will be attenuated to the extent that they are, or are substantially, inaudible in the output signal S3 of the filter 42. Due to the increasing attenuation of the filter 42 with increasing frequency, higher order harmonics, which are more objectionable in terms of sound quality, will have increasing levels of attenuation.
Typically, when the filter 42 is a band-pass filter, it will have a centre frequency between 50 Hz and 100 Hz and a bandwidth between 50 Hz and 100 Hz. Ideally, the filter bandwidth is chosen so that signals of 3-times the centre-frequency (corresponding to the third-harmonic of the amplified input signal SIN) and above are well attenuated.
The signal adder 44 then adds signal S3 and SIN to produce output signal SOUT.
Referring now to
In this embodiment, the other branch comprises a second filter 58 with an input connected to the audio signal input terminal 48 and an output terminal connected to another input terminal of the adder 56. The second filter 58 is, in this particular embodiment, a notch filter, which filters out of the signal frequencies between a pre-defined range of frequencies to produce audio signal S4. As such, when the respective output signals S4 and S3 from the notch filter 58 and the band-pass filter 54 are combined by the adder 56 to generate the output signal SOUT of the bass-boost circuit 46, the full range of frequencies are outputted in the output signal SOUT but with an amplified, or boosted, bass range.
In both of the above embodiments shown in
The respective band-pass filters 42, 54 and notch filter 58 used in the embodiments of
One potential problem with the circuit of
A third embodiment of the present invention, which mitigates this potential problem, is shown in
As described in previous embodiments, the first and second filters, respectively 70 and 74, in the embodiment of
A fourth embodiment of the present invention is shown in
In this fourth embodiment, the static gain control provided by amplifiers 38, 50, 66 of the previous three embodiments is replaced by a dynamic gain controller 80 that is capable of providing both variable and static gain control. The dynamic gain controller 80 prevents signal distortion by automatically reducing its gain if the level of the input signal SIN that is boosted by its static gain setting exceeds a predefined threshold. The programmable static gain represents the target gain of the bass boost circuit, set, for example, by the user.
The dynamic gain controller 80 can replace amplifiers, or gain blocks, if implemented digitally, 38, 50, 66 of any of the previous three embodiments. Several methods for implementing the dynamic range control are possible.
For example, as shown in
Alternatively, as shown in
If the dynamic (variable) gain has been reduced by the gain control stage 100 due to a previous signal occurring above the limiter threshold, and, for example, following this a user reduces the input gain 106, the dynamic gain will increase at a rate set by the decay rate, until either the static gain is reached or the signal level reaches the threshold.
Due to the slow rate of decay, the user may potentially experience a gradual increase in bass level which may not sound acceptable. Therefore, to mitigate this potential problem, when the input gain is reduced, and if the signal is below the limiter threshold, the dynamic gain automatically switches into a faster decay rate. This faster decay rate is maintained until either the signal level reaches the limiter threshold, or the gain reaches the static gain, after which the slower decay rate is again selected.
It will be appreciated by those skilled in the art that that in both embodiments of the control circuits 90 and 98 that an RMS type detector, or other type of signal detector, may be used instead of the peak-detector.
It will be further appreciated by those skilled in the art that any implementation may be carried out by using digital signals and circuits or analogue signals or circuits or indeed a mixture of both analogue and digital signals and circuits.
It should be noted that where a claim recites that elements are “coupled”, this is not to be interpreted as requiring direct coupling to the exclusion of any other element, but rather the elements are coupled or connected sufficient to enable those elements to function as described. The skilled reader will appreciate that a good, practical design might include many auxiliary components not mentioned here, performing, for example, start-up and shutdown functions, sensing functions, fault protection or the like, none of which detract from the basic functions characteristic of the invention in its various embodiments described above and in the claims.
In addition, although the invention is described in relation to a single audio signal or channel, it can be also applied to multiple channels, such as left and right channels of headphones or surround sound type systems. Furthermore, where the invention is applied to a plurality of audio signals or channels, some elements may provide common functions to those plurality of signals/channels. For example, the dynamic gain control may apply a common gain to more than one audio signal but each audio signal has an individual band-pass filter.
The response of the human ear is such that distortions at higher frequencies are more perceptible than distortions at lower frequencies. As alluded to above, circuits such as those described with reference to
Furthermore, the above described embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the spirit or scope of the appended claims and drawings. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single element may fulfil the functions of several elements recited in the claims. It should also be noted that the attenuation, or decrease, of a signal amplitude is a form of amplification, thus the word “amplify”, amplifying”, “amplified” and the like can be taken to mean an increase or a decrease in the amplitude of a signal. Similarly any reference to “gain” applied may refer to a gain less than unity being applied (that is the effect of applying “gain” to a signal may result in its attenuation). The terms “gain” and “amply” are intended to be interchangeable. Also any reference to “addition”, “add” or “adding” may equally mean subtraction. Any reference signs in the claims shall not be construed so as to limit their scope.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0625516.0 | Dec 2006 | GB | national |
