Group delay correction in acoustic transducer systems
Methods are provided for equalizing the group delay of a sound reproduction system, in particular a system comprising acoustic transducers with at least one crossover between a lower-frequency and a higher-frequency range. A correction is applied to a signal in the lower-frequency range, including the crossover region, to substantially equalize the group delay for the lower-frequency range, and a signal delay is applied to a signal in the higher-frequency range to bring it into closer alignment with the equalized lower-frequency range signal. The methods may be implemented in the design of an acoustic transducer system and also via a computer program product, which can be implemented as an update or enhancement to an existing digital signal processor loudspeaker system.
1. A method for equalising overall group delay in the response of an acoustic transducer system having a crossover for dividing an input signal between a lower-frequency range and a higher-frequency range, the method comprising the steps of:
obtaining a first signal and a second signal derived from the input signal;
equalising the first signal in the lower frequency range by applying a correction to the first signal in the lower-frequency range, including the crossover region, to equalise a group delay of the lower-frequency range when subject to the response of the acoustic transducer system; and
aligning a timing of the second signal in the higher-frequency range into closer alignment with a timing of the equalised first signal in the lower-frequency range when subject to the response of the acoustic transducer system by applying a signal delay to the second signal.
2. A method according to claim 1 , further comprising joining the equalised first signal and the aligned second signal to generate an output signal, and wherein the obtaining comprises splitting the input signal into the lower-frequency range and the higher-frequency range to derive the first and second signal, respectively.
3. A method according to claim 2 , wherein the splitting is performed using a bandsplit process, and the joining is performed using a band-join process.
4. A method according to claim 1 , wherein the applying the correction to the first signal in the lower-frequency range comprises applying at least one all-pass filter to the first signal in the lower-frequency range, including the crossover region, to equalise the group delay of the lower-frequency range.
5. A method according to claim 4 , wherein the applying the correction to the first signal in the lower-frequency range comprises applying a combination of all-pass filters to the first signal in the lower-frequency range, including the crossover region, to equalise the group delay of the lower-frequency range.
6. A method according to claim 1 , wherein the applying the correction to the first signal in the lower-frequency range comprises applying a combination reverse-block processing to the first signal in the lower-frequency range, including the crossover region, to equalise the group delay for of lower-frequency range.
7. A method according to claim 6 , wherein the combination reverse-block processing comprises reverse-block processing the first signal in the lower-frequency range, including the crossover region, to determine an all-pass filter combination which would substantially equalise the group delay of the low-frequency range, and applying the determined all-pass filter combination to said first signal in the lower-frequency range signal to equalise the group delay.
8. A method according to claim 7 , further comprising down-sampling the first signal in the lower-frequency-range prior to the combination reverse-block processing.
9. A method according to claim 8 , further comprising up-sampling the equalised first signal to its original or a different sample rate.
10. A method according to claim 6 , wherein the combination reverse-block processing of the first signal utilises a plurality of buffers.
11. A method according to claim 6 , wherein the applying the signal delay to the second signal comprises buffering the second signal in the higher-frequency range during the combination reverse-block processing of the first signal.
12. A non-transitory computer-readable medium storing instructions which, when executed by a processor, cause the processor to perform operations for equalizing an overall group delay in a response of an acoustic transducer system having a crossover for dividing an input signal between a lower-frequency range and a higher-frequency range, the operations comprising:
obtaining a first signal and a second signal derived from the input signal;
equalising the first signal in the lower frequency range by applying correction to a the first signal in the lower-frequency range, including the crossover region, to equalise a group delay of the lower-frequency range when subject to the response of the acoustic transducer system; and
aligning a timing of the second signal in the higher-frequency range into closer alignment with a timing of the equalised first signal in the lower-frequency range when subject to the response of the acoustic transducer system by applying a signal delay to the second signal.
13. A non-transitory computer-readable medium according to claim 12 , wherein the instructions are implemented as an update or an enhancement to an existing digital signal processor (DSP) loudspeaker system.
14. A non-transitory computer-readable medium according to claim 12 , wherein the instructions are implemented as an update or an enhancement to an existing multichannel or stereo audio processor.
15. An acoustic transducer system having a crossover for dividing an input signal between a lower-frequency range and a higher-frequency range, the acoustic transducer system comprising:
a unit for obtaining a first signal and a second signal derived from the input signal;
a correction component to equalise the first signal in the lower-frequency range, including the crossover region, by applying correction to equalize a group delay of the lower-frequency range; and
a delay component to align a timing of the second signal in the higher-frequency range into closer alignment with a timing of the equalized first signal in the lower-frequency range by applying a signal delay to the second signal.
16. An acoustic transducer system according to claim 15 , comprising low and upper range sections of a multi-way loudspeaker or other transducer combination.
17. An acoustic transducer system according to claim 15 , further comprising a frequency-adjacent pair of driver ranges of a multi-way system, not including the lowest-frequency range.
18. An acoustic transducer system according to claim 15 , further comprising a low-frequency system in a separate enclosure or cabinet and one or more loudspeakers, wherein there exists a crossover.
19. An acoustic transducer system according to claim 15 , further comprising a compensating signal-processing device which comprises:
a band splitter for splitting the input signal into the lower-frequency range and the higher-frequency range to derive the first signal and the second signal respectively; and
a band joiner for joining the equalised first signal and the aligned second signal to provide a composite signal which equalises the overall group delay of the acoustic transducer system.
20. A compensating signal-processing device for equalising overall group delay in the response of an acoustic transducer system having a crossover for dividing an input signal between a lower-frequency range and a higher-frequency range, the device comprising:
a band splitter for splitting the input signal into the lower-frequency range and the higher-frequency range to derive a first signal and a second signal respectively;
a correction component to equalise the first signal in the lower-frequency range, including the crossover region, by applying correction to equalize a group delay of the lower-frequency range when subject to the response of the acoustic transducer system;
a delay component to align a timing of the second signal in the higher-frequency range into closer alignment with a timing of the equalized first signal in the lower-frequency range when subject to the response of the acoustic transducer system by applying a signal delay to the second signal; and
a band joiner for joining the equalised first signal and the aligned second signal to provide a composite signal which equalises the overall group delay of the acoustic transducer system.