Addition of virtual bass
Provided are, among other things, systems, methods and techniques for processing an audio signal to add virtual bass. In one representative embodiment, an apparatus includes: an input line that inputs an original audio signal; an estimator, coupled to the input line, that estimates a fundamental frequency of a bass sound within the original audio signal; a bass extraction filter, coupled to the input line, that extracts a bass portion of the original audio signal that is at least 1 octave wide and includes the fundamental frequency; a frequency translator, coupled to the bass extraction filter, that shifts the bass portion, in its entirety, by a positive frequency increment that is an integer multiple of the fundamental frequency estimated by the estimator, thereby providing a virtual bass signal; and an adder having 1) inputs coupled to the original audio signal and to the virtual bass signal and 2) an output.
1. An apparatus for processing an audio signal, comprising:
(a) an input line that inputs an original audio signal;
(b) an estimator, coupled to the input line, that estimates a fundamental frequency of a bass sound within said original audio signal;
(c) a bass extraction filter, coupled to the input line, that extracts a bass portion of said original audio signal, said bass portion being at least 1 octave wide and including the fundamental frequency;
(d) a frequency translator, coupled to the bass extraction filter, that shifts said at least 1 octave wide bass portion, in its entirety, by a positive frequency increment that is an integer multiple of the fundamental frequency estimated by said estimator, thereby providing a virtual bass signal; and
(e) an adder having (i) inputs coupled to said original audio signal and to said virtual bass signal and (ii) an output.
2. An apparatus according to claim 1 , wherein the bass extraction filter has a high-end cutoff frequency within a range of 100 Hz-200 Hz.
3. An apparatus according to claim 2 , wherein the bass extraction is a bandpass filter having a low-end cutoff frequency of at least 10 Hz.
4. An apparatus according to claim 1 , wherein said bass extraction filter is a bandpass filter having a low-end cutoff frequency of at least 15 Hz.
5. An apparatus according to claim 1 , wherein said bass portion is based on the fundamental frequency estimated by said estimator.
6. An apparatus according to claim 1 , wherein said bass extraction filter is a filter having a passband of at least 2 octaves.
7. An apparatus according to claim 1 , further comprising a loudness controller that adjusts a strength of said virtual bass signal based on a first estimate of a perceived loudness of said bass portion and a second estimate of a perceived loudness of said virtual bass signal.
8. An apparatus according to claim 7 , wherein the first estimate is based on an estimate of at least one of a sound pressure level (SPL) or a power of the bass portion.
9. An apparatus according to claim 7 , wherein said loudness controller also adjusts the strength of said virtual bass signal based on user input.
10. An apparatus according to claim 1 , wherein the fundamental frequency of said bass sound is constrained to fall within a specified one-octave range.
11. An apparatus according to claim 10 , wherein an initial estimate of the fundamental frequency that is outside the specified one-octave range is shifted into said specified one-octave range by changing an octave of said initial estimate of the fundamental frequency.
12. An apparatus according to claim 1 , wherein the frequency translator uses single-sideband (SSB) modulation.
13. An apparatus according to claim 1 , further comprising a high-pass filter that suppresses frequencies within said original audio signal that are not capable of being efficiently converted into sound by said audio output device.
14. An apparatus according to claim 1 , further comprising a delay element coupled between said input line and said adder that time-aligns the original audio signal with said virtual bass signal.
15. An apparatus according to claim 1 , wherein the estimator and the frequency translator operate in a frequency domain.
16. An apparatus according to claim 1 , wherein the bass extraction filter, the estimator and the frequency translator operate in a time domain.
17. An apparatus according to claim 1 , wherein the bass extraction filter, the estimator, the frequency translator and the adder are digital devices, and further comprising a digital-to-analog converter disposed between the adder and the audio output device.
18. An apparatus according to claim 1 , further comprising an audio output device coupled to the output of said adder.