This process is usually applied through external equipment or embedded in the speaker system using a digital signal processor.
By boosting high frequencies and overdriving the speaker or amplifier slightly, higher harmonics can be generated.
By low pass filtering the lowest octave and half-wave rectifying a waveform is created with a fundamental half of the original frequency.
Using such a subharmonic synthesizer the essential frequency band between 125 – 250 Hz is recreated, adding weight to the signal.
To extend the high-frequency bandwidth, we can isolate the top octave using high pass filtering and then generating harmonics of this.
The primary design goal of this algorithm is to reconstruct the high band spectrum without introducing any aliasing artifacts and to provide good spectral and time resolution.
At the encoder, the filterbank is used to obtain energy samples of the original input signal's high band.