AudioQAM
1. Overview of AudioQAM
AudioQAM simultaneously modulates phase and amplitude of audio input, and as a result, creates unexpected sound.
Quadrature Amplitude Modulation (QAM) is widely used in digital communications. AudioQAM is derived from digital quadrature amplitude modulators and refined for audio processing. AudioQAM supports sine as well as triangle, square, and sawtooth waveforms for carrier waves.
In the case of the sine waveform carrier, AudioQAM is essentially equivalent to the phase shift network and thus works as the well-known Bode frequency shifter. However, the low-frequency shift of conventional frequency shifters results in the spectral inversion band. Since it typically creates noisy sounds, AudioQAM supports automatic input band-limiting that suppresses the spectral inversion band.
In the triangle, square, and sawtooth waveform carrier cases, the output spectrum is the convolution of the carrier wave and audio input spectrums, therefore creating unknown sound. AudioQAM automatic input band-limiting also optimizes superimposed spectrum by the convolution, thus it could create clearer sound.
AudioQAM is provided as a VST 3 plug-in for digital audio workstations and supports any sampling rates. OS environment is 64bit Windows 10 and later.
AudioQAM binary distribution is licensed under Creative Commons Attribution 4.0 (CC BY 4.0) at no charge.
AudioQAM source code distribution is licensed under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 (CC BY-NC-SA 4.0) at no charge.
2. Block diagram of AudioQAM
A single-channel block diagram of AudioQAM is shown in Fig. 1.
The structure of AudioQAM is simple. A pair of HPF and LPF are equipped at input and output respectively. Hilbert Transformer outputs the input signal and π/2 phase lag signal of the input. Direct Digital Synthesizer generates highly accurate broadband sine, triangle, square, and sawtooth wave signals and π/2 phase lag signals of these. The amplitudes of these quadrature signals are modulated by Ring Modulators.
3. Operation of AudioQAM
User interface of AudioQAM is shown in Fig. 2.
All of the blue-letter numerical numbers are directly editable.
The frequency of the carrier wave could also be set from the blue slider.
The scale of the slider is selected from linear or logarithmic.
The range of the slider is selected from between ±50Hz and ±3,200Hz.
The waveform of the carrier is selected from the sine, triangle, square, or sawtooth wave.
HPFs and LPFs of input and output could be set cutoff frequencies.
In the case of the sine waveform carrier in the automatic band-limiting mode, the cutoff frequency of the input HPF is automatically set based on the carrier wave frequency. Similarly, in the triangle, square, and sawtooth waveform carrier cases in the mode, the cutoff frequency of the input LPF is automatically set. In the latter cases, the most important filter is input LPF. Especially, in cases of square and sawtooth carrier waveforms, impressive interesting sounds may result from setting the cutoff frequency to a roughly absolute value of the carrier frequency.
The input HPF is effective for low-frequency noise reduction. The output LPF is also effective for high-frequency noise reduction, especially in cases of square and sawtooth carrier waveforms.
The input and output mix ratio is set from wet/dry.
VST is a registered trademark of Steinberg Media Technologies GmbH.
