Raised-Cosine Signal Generation for Audio Pop and Click Reduction
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Solution Overview
Problem
Audio amplifiers often produce audible pops and clicks during power switching or mute/unmute operations, which existing technologies have not adequately addressed.
Innovation Solution
A digital signal generator comprising a counter, comparator, and integrator that generates an approximation of a raised cosine waveform by counting up or down from an initial value, with the count direction determined by a threshold comparison, and integrates the counter output to produce a digital signal for application to the amplifier stage during power or mute state changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a linear ramp voltage is applied to gradually increase or decrease the amplifier voltage, then audio artefacts are reduced, but the transition smoothness is insufficient compared to raised cosine waveforms
Solution Approach 1:
The patent changes the voltage transition parameter from a linear ramp to an approximated raised cosine waveform. This is achieved by using a counter to generate a triangle wave that is then integrated to produce the smooth raised cosine shape, fundamentally changing the mathematical parameter of the transition waveform to achieve better audio artefact reduction.
Solution Approach 2:
The integrator circuit acts as an intermediary between the counter output and the final voltage transition signal. By integrating the triangle wave from the counter, the system generates the smooth raised cosine waveform without requiring complex direct generation circuitry, thus achieving high transition smoothness through an intermediate processing step.
2Reliability
If a true raised cosine waveform is generated, then optimal audio artefact reduction is achieved, but hardware complexity increases
Solution Approach 1:
Instead of generating a mathematically precise raised cosine waveform, the patent creates a simplified copy or approximation using basic digital circuits. The counter and integrator produce a waveform that closely mimics the beneficial characteristics of a true raised cosine without requiring the complex computation and circuitry needed for an exact implementation.
Solution Approach 2:
The patent replaces expensive, complex raised cosine generation hardware with inexpensive, simple digital logic components (counter and integrator). This substitution achieves comparable audio artefact reduction performance using much simpler, cheaper circuitry that is easier to implement in practical audio amplifier systems.
3Device complexity
If the counter counts continuously in one direction, then the circuit is simple, but the waveform cannot achieve the smooth transition characteristics of raised cosine
Solution Approach 1:
The counter is configured to periodically reverse direction, creating a triangular waveform through periodic up-counting and down-counting cycles. This periodic bidirectional action of the counter, combined with integration, generates the smooth raised cosine waveform characteristics needed for optimal audio transitions while maintaining relatively simple circuit implementation.
Data Source
Figure 1~2B
Figure 3A
Figure 3B~3C
AI summary
A digital signal generator apparatus and method is described. The digital signal generator (400) includes a counter (410), an integrator (414) and a comparator (408). The counter counts up or down from an initial counter value dependent on a counter control input. The comparator has a first input coupled to the counter output (418), a threshold input (416) and a comparator output (412) coupled to the counter control input. The integrator has an input (418) coupled to the counter output and an output coupled to the digital signal generator output (428). The digital signal generator determines the count direction after the initial direction dependent on the comparison between a threshold value applied to the threshold input and the counter output value. The digital signal generator may implement the generation of a waveform having an approximation to a raised cosine function. The generated waveform may be used for audio artefact reduction in an audio amplifier during mute or unmute operations or during power up power down operations.