DSP Room Mode Cancellation Without Altering Original Sound
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Solution Overview
Problem
Existing methods for eliminating room modes in a room, which distort the original sound, require high computing power and are limited to canceling only a few specific modes, failing to address the multitude of room modes present in different orientations.
Innovation Solution
A two-stage characteristic measurement using a digital signal processor to generate and store filters W*(z) and S'*(z) that map the sound changes of a user signal and a correction signal, respectively, allowing them to cancel out room modes without altering the original sound, using separate or combined loudspeakers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a filter adjusts the volume of problematic tones to eliminate room modes, then room modes are reduced, but the original sound is altered
Solution Approach 1:
The invention applies preliminary anti-action by generating an anti-phase signal before the room modes fully develop. The correction signal is created in advance with opposite polarity to the anticipated room modes, allowing them to cancel each other out before the modes can significantly distort the original sound. This prevents the harmful effect rather than correcting it after the fact.
Solution Approach 2:
The system performs preliminary action by measuring the room impulse response in advance and calculating the appropriate correction filter coefficients before actual audio playback. The filter W*(z) is pre-computed based on the room's acoustic characteristics, allowing the correction signal to be ready and applied immediately when music is played, ensuring the original sound remains intact while room modes are suppressed.
2Object-affected harmful factors
If a time-delayed correction signal is used to cancel room modes, then specific modes are eliminated, but only a small subset of all modes can be addressed
Solution Approach 1:
The invention applies parameter changes by using a frequency-dependent correction filter W*(z) that adjusts different frequency components differently. Instead of a single time delay for all frequencies, the filter modifies the amplitude and phase parameters across the frequency spectrum, allowing effective cancellation of multiple room modes at different frequencies simultaneously. This transforms the correction approach from a single-parameter time delay to a multi-parameter frequency-selective filter.
Solution Approach 2:
The correction filter W*(z) serves multiple functions simultaneously: it cancels longitudinal room modes, transversal modes, vertical modes, and their overtones across different frequency ranges. This universal filter approach replaces the need for separate correction mechanisms for each type of room mode, making the system versatile against all room mode types without requiring multiple specialized processors.
3Measurement precision
If precise time delay is determined for ANC signal superposition, then cancellation accuracy is improved, but computing power requirements become very high
Solution Approach 1:
The invention applies copying by creating a digital model of the room's acoustic impulse response and using this copied representation to calculate correction filters. Instead of performing complex real-time calculations for precise time delay determination, the system copies the room's acoustic characteristics into a filter model W*(z) that can be pre-computed and then applied efficiently during playback. This reduces the real-time computing burden while maintaining cancellation accuracy.
Solution Approach 2:
The system replaces the mechanical approach of precise time delay adjustment with an electronic filter-based solution. Instead of mechanically adjusting time delays to achieve cancellation, the invention uses digital signal processing with filter W*(z) that automatically provides the necessary phase and amplitude corrections across all frequencies. This substitution reduces the need for high-speed real-time time delay calculations while achieving superior cancellation accuracy.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method ensures the original sound is fully audible while effectively canceling out long-lasting room modes with relatively low computing power, suitable for use in consumer devices.
Implementation Method 1
The generated correction signal is played back by the correction loudspeaker. The two signals cancel each other out in the room.
Data Source
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AI summary
The invention relates to a method for eliminating room modes Ñ. The method consists substantially in using a digital signal processor (60) to generate and store a filter W*(z) in a two-stage characteristic measurement, which filter characterises and maps the sound changes of a user signal N emitted into a room (10) by a main loudspeaker (21), including the room modes Ñ generated in this way. The filter W*(z) generates a changed signal Ñ- from the digitally available user signal N, which is played by the main loudspeaker (21), to eliminate the room modes Ñ, which changed signal is played by a correction loudspeaker. The two signals cancel each other out in the room (10). Because passage through the filter W(z) requires a certain time dt, the original signal N remains completely audible in the room (10) and cannot be eliminated. The signal Ñ- can therefore eliminate only the portion of the soundwaves in the room (10) that is still present after this time dt. This ensures that the original signal N is audible in the room (10) completely and unchanged in comparison with the source, while the long-lasting room modes Ñ are eliminated after a short time dt. The main and correction loudspeakers (21, 22) may be the same loudspeaker (23). The invention also relates to a digital signal processor (60) provided for this purpose and to a loudspeaker (21, 22, 23).