System for limiting receive audio
Patent No. US7894598 (titled "System for limiting receive audio") on Dec 14, 2004. The application was issued on Feb 22, 2011.
What is this patent about?
'598 is related to the field of audio processing, specifically echo cancellation in communication systems like speakerphones or hands-free devices. These systems often suffer from acoustic feedback where the loudspeaker's output is picked up by the microphone, creating an echo that degrades communication quality. Traditional echo cancellation techniques struggle when the audio signal is too loud, leading to clipping at the microphone.
The underlying idea behind '598 is to prevent microphone clipping by proactively limiting the audio signal sent to the loudspeaker. This is achieved by estimating the power of the sound that will reach the microphone if the signal is played at full volume. If this estimated power exceeds a certain threshold known to cause clipping, the signal is attenuated before it reaches the loudspeaker.
The claims of '598 focus on a system that includes an echo power estimator to predict the audio power at the microphone, and a limiter that reduces the loudspeaker's output signal when the estimated power exceeds a predefined clipping threshold. The claims also cover communication systems and echo cancellation systems incorporating this limiting functionality, as well as a method for performing echo cancellation by estimating power and limiting the audio signal.
In practice, the system uses a low-order Infinite Impulse Response (IIR) filter to model the loudspeaker-enclosure-microphone (LEM) system's transfer function. This filter provides a simplified estimate of how the loudspeaker's output will be perceived by the microphone. Based on this estimate, a short-term power calculation determines if the signal needs to be attenuated. A soft limiter then adjusts the gain of the audio signal to prevent clipping, ensuring the LEM system remains in its linear operating range.
Unlike prior approaches that react to clipping after it occurs, '598 anticipates and prevents it. By modeling the LEM system and estimating the power at the microphone, the system can proactively limit the loudspeaker's output. This ensures that the echo cancellation filter receives a clean, unclipped signal, leading to more effective echo removal and improved audio quality. The use of a low-order IIR filter provides a computationally efficient way to estimate the power without requiring a highly detailed model of the LEM system.
How does this patent fit in bigger picture?
Technical Landscape
In the mid-2000s when ’598 was filed, acoustic echo cancellation was typically implemented using linear adaptive filters that relied on a stable relationship between the loudspeaker output and the microphone input. At a time when hands-free communication systems were increasingly integrated into confined environments like automobile cabins, hardware constraints made managing signal non-linearities non-trivial. Systems commonly relied on standard subtraction of an estimated echo signal from the microphone pickup, but these models often failed when high volume levels or physical proximity caused the microphone or the analog-to-digital gain stage to clip. Because such clipping introduced unpredictable distortions into the signal path, maintaining a linear transfer function was a primary engineering challenge for ensuring clear duplex communication.
Prosecution Position
The examiner allowed the application because the prior art did not suggest a system that automatically limits an incoming microphone signal based on a specific calculation. Specifically, the examiner noted that it was not obvious to use an echo estimate to determine the exact amount of pure echo signal that would cause a microphone to overload, and then use that threshold to proactively limit the audio in a communication terminal.
Claims
This patent contains 29 claims, with independent claims 1, 8, 15, 21, and 26. The independent claims generally focus on systems and methods for limiting output signals to a speaker or canceling echo in a communication system by estimating audio power or echo signals and limiting the output based on a threshold. The dependent claims generally elaborate on specific components, features, or implementations of the systems and methods described in the independent claims, such as specific types of filters, models, or frequency sub-band limitations.
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US7894598
- Application Number
- US11012079A
- Filing Date
- Dec 14, 2004
- Publication Date
- Feb 22, 2011
- External Links
- Slate, USPTO, Google Patents