Adaptive Low-Pass Filter for Speech Signal Bandwidth Transition
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Solution Overview
Problem
Existing speech codecs experience noticeable distortions when switching sampling rates, particularly during periods of speech inactivity or in the presence of background noise, due to sudden changes in audio bandwidth, which disrupt the conversation quality.
Innovation Solution
Implementing an adaptive low-pass filter with a variable cut-off frequency to smoothly transition between sampling rates, ensuring that the audio bandwidth changes gradually, masking the instant switch and maintaining conversation quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the sampling rate is switched immediately to adapt to network bandwidth changes, then the speech quality can be improved without exceeding available bandwidth, but the audio bandwidth changes suddenly causing noticeable distortion and disturbing the conversation
Solution Approach 1:
The patent applies preliminary action by gradually changing the cut-off frequency of the low-pass filter before the sampling rate switch occurs. This pre-adjustment of the audio bandwidth ensures that when the sampling rate changes, the transition is smooth and imperceptible to users. The filter frequency is adjusted in advance during speech activity periods, preparing the signal path for the upcoming sampling rate change without causing sudden audio bandwidth shifts that would be noticeable during speech inactivity.
2Manufacturing precision
If the sampling rate is switched during speech inactivity to minimize distortion, then the distortion in the frame is reduced, but the sudden change in audio bandwidth is still noticeable particularly during subsequent speech activity
Solution Approach 1:
The patent applies preliminary action by performing filter frequency adjustment during speech activity periods rather than during speech inactivity. This ensures that the audio bandwidth transition occurs when speech is present, making the change imperceptible to users. The system monitors speech activity and schedules the filter frequency change to coincide with active speech, thereby avoiding noticeable transitions during silent periods while maintaining encoding precision.
Solution Approach 2:
The patent applies parameter changes by dynamically adjusting the cut-off frequency of the low-pass filter based on speech activity detection. The filter frequency parameter is modified gradually over time rather than instantaneously, and this modification is timed to occur during speech activity when users are less sensitive to parameter changes. This dynamic parameter adjustment resolves the contradiction between maintaining encoding precision and avoiding noticeable speaker characteristic changes.
3Measurement precision
If a greater sampling rate is used to achieve higher speech signal quality, then the speech quality is improved, but the network bandwidth required to transmit the signal is increased
Solution Approach 1:
The patent applies dynamics by implementing a hybrid speech codec that can dynamically switch between multiple internal sampling rates based on available network bandwidth. The system monitors network conditions and adjusts the sampling rate accordingly, allowing the speech quality to adapt to current bandwidth availability. This dynamic adjustment enables the system to maintain high speech quality when bandwidth permits while reducing quality (and thus bandwidth consumption) when network conditions deteriorate, resolving the contradiction between speech quality and network bandwidth requirements.
Data Source
AI summary
A method and apparatus for transmitting an audio signal over a communication channel comprising encoding the audio signal with an encoder 204 using a first sampling rate, filtering the audio signal using a first cut off frequency, the first cut off frequency being chosen in dependence upon the first sampling rate, and transmitting the encoded and filtered audio signal over the communication channel. The presence of a condition in which the sampling rate of the encoder 204 is to be switched to a second sampling rate at a switching time is determined and if the condition has been determined to be present, the cut off frequency used in the filtering step is gradually changed from the first cut off frequency to a second cut off frequency, the second cut off frequency being chosen in dependence upon the second sampling rate, such that the audio bandwidth of the transmitted signal changes gradually when the sampling rate is switched to the second sampling rate.