In a packet communication system, the delay time needed in a jitter buffer is determined, enabling a smooth data feed to an application without excessive delays, by methods and apparatus that vary the size of the jitter buffer based on an estimated variation of packet transmission delay derived from the times of arrival of stored packets. A variance buffer stores variances of the times of arrival of stored packets, and the estimated variation of packet transmission delay is derived from the stored variances. The size of the jitter buffer can be changed preferentially during periods of discontinuous packet transmission.

Adaptive jitter buffering

A communication system having an echo canceller is disclosed. One embodiment of the echo canceller includes an adaptive filter used to provide an estimate of reflected echo which is removed from the send signal. The echo canceller may also include a near-end talker signal detector which may be used to prevent the adaptive filter from adapting when a near-end talker signal is present. The echo canceller may also include a nonlinear processor used to further reduce any residual echo and to preserve background noise. The echo canceller may also include a monitor and control unit which may be used to monitor the filter coefficients and gain of the adaptive filter to maintain stability of the echo canceller, estimate pure delay, detect a tone, and inject a training signal. The echo canceller may also include a nonadaptive filter used to reduce the length of the adaptive filter.

Monitoring and control of an adaptive filter in a communication system

Method and apparatus for performing adaptive filtering

Milling exhibits forced vibrations at tooth passing frequency and its harmonics, as well as chatter vibrations close to one of the natural modes. In addition, there are sidebands, which are spread at the multiples of tooth passing frequency above and below the chatter frequency, and make the robust chatter detection difficult. This paper presents a novel on-line chatter detection method by monitoring the vibration energy. Forced vibrations are removed from the measurements in discrete time domain using a Kalman filter. After removing all periodic components, the amplitude and frequency of chatter are searched in between the two consecutive tooth passing frequency harmonics using a nonlinear energy operator (NEO). When the energy of any chatter component grows relative to the energy of forced vibrations, the presence of chatter is detected. The proposed method works in discrete real time intervals, and can detect the chatter earlier than frequency domain-based methods, which rely on fast Fourier Transforms. The method has been experimentally validated in several milling tests using both microphone and accelerometer measurements, as well as using spindle speed and current signals.

On-line energy-based milling chatter detection

Teager–Kaiser energy methods for signal and image analysis: A review

Based on physical considerations, Kaiser (1990) proposed a new energy function for discrete time signals, known as the Teager-Kaiser energy function (TKEF). An interesting property of the TKEF is that if the input signal consists of two closely-spaced tones (amplitude modulated signal), the TKEF produces the difference frequency tone (envelope signal). However, a drawback is that the TKEF is highly sensitive to additive noise. We propose a generalized TKEF (GTKEF) to reduce the noise sensitivity. Fortunately, it turns out that the generalization can be used to enhance the sum frequency tone as well. This enables us to apply the GTKEF to resolve of two closely-spaced tones. The result can be viewed as an interesting nonlinear preprocessing scheme that can be used to transform a signal consisting of two closeby frequencies at f/sub 1/ and f/sub 2/ into a signal consisting of frequencies at (f/sub 1/-f/sub 2/) and (f/sub 1/+f/sub 2/). Clearly, it is much easier to resolve the latter frequencies, whichever spectral analysis method is used.

A generalization to the Teager-Kaiser energy function and application to resolving two closely-spaced tones

A tone blocking system and method for use preferably in conferencing systems in order to prevent control tones from being transmitted to other conferees is disclosed. The buffer length used to process the signal and detect tones is varied, being increased when a tone is suspected to allow for sophisticated tone detection algorithms, and being decreased when silence is present by trimming the silence away. The technique minimizes delay, and its degrading effect on echo, but nonetheless provides for a lengthy buffer required to do reliable tone detection. The invention is applicable to any signal other than tone as well.

Wei Lin

Papers

A generalization to the Teager-Kaiser energy function and application to resolving two closely-spaced tones

Tone blocking using variable delay buffer