NOISE CANCELLATION NOISE CANCELLATION AND SUPPRESSION AND SUPPRESSION TECHNIQUESTECHNIQUES

-Shruti Dasgupta

NOISE CANCELLATION IS NOISE CANCELLATION IS SIGNAL PROCESSINGSIGNAL PROCESSING Signals are carriers of information, both

useful and unwanted. Thus, extracting or enhancing the useful

information from a mix of conflicting information is a simplest form of signal processing

More generally, signal processing is an operation designed for extracting, enhancing, storing, and transmitting useful information.

A Noise Cancellation algorithm is to pass the corrupted signal through a filter that tends to suppress the noise while leaving the signal unchanged.

A noise-cancellation system generally emits a signal with the same amplitude but with inverted phase (also known as anitphase) to the original noise signal. The waves combine to form a new wave, in a process called interference, and effectively cancel each other out - an effect which is called phase cancellation. Depending on the circumstances and the method used, the resulting signal may be so faint as to be inaudible to human ears.

A noise-cancellation system may be co-located with the sound source to be attenuated. It must have the same audio power level as the source of the unwanted sound. Alternatively, the transducer emitting the cancellation signal may be located at the location where sound attenuation is wanted (e.g. the user's ear). This requires a much lower power level for cancellation but is effective only for a single user.

The active methods (above) differs from passive noise control methods in that a powered system is involved, rather than unpowered methods such as insulation, sound-absorbing ceiling tiles.

The advantages of active noise control methods compared to passive ones are that they are generally:

More effective at low frequencies. Less bulky. Able to block noise selectively.

TYPES OF NOISETYPES OF NOISE In general, noise that affects the speech signals can be

modeled using any one of the following: 1. White Noise: is a random signal with a flat power

spectral density. 2. Coloured Noise: refers to noise that may contain a

wide audible spectrum but shows a greater intensity in a narrow band of frequencies. An example is “whistling” wind.

3. Impulsive Noise: is a category of (acoustic) noise which includes unwanted, almost instantaneous sharp sounds (like clicks and pops). Noises of the kind are usually caused by electromagnetic interferences, scratches on the recording disks, and ill synchronization in digital recording and communication.

4.. Acoustic Echo: Acoustic echo arises when sound from a speaker—for example, the earpiece of a telephone handset—is picked up by the micraphone in the same room. The problem exists in any communications scenario where there is a speaker and a microphone.

WHITE NOISE SUPPRESSION WHITE NOISE SUPPRESSION TECHNIQUETECHNIQUE

Consider, the case when the Pi are quantizers for signal compression. A commonly used quantizer model replaces Pi each by an additive white noise of variance fi(bi)σi².

Here bi is the number of bits allotted to the quantizer, σi² is its input variance and fi is the normalized quantizer function which is assumed not to depend on the input statistics. If the quantization noise processes are uncorrelated to each other, the overall mean-square reconstruction error can be shown to be

The minimization objective here is this error g . For any given set of bits bi , the best Filter Bank (FB) is a Principal Component FB (PCFB) for the class for the given input spectrum .

Suppose the FB input x(n) in above figure is a signal corrupted by uncorrelated additive white noise, and the subband processors Pi are aimed at rejecting the noise components in their inputs.

Then Pi are chosen as any combination of constant multipliers, zeroth-order Wiener filters, and hard thresholds; the PCFB is the optimum FB.

IMPULSIVE NOISE IMPULSIVE NOISE COMPENSATIONCOMPENSATION A simple method of reducing the adverse effect of

impulsive noise is to precede conventional OFDM demodulator with a limiting nonlinearity.

The non-linearity reduces the effect of large received signal values as these are assumed to be the result of impulsive noise. The blanking nonlinearity gives a slightly better results, because in average replacing affected samples by zero is likely to introduce the least error energy. A major advantage of this method is that it is very simple to implement and gives an improvement over conventional OFDM demodulator in impulsive noise.

ACOUSTIC ECHO ACOUSTIC ECHO CANCELLATIONCANCELLATIONThe duration of the impulse response

of the acoustic echo path is usually 100-400ms and may change rapidly at any time.

The subband echo canceller is used for acoustic echo cancellation .

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The input and output signals of the echo path are passed through identical analysis filter banks A producing vectors of N subband signals which are sampled at a reduced rate.

The cancellation structure C forms a vector of subband signals Y to approximate the corresponding subband echo signal Y.

The resulting subband errors E are passed through a synthesis system S to give full band signal E, which is then transmitted back to the remote speaker.

The adaptation block utilizes the vector of subband error signals and input signals to adjust the input-output characteristics of the cancellation unit so as to drive the vector signal towards zero.

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