DIGITAL PROCESSING OF SPEECH SIGNAL USING THE PRE-EMPHASIS FILTER AND BAND-PASS FILTER

CHAPTER 1

AIM:

(a) Load, store display and manipulations of Speech signals.

(b) Plot the speech signals in time domain and frequency domain.

(c) Design of Pre-emphasis filter and Band-pass filters.

(d) Compute and display the response of Pre-emphasis and Band-pass filters.

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INTRODUCTION:

The project is about processing a speech signal using pre-emphasis and band pass filter. In processing electronic audio signals, pre-emphasis refers to a system process designed to increase (within a frequency band) the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system, and the system as a whole is called emphasis. Bandpass is an adjective that describes a type of filter or filtering process. it is to be distinguished from pass band, which refers to the actual portion of affected spectrum. Hence, one might say. A dual band pass filter has two pass bands." A band-pass filter is a signal containing a band of frequencies not adjacent to zero frequency, such as a signal that comes out of a band pass filter.

1.1 Pre Emphasis Filter:

In processing electronic audio signals, pre-emphasis refers to a system process designed to increase (within a frequency band) the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system, and the system as a whole is called emphasis.

Pre-emphasis is achieved with a  pre-emphasis network which is essentially a calibrated filter. The frequency response is decided by special time constants. The cut-off frequency can be calculated from that value.

Pre-emphasis  commonly used in telecommunications, digital audio recording, record cutting, in FM broadcasting transmissions, and in displaying the spectrograms of speech signals.

One example of this is the RIAA equalization curve on 33 rpm and 45 rpm vinyl records. Another is the Dolby noise-reduction system as used with magnetic tape.

In high speed digital transmission, pre-emphasis is used to improve signal quality at the output of a data transmission. In transmitting signals at high data rates, the transmission medium may introduce distortions, so pre-emphasis is used to distort the transmitted signal to correct for this distortion. When done properly this produces a received signal which more closely resembles the original or desired signal, allowing the use of higher frequencies or producing fewer bit errors.

Pre-emphasis is employed in frequency modulation or phase modulation transmitter to equalize the modulating signal drive power in terms of deviation ratio.

1.2 Band Pass Filter:

Bandpass is an adjective that describes a type of filter or filtering process. it is to be distinguished from pass band, which refers to the actual portion of affected spectrum. Hence, one might say. A dual band pass filter has two pass bands." A band-pass filter is a signal containing a band of frequencies not adjacent to zero frequency, such as a signal that comes out of a band pass filter.

An ideal band pass filter would have a completely flat pass band (e.g. with no gain/attenuation throughout) and would completely attenuate all frequencies outside the pass band. Additionally, the transition out of the pass band would be instantaneous in frequency. In practice, no band pass filter is ideal. The filter does not attenuate all frequencies outside the desired frequency range completely, in particular, there is a region just outside the intended pass band where frequencies are attenuated, but not rejected. This is known as the filter roll-off, and it is usually expressed in dB of attenuation per octave or decade of frequency. Generally, the design of a filter seeks to make the roll-off as narrow as possible, thus allowing the filter to perform as close as possible to its intended design. Often, this is achieved at the expense of pass-band or stop-band ripple.

The bandwidth of the filter is simply the difference between the upper and lower cutoff frequencies. The shape factor is the ratio of bandwidths measured using two different attenuation values to determine the cut-off frequency, e.g., a shape factor of 2:1 at 30/3 dB means the bandwidth measured between frequencies at 30 dB attenuation is twice that measured between frequencies at 3 dB attenuation.

Optical band-pass filters are common in photography and theatre lighting work. These filters take the form of a transparent coloured film or sheet.

CHAPTER  4

APPENDIX

SOURCE CODE:

4.1 Program to pass a speech signal into a Pre-Emphasis filter

[y,fs] = wavread(‘C:\Users\My Laptop\Desktop\Somebody_39_s_Me.wav’);

y1=y(100000:100100);

bpre = [ 1 -0.95 ];

apre = [ 1 ];

fvtool(bpre,apre);

ypre = filter(bpre,apre,y1);

soundsc(y,fs);

soundsc(ypre,fs)

4.2 Program to pass the Pre-Emphasised signal into a Band Pass filter

T=0:100;

Fp=1000;

Fs=1400;

2Fm=8000;

Wp=(fp/fm);

Ws=(fs/fm);

Wn=[wp ws];

N=8;

Y=fir1(N,wn,'bandpass');

z=filter(Y,1,ypre)

Plot(t,z)