Learning Introductory Signal Processing Using Multimedia 1 Outline Overview of Information and...

Learning Introductory Signal Processing Using Multimedia

1

Outline•Overview of Information and Communications

•Some signal processing concepts

•Tools available for use in the laboratory

•A selection of exercises


Roger Browne

Institute of Information Sciences and Technology Massey University


2

Overview of Information and Communications

The learning outcomes involve describing:

• the nature of information

• how information can be measured

• a basic communication systems model

• the nature of noise and its affect on communications

• A/D and D/A conversion, signals, modulation and Fourier theory

• some common coding schemes and how they can be used to combat the affect of noise

•application of discrete mathematics to simple linear systems

A second year paper that underpins the BTech (Information Engineering) and BE (Information and Telecommunication Engineering) degrees


3

Overview of Information and Communications

The learning outcomes involve describing:

• the nature of information

• how information can be measured

• a basic communication systems model

• the nature of noise and its affect on communications

• A/D and D/A conversion, signals, modulation and Fourier theory

• some common coding schemes and how they can be used to combat the affect of noise

• application of discrete mathematics to simple linear systems

The relevant sections are:


4

Some signal processing concepts

Source Encoder Tx Rx Decoder Output

Channel

+

Noise Usually electronic noise

Standard communications model:


5


Time domain and frequency domain views of signals

Time domain:


6


Time domain and frequency domain views of signals

Frequency domain:


7


Conversion from analogue to digital involves:

• sampling

• aliasing


8

Tools

• PC and its multi-media facilities (sound card, microphone, speakers)

• Signal processing software (Matlab)

• Very sophisticated personal signal processing facilities:

(our ears)


9

Exercise One: Spectrum Analysis

Square wave:

This has been studied using Fourier analysis in lectures


10

Sound Recorder

Document

Sound of a square wave:

Spectrum of a square wave


11

Spectrum of a square wave

A filter selects a range of frequencies.

For instance, selecting frequencies between 500Hz and 2000Hz results in a different sound:

Sound Recorder

Document

Filtered signal at original volume:

Amplified version of filtered signal:


12

Viewing this in frequency space: Sound Recorder

Document


13

Exercise Two - Noise in Communications

Noise usually refers to electronic noise, but could be acoustic noise.

Sound Recorder

Document

Original signal:

SNR = 0dB

SNR = -10dB

Sound Recorder

Document

Sound Recorder

Document


14

Sound Recorder

Document

SNR = -20dB

SNR = -30dB Sound Recorder

Document

•The signal frequency is 1300Hz

•The noise is ‘wide-band’ or ‘white’ noise, occupying the full frequency spectrum.

Objective: design a filter to retrieve the signal in the case of SNR = -30dB

Noise in Communications


15


Technique: develop a narrow-band filter centred on 1300Hz.

Sound Recorder

Document

Filter = 1200Hz to 1400Hz:



Sound Recorder

Document

Sound Recorder

Document


16

Sound Recorder

Document

What about noisy speech signals?


Original:

SNR = 0dB:

Band-pass filter, 200Hz to 1500Hz:

Sound Recorder

Document

Sound Recorder

Document

No amount of filtering achieves a clear speech signal


17


The way in which the frequencies are spread across the spectrum can be viewed pictorially:

Hence a narrow-band filter is not effective.


18

Exercise Three - Aliasing

This is a concept that students often find difficult.

It is a characteristic of the process of converting from an analogue signal to a digital signal.

A fundamental law in signal processing states that the sampling frequency must be at least twice the highest frequency present in the signal.

For example, CDs are sampled at 44.1kHz so the highest frequency that can be present in the signal is 22kHz.


19

Aliasing

If a higher frequency is present it ‘folds over’ into the lower frequencies.

This can be illustrated by taking a simple signal of 1000Hz and steadily reducing the sampling frequency.

The minimum sampling frequency for correctly recording the sound is 2000Hz.


20

Aliasing

Sound Recorder

Document

Original:

Sampled at 3000Hz:

Sampled at 1800Hz:

Sampled at 1500Hz:

Sampled at 1200Hz:

Sound Recorder

Document

Sound Recorder

Document

Students are asked to estimate the frequency in each case.

Sound Recorder

Document

Sound Recorder

Document

(1000Hz)


21

Aliasing

Their estimates are plotted against sampling frequency:

1000Hz 2000Hz

Sampling frequency

Est

imat

ed f

requ

ency

1000Hz

normal

aliased


22

Aliasing

What is the affect on speech?

Original:

Sampled at 2000Hz:

Sampled at 1000Hz:

Filtered 0-500Hz:

Sound Recorder

Document

Sound Recorder

Document

Sound Recorder

Document

Frequencies greater than 500Hz sound as lower frequencies, creating distortion


23

• The modern multimedia computer offers excellent facilities for basic experiments in signal processing.

• By making use of:

� audible signals

� the sophisticated signal processing of the human ear,

a number of basic concepts can be exemplified in a concrete and readily-assimilated form.

Conclusions

Learning Introductory Signal Processing Using Multimedia 1 Outline Overview of Information and...

Documents

Transcript of Learning Introductory Signal Processing Using Multimedia 1 Outline Overview of Information and...