MULTIMEDIA TUTORIAL

PART - III

SHASHI BHUSHAN

SOCIS, IGNOU

Introduction to Sound

When something vibrates in the air it creates waves of pressure. When these waves reach your ear, the eardrum vibrates, causing the tiny bones of the inner ear to vibrate along with it, sending nerve pulse to the brain. These pulses are perceived as sound by the listener.

Introduction to Sound

In a similar way when an acoustic wave strikes a microphone, the microphone generates an electrical signal, representing the sound amplitude as a function of time.

Introduction to Sound

The representation of sound, processing, storage and transmission of such audio signals are a major part of study of multimedia systems.

Introduction to Sound

Except air, vibrations can also travel through other materials such as

Wood

Metal

Water

The speed of travel of the vibrations differ from material to material.

Introduction to Sound

Sound involves the followings

Vibrating objects (Sound source)Transmission medium (air/water/

conducting medium

A receiver (ear)

A preceptor (brain)

Introduction to Sound

Sound wave vary in loudness (measured in decibels or dB) and in frequency or pitch (vibration per second, measured in Hertz or Hz.

Introduction to Sound

When a wave travels, only the energy of wave motion is propagated through the medium; no portion of the medium itself actually moves very far.

Introduction to Sound

In Human learning takes places whenever vibrations of frequencies between 20 hertz and 20 KHz reach inner ear.

Introduction to Sound

Acoustics is the science of sound. Sound pressure levels (loudness or volume) are measured in decibels or dB; decibel measurements is actually ratio between a chosen reference point on logarithmic scale and a level that is actually experienced.

Introduction to Sound

dB = 20 log10100(A/B)

When you make the sound 100 times more intense, the increase in dB not hundred fold, but only 20 dB.

Audio Basic Concepts

Some of the important features are as follows :

Waveforms Sound Attribute (pitch, amplitude, quality)

Noise

Frequency

Intensity / Volume

Audio Basic Concepts

Waveforms : A waveform that repeats itself at regular intervals is called a periodic waveform. The unit of repetition in a periodic waveform a called a cycle.

Audio Basic Concept

Some periodic waveforms such as sine wave, square wave, have only one frequency component , complex waveforms contain several frequencies simultaneously complex waveform can always be analyzed as a sum of constituent sine wave.

Attribute of Sound

Sound is described in terms of sound characteristics pitch, amplitude and quality.

Attribute of Sound: Pitch

Pitch is basically vibrations per second measured in Hertz. Some Objects like strings vibrate as discrete well defined frequencies is discernable to humans as pitch. Humane pitch perception lies between 20 and 20,000 cycles/s.

Attribute of Sound: Amplitude

The perception of amplitude is called loudness. Loudness is usually measured in dB. The greater the amplitude of the wave, the louder the sound is perceived.

Attribute of Sound: Amplitude

Decibels describe a logrithmic relationship between the amplitudes of two sounds where a doubling of loudness corresponds to an increase of 6dB.

Amplitude

Amplitude of a sample is stored as a set of bits the higher the number of bits used to store a sound sample, the greater the amplitude. In an 8 bit sound card, 256 different levels of sound are recorded, while a 16 bit card provides 65,536 different, levels. For a high fidelity, 16 bit card is preferable.

Dynamic Range

The difference between the loudest and softest amplitudes that an audio system can produce is called its dynamic range. Good audio system have a dynamic range of around 96 dB.

Noise

Periodic waveforms that do not exhibit discernable regularity are called noise. The signal to noise ratio the ratio in dB between the dynamic range and the ambient noise produced by the audio system itself. The higher the signal to noise number, the better the audio system.

Frequency

Refers to the speed of vibrations that causes a sound which is measured in cycle per second (Hz). In other words the frequency of a sound is the number of the cycles or oscillations completed in a given time.

Frequency

The frequency of sound cards various from 11Khz to 44Khz the higher the frequency, the greater the fidelity of sound.

Intensity/Volume

The distance at which a sound can be heard depends on its intensity, which is the average rate of flow of energy per unit area perpendicular to the direction of propagation.

Analog versus digital

Communications can be classified in the two categories

(i) Analog

(ii) Digital

Analog versus digital

In the analog form of electronic communication information is represented as a continuous electromagnetic wave form. Digital communication represents information in binary form through a series of discrete pulses.

Analog

Analog signal is a signal that is transmitted as a continuous wave. Analog signal changes all the time. These signals are generally represented as a wave

Analog

Analog audio track contain sound signals and control tracks, contain signals that keep the recorded images from tilting or merging into one another when they appear the on the TV Screen

Analog Audio

Analog audio tracks and control tracks are recorded by separate stationary heads. The tracks are recorded vertically. Audio tracks run along one edge of the tape and control tracks along the other

Digital Audio

Audio waves can be converted to digital form by an ADC (Analog Digital Converter) An ADC takes an electrical voltage as input and generates binary number output

Digital Audio

You can digitize sound from microphone, a synthesizer, existing tape recordings, live Radio and Television broadcast. In fact you can digitize sound from any source, natural or prerecorded.

Digital Audio

Digitized sound is sampled sound, Every nth fraction of a second, a sample of sound is taken and stored as digital information in bits & bits.

Digital Audio

How often the samples are taken is the sampling rate and amount of information stored about each sample is the sample size.

Digital Audio

The more often you take a sample and the more data you store about that sample, the finer the resolution and quality of the captured sound when it is played back.

Digital Audio

The three sampling frequencies used in multimedia are CD audio quality 44.1 KHz, 22.05 KHz and 11.025 KHz. Sample sizes and either 8 bits or 16 bits.

Digital Audio

The larger the sample size, the better the data describes the recorded sound. An 8 bit sample size provides 256 equal units to describe the dynamic range or amplitude (the level of sound at the time) of the slice of the sound compound.

Digital Audio

A 16 bit sample size on the other hand provides a staggering 65,536 equal units to describe the dynamic range.

Digital Audio

Slices of analog waveform are sampled at various frequencies and each discrete sample is then stored either as 8 or 16 bit of data.

Digital Audio

Digital samples are never exact 3 bit samples allows only 8 values, from –1.00 to +1.00 in steps of 0.25

Digital Audio

An 8 bit sample would allow 256 distinct values. A 16 bit sample would allow 65,536 distinct values.

Digital Audio

The value of each sample is recorded to the nearest integer (also called quantization) and if the amplitude is greater than internal available, greater than the internal available clipping of the tops and bottoms of the waves occurs.

Digital Audio

Two well known exampled sound are telephone and audio CD, PCM, as used within telephone system, uses 7 bit (North America & Japan) or 8 bit (Europe) samples 8000 times per seconds. This gives data rate of 56,000 bps or 64,000 bps

Digital Audio

Audio CDs are digital with a sample rate of 44,100 samples/sec, enough to capture frequencies upto 22,050 Hz which is good for people.

File Size Vs. Quality

Remember that the sampling rate determines the frequency make up of the recording. Sampling at higher rates more accurately captures the high frequency content of your sound.

File Size Vs. Quality

Resolution determines the accuracy with which a sound can be digital. Using more bit yields a recording that sounds more like its original.

Digital Audio

Remember that the sampling that data the frequency make up of the recording. Sampling at higher rates more accurately captures the high frequency context of you sound.