UNIT III: Multimedia Audio
Nature of sound waves,
characteristics of sound waves,
elements of audio systems: Microphone, amplifiers, speakers,
synthesizer, MIDI, digital audio, CD formats.
Audio File Format: WAV, AIFF, VOC, AVI, AVO, MPEG-I, MPEG-II,
MPEG-III, MPEG-IV, RMF, WMA, MPC.
Sound is a form of energy similar to heat and light. Sound is
generated from vibrating objects and can flow through a material
medium from one place to another. During generation, the
kinetic energy of the vibrating body is converted to sound
Eg. spreading wave over the surface of water.
Acoustics is the branch of science dealing with the study of sound
and is concerned with the generation, transmission and
reception of sound waves.
Nature of sound waves,
Sound is an alteration in pressure, particle displacement or
particle velocity propagated in an elastic material
Characteristics of sound
1. Amplitude – it is the maximum displacement of a particle in the
path of a wave and is a measure of the peak to peak height of the
wave. The physical meaning of amplitude is the intensity of energy
of the wave. For sound waves this corresponds to the loudness of
sound (measured in decibel – dB).
Larger the energy of the sound wave, more is its amplitude and
louder its perception in our ears.
Characteristics of sound
2. Frequency measures the number of vibrations of a particle in the
path of a wave, in one second. The physical meaning of frequency
of a sound wave is the pitch of sound. A high pitched sound
(whistle) has higher frequency than a dull flat sound (drum).
Frequency is measured in Hertz (Hz)
3. Waveform indicates the actual shape of the wave when
represented pictorially. Shapes of waves can be sinusoidal, square,
triangular. Complex sounds can be arbitrary or irregular in shape.
Physical meaning of waveform is quality of sound (guitar or violin).
Characteristics of sound
4. Speed depends on the medium through which the sound travels
and the temperature of the medium but not the pressure.
Psycho acoustics is the branch of acoustics which deals with the
human auditory perception ranging from the biological design of the
ear to the brain's interpretation of the aural information.
1. Decibel (dB) – level of loudness is logarithmic.
2. Sound Measurement – 2 popular methods.
Direct Method (measuring a set of environmental factors like
temperature, humidity, viscosity etc to compute sound energy
Indirect Method (measuring sound pressure level of a
reference sound of known energy levels and comparing those
pressure levels with the sound measured.)
Sound pressure is the amount of pressure the sound waves exert
on a surface eg diaphragm of a microphone and provides a way to
compute its energy content. (SPL)
SPL (dB) = 20log10 (Pressure A / Pressure B)
3. Hearing Threshold and Masking
2 fundamental phenomenon that govern human hearing are the
minimum hearing threshold(least audible sound) and the amplitude
masking(occurs because an audible sound has a tendency to
distort the threshold curve and shift it upwards).
Elements of Audio System
Microphone – input of sound to the audio system. Converts sound
energy to electrical energy. Sound pressure exists as patterns of air
pressure. The microphone changes this information into patterns of
electrical current. There are 2 types of microphones :–
1. moving coil type
Consists of thin metallic diaphragm and an attached coil of wire.
A magnet produces a magnetic field which surrounds the coil. As
sound impinges on the diaphragm attached to the coil, it causes
movement of the coil producing a proportionate intensity current.
2. condenser type
The diaphragm is actually the plate of a capacitor, which moves
on the receipt of sound by changing the capacitance and generating
Based on directional properties, microphones are of 3 types :
1. Omni Directional Microphone – sensitive to sound coming
from all directions. Eg – environmental sounds in a wild life video
2. Bi Directional Microphone – sensitive to sound coming from
two directions ie front and rear. Eg – 2 persons sitting on
opposite sides of a table.
3. Uni Directional Microphone – records sound from a single
Note : Polar Plot (cardiod) of a microphone is a graph plotting the
output level of the microphone against the angle at which the
incident sound is produced.
90 90 90
Polar Plots of omni directional, bi directional and uni directional
It is a device in which varying signal controls a flow of energy to
produce an output signal that varies in the same way but has a
larger amplitude. Input signal may be current, voltage, mechanical
motion etc. output signal is usually of the same nature.
Audio amplifiers are electronic in nature and use a series of
transistors as their principal components connected in a printed
circuit board. Amplification is not 100% due to noise and produces
some distortion of the signal.
The ratio of the output amplitude to the input amplitude is known as
the gain of the amplifier.
Amplifier circuits are designated as A, B, AB and C (for analog
designs) and D and E (for digital designs)
Elements of Audio System...
Class A - use 100% of the input cycle for generating the output,
although only 50% efficiency is obtained. The amplifying
transistor is biased such that the device is always conducting,
due to which more power is wasted.
Adv : it gives the best quality sound due to the
fact that the transistor operates on the most
linear portion of its characteristic curve which
produces minimum distortions of the output
Disadv : Generation of large amount of heat.
Class B – Class B amplifiers only use half of the input cycle for
amplification. They are used in RF power amplifiers where
distortion is unimportant. They are mostly used in pairs in a push
pull configuration ie each element is used to amplify the opposite
half of the input wave after which they are joined together in the
output. This produces cross over distortion.
Disadv : the produce lot of distortion
Adv : they are efficient compared to Class A
as the amplifying element is switched off
during half of the cycle.
Class AB – In order to avoid cross over distortion, the elements are
kept on even after the corresponding half cycles are over. The
elements are biased in such a way that they operate over a linear
portion of their characteristic curve during a half cycle, but still
conduct a small amount of current in the
Uses – mostly used for audio amplifiers.
Class C – they use less than half of the input cycle for amplification.
They are used for filtering or suppressing certain unwanted
harmonics from the input wave.
Adv : Very efficient.
Disadv : huge amount of distortion.
Distortions (noise) in the output wave is an inherent problem due to
the non linearity of amplifying elements. There is a limit to reducing
the distortion. Negative feedback helps in reducing the distortion by
feeding back a portion of the output back to the input so that it is
subtracted from the original input.
The negative distortion combine with the positive distortion to make
the output signal more or less linear.
Class D – they use a series of transistors as switches. The input
signal is sampled and converted to digital pulses using an ADC.
The pulses are then used to switch the transistors on and off
(therefore more efficient).
The sampling rate must be at least double that of the highest input
frequency value (called the Nyquist frequency) otherwise it
produces low frequency distortions called aliasing.
Quantization error results due to the finite number of transistor
Class E –They use Pulse Width Modulation (PWM) to produce
output waves whose widths are proportional to the desired
amplitudes. This requires a single transistor for switching and a fast
clock to enable the switching operation. Generally used for
controlling small DC motors.
Adv : cheap compared to others.
Disadv : abrupt switching off / on can produce severe distortion.
It is a device that converts electrical energy back to acoustic
energy. There are 2 types :
1. Dynamic Loudspeaker – made of wire coil and diaphragm
and a permanent magnet.
2. Woofers and tweeters – loudspeaker is divided into
- smaller units called woofers to handle low frequencies
- mid range for handling middle frequencies and
- tweeters for handling high frequencies.
They are electronic instruments which allow us to generate digital
samples of sounds of various instruments synthetically ie without
the actual instrument being present. The core of a synthesizer is
a special purpose chip or IC for producing sound.
Types of Synthesizers :
1. FM Synthesizer – generate sound by combining elementary
sinusoidal tones (using Fourier transform) to build up a note
having the desired waveform.
2. Wavetable Synthesizer – produce sound by retrieving high
quality digital recordings of actual musical instruments from
memory and playing them on demand.
Characteristics of a Synthesizer
Polypony – refers to its ability to play more than one note at a time.
Previously synthesizers were monophonic ie they could play only one
note at a time.
A synthesizer is said to be multitimbral if it is capable of producing 2 or
more different instrument sounds simultaneously.
MIDI-Musical Instrument Digital Interface
It is a protocol or set of rules for connecting digital synthesizers to
each other or to digital computers. 2 synthesizers communicate
MIDI information tells a synthesizer in its most basic mode, when to
start and stop playing a specific note.
MIDI information can also be more hardware specific. It can tell a
synthesizer to change sounds, master volume, modulation
devices and even how to receive information.
It can also be used to indicate the starting and stopping points of a
Digital form is a degraded version of the analog form because of
quantization error, however quality can be improved by
increasing sampling rate. The upper value of sampling rate is
limited by the hardware / software capabilities and economic
considerations, while the lower value is decided by the Nyquist
Important parameters for digital audio.
1. Practical Audio Sampling Parameters – bit depths of 8 bits
and 16 bits are sufficient.
2. Aliasing – if audio frequency > Nyquist frequency => errors
3. Bit Rate and File Size
4. Streaming Audio – for downloading files on internet.
5. High Fidelity (HiFi) – reproduction of sound and image
almost identical to the original in quality.
A compact disc is an optical disc used to store digital data,
originally developed for storing digital audio.
1. CD-Digital Audio (CD DA)
2. CD-ROM Digital Data (CD ROM)
3. CD-Interactive (CD-i)
4. CD-ROM Extended Architecture (CD-ROM XA)
6. Video-CD (VCD)
7. CD-Recordable (CD-R)
8. CD-Rewritable (CD-RW)
9. CD+Graphics (CD+G)
Audio File Format
WAV – Waveform Audio
AIFF – Audio Interchange File Format
VOC – Voice
AVI – Audio Video Interleave
AVO – more commonly referred to as AVI
MPEG-I : Motion Pictures Experts Group I
MPEG-II : Motion Pictures Experts Group II
MPEG-III : Motion Pictures Experts Group III
MPEG-IV : Motion Pictures Experts Group IV
RMF – Rich Music Format
WMA – Windows Media Audio
MPC – Musepack