1.  Explain the theory of audio.

2. In This Chapter, you’ll learn on:
 What is Audio?
 What is Analog and Digital Audio?
 What is Bandwidth?
 How to digitize analog audio?
 Sampling Theory of digital audio.

3.  The range of frequencies detectable by the human
ear — approximately 20Hz to 20kHz. 20Hz is the
lowest-pitched (bassiest) sound we can hear, 20kHz
is the highest pitch we can hear.
 Audio work involves the production, recording,
manipulation and reproduction of sound waves. To
understand audio you must have a grasp of two
things:


4.  1. Sound Waves:
 Sound waves exist as variations of pressure in a
medium such as air. They are created by the
vibration of an object, which causes the air
surrounding it to vibrate. The vibrating air then
causes the human eardrum to vibrate, which the
brain interprets as sound.
 Sound waves travel through air in much the same
way as water waves travel through water. In fact,
since water waves are easy to see and understand,
they are often used as an analogy to illustrate how
sound waves behave.


5.  Sound Waves:


6.  2. Sound Equipment:
 A piece of audio equipment is any device designed
principally to reproduce, record or process sound.
This includes microphones, radio receivers, AV
receivers, CD players, tape recorders, amplifiers,
mixing consoles, effects units, and loudspeakers.
 Audio theory is simpler than video theory and once
you understand the basic path from the sound
source through the sound equipment to the ear, it all
starts to make sense.

7.  The Field of Audio Work
 The field of audio is vast, with many areas of
specialty. Hobbyists use audio for all sorts of things,
and audio professionals can be found in a huge
range of vocations. Some common areas of audio
work include:
Studio Sound
Engineer
Live Sound Engineer
Musician
Music Producer
DJ
Radio technician
Film/Television Sound
Recordist
Field Sound Engineer
Audio Editor
Post-Production Audio Creator

8.  What is Analog Audio?

 Analog audio is a representation of a sound that is
analogous to the air pressure waves of the sound.
Sound is waves of air molecules. Analog audio is a
representation of the intensities of those waves in a
different form, such as voltages on a wire or
magnetized particles on a cassette tape.
In audio, an analog signal is a smooth-flowing
representation of music or sound. Like a bow on a
violin string, analog sound is marked by a
continuously flowing sound.

9.  What is Analog Audio?
Examples of analog audio recording are:,
Compact Cassette magnetic tape
recorder
Gramophone record (also known as a phonograph
record, vinyl, etc.)

10.  What is Analog Audio?
 In an analog audio system, sounds begin as physical waveforms in
the air, are transformed into an electrical representation of the
waveform, via a transducer (for example, a microphone), and are
stored or transmitted. To be re-created into sound, the process is
reversed, through amplification and then conversion back into
physical waveforms via a loudspeaker.
 Although its nature may change, analog audio's fundamental
wave-like characteristics remain the same during its storage,
transformation, duplication, and amplification.
 Analog audio signals are susceptible to noise and distortion,
unavoidable due to the innate characteristics of electronic circuits
and associated devices. In the case of purely analog recording
and reproduction, numerous opportunities for the introduction of
noise and distortion exist throughout the entire process.

11.  What is Digital Audio?
 Digital audio has emerged because of its usefulness in the
recording, manipulation, mass-production, and distribution of
sound. Modern distribution of music across the Internet via on-line
stores depends on digital recording and digital compression
algorithms. Distribution of audio as data files rather than as physical
objects has significantly reduced the cost of distribution.
 When audio is digitized, distortion and noise are introduced only by
the stages that precede conversion to digital format, and by the
stages that follow conversion back to analog.

12.  What is Digital Audio?
 Digital audio takes advantage of some peculiarities of acoustics
and the human ear. When sound is converted from analog into
digital audio, the hardware "samples" the level of the waveform at
a specific interval. For CD audio, this interval is 1/44,100th of a
second. In other words, 44,100 times each second a special chip
calculates a value for analog input and sends it off for use or
storage. This process is called "digitizing" a sound.

13.  What is Digital Audio?
 The result, if we were to graph it as we have with analog
waveforms, would look quite different. Instead of smooth, gradual
changes we would see stair steps as the line jerked from sampling
data point to sampling data point. Here’s a picture showing the
difference:

14.  What is Digital Audio?
 There are two useful terms here- the "sampling rate" and
the "sample size".
 The sample rate is the number of times per second that
the analog signal is measured.
 The sample size tells us what number is associated with
the maximum value.
 The maximum value of the analog data doesn’t change-
if you try to add power past a certain point you just start
blowing up hardware.
 But if the range is from 0 to 1000 the values stored will
represent the analog data more closely than if they
range from 0 to 10.

15.  What is Digital Audio?
 Still, if analog data is smooth and digital data is made up
of stair steps, why doesn’t digital audio sound bad? The
answer is fairly technical, but what it boils down to is that,
as long as the samples are taken often enough, the noise
created by the stair stepping is too high in frequency for
us to hear. According to the theory, the frequency of this
noise will always be at least twice the sampling
frequency. This is called the Nyquist Limit.
 Very few if any humans can hear above about 20,000
cycles per second. Note that the speed chosen for audio
CDs is 44,100 cycles per second. It is no coincidence that
CD sampling rate is just over twice what our ears can

16.  What is Digital Audio?
 The conversion of analog data to digital and back to
analog is accomplished by special chips. A chip that
converts analog to digital is called an ADC- an Analog to
Digital Converter. The ADC measures the amount of
current at each sampling interval and converts it to a
binary number. This is called "digitizing" the sound. On the
other end is a chip called a DAC- a Digital to Analog
Converter. This chip takes a binary number and converts
it to an output voltage.

17.  What is Digital Audio?
 Here's what happens if you record your voice using a
microphone plugged into your computer, then edit it and play
it back over your speaker system: the microphone generates
an analog waveform corresponding to the compression and
rarefaction cycles generated by your voice. This smooth
analog waveform is converted into a series of binary values by
the ADC which are then transferred into the memory of your
computer. Once you are done editing (if you’ve ever tried
editing analog tape you’ll appreciate how much easier digital
editing is!) the computer sends the resulting series of binary
numbers to the DAC, which converts them to a (relatively)
smooth analog waveform that drives your speaker.

18.  What is Digital Audio?
 Digital, or digitized, sound is easier to reproduce and
manipulate without loss in quality. Some question whether the
quality is quite as good as analog sound, but it can be very
good indeed, and CDs don’t wear out like records used to.
Digital audio can also be compressed much more easily than
analog, which is why MP3 is a digital format.
 Digital is not necessarily better, but it is different, and offers
advantages to engineers and end users that will increase its
dominance in the coming years.

19.  What is Bandwidth?
 In computer networks, bandwidth is often used as a synonym
for data transfer rate - the amount of data that can be carried
from one point to another in a given time period (usually a
second). This kind of bandwidth is usually expressed in bits (of
data) per second (bps). Occasionally, it's expressed as bytes
per second (Bps). A modem that works at 57,600 bps has twice
the bandwidth of a modem that works at 28,800 bps. In
general, a link with a high bandwidth is one that may be able
to carry enough information to sustain the succession of
images in a video presentation.

20.  What is Bandwidth?
 In electronic communication, bandwidth is the width of the
range (or band) of frequencies that an electronic signal uses
on a given transmission medium. In this usage, bandwidth is
expressed in terms of the difference between the highest-
frequency signal component and the lowest-frequency signal
component. Since the frequency of a signal is measured in
hertz (the number of cycles of change per second), a given
bandwidth is the difference in hertz between the highest
frequency the signal uses and the lowest frequency it uses. A
typical voice signal has a bandwidth of approximately three
kilohertz (3 kHz); an analog television (TV) broadcast video
signal has a bandwidth of six megahertz (6 MHz) -- some 2,000
times as wide as the voice signal.

21.  How to Digitize Analog audio
 The term digitization is often used when diverse forms of information,
such as text, sound, image or voice, are converted into a single
binary code. Digital information exists as one of two digits, either 0 or
1. These are known as bits (a contraction of binary digits) and the
sequences of 0s and 1s that constitute information are called
bytes.[3]
 Analog signals are continuously variable, both in the number of
possible values of the signal at a given time, as well as in the number
of points in the signal in a given period of time. However, digital
signals are discrete in both of those respects – generally a finite
sequence of integers – therefore a digitization can, in practical
terms, only ever be an approximation of the signal it represents.

22.  Digitization occurs in two parts:
 Discretization
 The reading of an analog signal A, and, at regular time intervals
(frequency), sampling the value of the signal at the point. Each such
reading is called a sample and may be considered to have infinite
precision at this stage;
 Quantization
 Samples are rounded to a fixed set of numbers (such as integers), a
process known as quantization.
 In general, these can occur at the same time, though they are
conceptually distinct.
 A series of digital integers can be transformed into an analog output
that approximates the original analog signal. Such a transformation is
called a DA conversion. The sampling rate and the number of bits
used to represent the integers combine to determine how close such
an approximation to the analog signal a digitization will be.

23.  When we digitize sound, we need to consider some parameters
that determine the amount of information stored in a file and the
quality of the digital sound.
Sampling rate:
 A sampling rate is the number of times the analog sound is
sampled during each period (one complete waveform) and
converted into digital information. The most common sampling
rates are 44.1, 22.05, and 11.025 kHz(kilo-Hertz). A sampling rate of
44.1 kHz means that 44,100 samples of the analog audio will be
taken. Therefore, the more samples taken, the closer the digital
version will approximate the original analog version.

24. Bits per sample:
 Bits per sample describes how much information
(number of amplitudes) in each sample the computer
is collecting. It involves a process of converting a
sampled sound into an equivalent digital value. The
number of individual sounds that can be represented
depends on the number of bytes used to
 store the digital values. Naturally, more bytes, more
possible sound and higher quality of digitized sound.

25.  Mono vs. stereo:
 Mono describes a system where all the audio signals
are mixed together and routed through a single audio channel.
Stereo sound systems have two independent audio channels, and
the signals are reproduced by two channels separated by some
distance. It is difficult to decide which one is the best. The two
sound channels give the illusion that the sound is coming from a
certain location.
 However, going to mono will reduce the file size by half.
Sometimes a welldesigned mono system is better than a low
quality stereo system.
 Note: It is important to balance the sampling rate, bits per sample
to come up with an acceptable quality of the sound with the
minimum file size. The following comparison is provided by Michael
Kennedy (2000):

26. Sound quality
Sampling rate
(kHz)
Bit Depth File size
(MB)
Stereo/Mono
Compact disc
quality
44.100 16 30.2 stereo
good quality 44.100 16 15.1 mono
44.100 8 15.1 stereo
22.050 16 15.1 stereo
22.050 16 7.5 mono
22.050 8 7.5 stereo
low quality 11.025 8 3.7 stereo
8.000 8 1.3 mono

27.  How to digitize sound?
 1st step: connect the output source to the computer: This can
be done by connecting a mini audio cable from the
headphone output on the tape recorder to the sound input
on the computer. (DAT and CD work in essentially the same
way.)
 2nd step: connect headphones or speakers for monitoring into
the computer’s headphones jack.
 3rd step: open up software, such as Sound Edit 16 or CoolEdit.
 4th step: Start playing the tape and listen to the sound.
Observe the sound level and make sure that it does not go
into the red zone.
 5th step: control the sound level with the tape recorder
volume controls.
 6th step: after digitizing the sound, save it in an appropriate
format.

28.  Sampling Theory of digital audio.
 Sounds from the real world can be recorded and digitized
using an analog-to-digital converter (ADC). As in the diagram
below, the circuit takes a sample of the instantaneous
amplitude (not frequency) of the analog waveform.
Alternatively, digital synthesis software can also create
samples by modeling and sampling mathematical functions or
other forms of calculation. A sample in either case is defined
as a measurement of the instantaneous amplitude of a real or
artificial signal.
 Frequencies will be recreated later by playing back the
sequential sample amplitudes at a specified rate. It is
important to remember that frequency, phase, waveshape,
etc. are not recorded in each discrete sample measurement,
but will be reconstructed during the playback of the stored
sequential amplitudes.

29.  Sampling Theory of digital audio.
Samples are taken at a
regular time interval. The rate
of sample measurement is
called the sampling
rate (or sampling frequency).
The sampling rate is
responsible for the frequency
response of the digitized
sound.