2. What are binary digits?
Computers use binary numbers, and
therefore use binary digits in place of
decimal digits.
The word bit is a shortening of the
words "Binary digit."
Whereas decimal digits have 10
possible values ranging from 0 to
9, bits have only two possible values: 0
and 1.
3. Decimal and binary numbers.
You can see that in binary numbers, each bit holds the value of
increasing powers of 2.
That makes counting in binary pretty easy E.g 1011 means
(1 * 23) + (0 * 22) + (1 * 21) + (1 * 20) = 8 + 0 + 2 + 1 = 11
Some more examples
10 = 1010
12 = 1100
13 = 1101
16 = 10000
5. Bits and bytes.
Bits are rarely seen alone in computers. They are
almost always bundled together into 8-bit
collections, and these collections are called bytes.
With 8 bits in a byte, you can represent 256
values ranging from 0 to 255, as shown here:
0 = 00000000
1 = 00000001
2 = 00000010
...
254 = 11111110
255 = 11111111
6. Bits and bytes continued
CD uses 2 bytes, or 16 bits, per sample. That
gives each sample a range from 0 to
65,535, like this:
0 = 0000000000000000
1 = 0000000000000001
2 = 0000000000000010
...
65534 = 1111111111111110
65535 = 1111111111111111
7. Analog at a glance
As a technology, analog is the process of
taking an audio or video signal (in most
cases, the human voice) and translating it
into electronic pulses.
Digital on the other hand is breaking the
signal into a binary format where the audio
or video data is represented by a series of
"1"s and "0"s.
8. A to D
Digital technology breaks your voice (or
television) signal into binary code—a
series of 1s and 0s—transfers it to the
other end where another device
(phone, modem or TV) takes all the
numbers and reassembles them into the
original signal. The beauty of digital is
that it knows what it should be when it
reaches the end of the transmission.
9. Is the duplication perfect?
But like any transferred
technology, digital has a few
shortcomings. Since devices are
constantly translating, coding, and
reassembling your voice, you won't get
the same rich sound quality as you do
with analog.
10. Can we use the digital phone
using an analog line?
There are digital-to-analog adapters that not
only let you use analog equipment in a digital
environment, but also safeguard against
frying the internal circuitry of your
phone, fax, modem, or laptop.
Some adapters manufactured come designed
to work with one specific piece of office
equipment: phone, modem, laptop, or
teleconferencer. Simply connect the adapter
in between your digital line and your analog
device.
11. Ancient way of recording the
analog way
In the Beginning: Etching Tin
Thomas Edison is credited with creating the
first device for recording and playing back
sounds in 1877. His approach used a very
simple mechanism to store an analog wave
mechanically. In Edison's original
phonograph, a diaphragm directly controlled
a needle, and the needle scratched an analog
signal onto a tinfoil cylinder .
http://communication.howstuffworks.com/analog-
digital1.htm
12. An analog wave
Image from
www.howstuffworks.com
Analog Wave
What is it that the needle in Edison's phonograph is
scratching onto the tin cylinder? It is an analog wave
representing the vibrations created by your voice.
For example, here is a graph showing the analog
wave created by saying the word "hello":
13. Analog recording contd….
The waveform was recorded electronically
rather than on tinfoil, but the principle is
the same.
What this graph is showing is the position of
the microphone's diaphragm (Y axis) over
time (X axis). The vibrations are very quick
-- the diaphragm is vibrating on the order
of 1,000 oscillations per second.
Notice that the waveform for the word
"hello" is fairly complex.
14. Getting in to the digital world
In a CD (and any other digital recording technology), the
goal is to create a recording with :
1. very high fidelity (very high similarity between the
original signal and the reproduced signal)
2. perfect reproduction (the recording sounds the
same every single time you play it no matter how
many times you play it).
15. To accomplish these two goals, digital
recording converts the analog wave into a
stream of numbers and records the
numbers instead of the wave.
The conversion is done by a device called
an analog-to-digital converter (ADC). To
play back the music, the stream of
numbers is converted back to an analog
wave by a digital-to-analog converter
(DAC).
The analog wave produced by the DAC is
amplified and fed to the speakers to
produce the sound.
16. CDs and DVDs.
http://electronics.howstuffworks.com/cd.ht
m
Exploring Sound: Digital Sound
– Laser discs such as CDs and DVDs carry
digital information, which is represented by
the binary code -- combinations of 1s and
0s. Any number can be represented in
binary code.
17. COMPACT DISCS (CD’S)
A CD is a fairly simple piece of plastic
about 1.2 mm thick.
The CD consists of a moulded piece of
plastic that is impressed with
microscopic bumps arranged as a
single, continuous spiral track of data.
A thin, reflective aluminium layer is
placed onto the top of the disc, to cover
the bumps.
18. A thin acrylic layer is sprayed over the
aluminum to protect it.
The label is then printed onto the acrylic.
19. A CD has a single spiral track of
data circling from the inside of
the disc to the outside.
The data track is incredibly small.
It is about 0.5 microns wide, with
1.6 microns separating one track
from the next.
20. Due to the extreme thinness, the total
length of the track squeezed onto this
small disc is about 8 km.
The information on the disc is read by
shining a laser beam from the underside
of the compact disc.
Thus the laser is seeing the
―bumps‖, not the ―pits‖.
21. The diagram below gives you some idea of
how small a CD ―bump‖ is compared to a
human hair.
22. The sequence of bumps, the length of the
bumps and the length of the spaces between
the bumps provides the information that the
CD player decodes.
23. The laser that is used is an infrared laser
emitting light at a wavelength of 780 nm.
The laser passes through the plastic and is
reflected off the aluminum coating on the
bumps and the land between them.
24. A very important point is that the height of the
―bumps‖ is approximately one quarter the
wavelength of the laser light.
When the laser light is passing over the
―land‖, all of the light is reflected off and it
travels back to photoelectric cell.
The photoelectric cell then produces an
electric current.
25. This electric current then goes on to
generate sound in a loudspeaker (see
loudspeaker application).
Now lets look at what happens when
the laser light approaches a ―bump‖.
When the light reaches a bump, half of
the light is reflected off the ―bump‖ and
half of the light is reflected off the ―land‖.
26.
27. Because the bump is ¼ of a wavelength in
height, the light being reflected off the land
travels one half a wavelength further.
The light reaching the photoelectric cell
coming from the ―land‖ and the ―bump‖ is out
of phase. This leads to partial cancellation
and a decrease in intensity.
This leads to decreased current being
produced.
28. As the laser moves along the track the
intensity of the light falling on the
photoelectric cell changes every time it
comes into approaches or leaves a
bump.
It is this change in intensity which
causes the fluctuation in electric
current, which causes the movement of
the loudspeaker and ultimately the
fluctuation in sound.
29. USING INTERFERENCE TO KEEP A
LASER ON TRACK
The musical data on the CD is read
from the inside out.
The CD spins above the laser.
After one revolution, the laser must
move to the outside exactly 1.6 microns
to remain on track.
30. This requires a very precise tracking
mechanism and an accurate correction
mechanism to move the laser back on
track if it should stray off the line.
The tracking correction is achieved by
first passing the laser beam through a
diffraction grating, before it reaches the
CD.
31. When the monochromatic light passes
through the diffraction grating a central
beam and a first order diffracted beam
will land on the CD.
32. The central beam is focused on the
track of the CD and passes over the
bumps while the two first order
diffracted beams are focused on the
land on either side of the bumps.
One diffracted beam is slightly ahead of
the other.
33. The laser beam is tracking correctly
when the central beam is varying in
intensity from 35% to 100% and the two
diffracted beams have a constant
intensity of 100%.
34. If the laser beam should stray to one side of
its correct position, then the variation in
intensity of the main beam is now reduced.
The leading tracking beam will also have a
variation in the intensity because some of it is
passing over the bumps.
The tracking mechanism ―senses‖ that it
must adjust the position of the laser down in
order to put it back on track.
35. If the laser beam were to stray to the other
side of its correct position, then the variation
in intensity of the main beam is again
reduced.
The TRAILING beam will now have a
reduction in intensity.
The tracking mechanism ―senses‖ that it must
adjust its position up in order to get back on
track.
36. CD’s and DVD’s
Data is stored digitally
– A series of ones and zeros read by laser light
reflected from the disk
Strong reflections correspond to constructive
interference
– These reflections are chosen to represent zeros
Weak reflections correspond to destructive
interference
– These reflections are chosen to represent ones
39. Reading a CD
As the disk rotates, the laser
reflects off the sequence of
bumps and lower areas into
a photodector
– The photodector converts the
fluctuating reflected light
intensity into an electrical string
of zeros and ones
The pit depth is made equal to
one-quarter of the
wavelength of the light
40. Question
If a laser emits light of wavelength
760nm calculate how deep the pits
would have to be for data to be stored
appropriately?
41. Solution
The depth of the pit is d
From the bottom of pit the light travels
an extra distance of 2d
For interference to occur the difference
in path travelled needs to be λ/2
d = λ/4
d = 760/4 nm
42. DVD’s
DVD’s use shorter wavelength lasers
– The track separation, pit depth and
minimum pit length are all smaller
– Therefore, the DVD can store about 30
times more information than a CD
If the pit depth on a DVD is 0.4micrometre
what laser wavelength is used?
43. In the case of CD sound, fidelity (the similarity
between the original wave and the DAC's
output ) is an important goal, so the sampling
rate is 44,100 samples per second and the
number of gradations is 65,536. At this
level, the output of the DAC so closely
matches the original waveform that the sound
is essentially "perfect" to most human ears .
44. Why is a CD’s capacity approximately
750 mb?
One thing about the CD's sampling rate and precision is
that it produces a lot of data.
On a CD, the digital numbers produced by the ADC are
stored as bytes, and it takes 2 bytes to represent
65,536 gradations.
There are two sound streams being recorded (one for
each of the speakers on a stereo system). A CD can
store up to 74 minutes of music, so the total amount
of digital data that must be stored on a CD is:
44,100 samples/(channel*second) * 2
bytes/sample * 2 channels * 74 minutes * 60
seconds/minute = 783,216,000 bytes (Convert to
kbs and then Mbs.)
45. Why store digitally?
There are five main reasons we prefer
storing data in a digital format rather
than analogue
Quality
Reproducibility
Portability
Manipulation
Retrieval speed
46. Quality
Digital is less likely to be corrupted with
time.
– Consider a magnetic tape recording, the
field strength will drop with time thus data
can be lost. If it was digitally sorted as
magnetic field and zero then the
boundaries will still be readable even if
strength diminishes.
48. Portability
Analogue storage is sequential and is
often large
Digital storage allows for large amounts
to be stored in pocket sized devices
What are the implications on society of
the ever increasing storage capacity?
49. Manipulation
Digital data is less prone to error in
calculations
Digital is easier to manipulate and
process
50. Capacitance
Capacitance is the quantity of charge that
can be stored per unit electric potential
C = Q/V
The unit of capacitance is the Farad, F
A Farad is one coulomb per unit potential
difference.
The capacitance of the Earth is 0.1F so
we usually deal with pF
54. CCD
A CCD is basically just like any other silicon
microchip except it has an array of pixels
which are exposed and are sensitive to light.
The size of such an array can vary from about
256 x 256 to 2048 x 2048.
Each pixel in this array counts the number of
photons which fall upon it. The number of
photons counted is stored as electric charge
in a capacitor by each pixel. Each time a
photon hits a pixel, a small amount of electric
charge is added to the capacitor.
55. CCD
This process continues until the stored number
counts in all of the capacitors are read. This
process, called 'readout', happens very
quickly in such a way that the data is read line
by line until the last line has been read.
The CCD is then 'flushed' which means that the
charges in all of the capacitors are reset to
zero. The charge storage process then
repeats.
56. Light and pixels
Photons have energy that is proportional to the wave
frequency:
E=hf,
where h is Planck's constant and f is the wave
frequency.
In a CCD being exposed, photons enter the chip from
the rear. When a photon strikes the silicon, it is very
likely to interact with an electron in the thin layer of
silicon (approximately 8µm) and give sufficient
energy to the electron to displace it from the silicon
lattice. This creates an electron-hole pair.
57. The displaced electron, or photoelectron, is
collected in the nearest potential well while
the hole created by the loss of the electron is
eventually filled by an electron from the
silicon substrate.
The more photons which strike the silicon, the
more electron-holes produced and so the
more electrons stored within each potential
well.
electrons captured
- - -
- liberated electrons
- silicon substrate
incident photon
incident photon
58. Photoelectric effect
How does the photoelectric effect apply
to CCDs?
How much charge would a pixel hold if
its capacitance was 30pF and the
potential changes by 0.2mV?
How many photons hit the pixel if the
time for the potential change is 10ms?
59. What is a digital image?
Essentially, a digital image is just a long string of 1s
and 0s that represent all the tiny colored dots -- or
pixels -- that collectively make up the image. If you
want to get a picture into this form, you have
two options:
You can take a photograph
You can directly sample … you can use a digital
camera
60. What is a digital image?
The first line shows the initial state with the coloured
bucket three from the left hand side.
The second line shows position of the coloured
bucket after the first swap. The free bucket at the
right hand side contains the charge which is then
digitised. The potential formed by the charge is
amplified and converted into a digital signal by an
Analogue to Digital Converter (ADC).
After each swap, the buckets move one place further
to the right.
61. Consider a CCD where the maximum output voltage
from a pixel, after amplification, is 0.8V. If this is to
be converted into a 3-bit binary number, then there
will be 8 different quantisation levels
e.g. The voltage at the output is 0.42 V. This falls in
the quantisation level represented by 4 and will give a
digital signal of 100.
62. Capturing image
The image sensor employed by
most digital cameras is a charge
coupled device (CCD). Some
A CMOS sensor
cameras use complementary
metal oxide semiconductor
(CMOS) technology instead. Both
CCD and CMOS image sensors
convert light into electrons.
A simplified way to think about these
sensors is to think of a 2-D array
of thousands or millions of tiny
solar cells.
63. Digitisation of the light
http://www.olympusmicro.com/primer/digitalimaging
/concepts/concepts.html
Electrode measures the potential difference
developed across the pixel and this is then
converted into a digital signal
Pixel position is also stored
64. Capturing Color
Unfortunately, each photosite is
colorblind. It only keeps track of the
total intensity of the light that strikes its
surface. In order to get a full color
image, most sensors use filtering to
look at the light in its three primary
colors. Once the camera records all
three colors, it combines them to create
the full spectrum
For illustrations and explanations visit:
http://electronics.howstuffworks.com/digital-camera3.htm
66. Quantum Efficiency
This is the ratio of the number of
photoelectrons emitted to the number of
photons incident on the pixel
This will never be 100% due to
scattering or non-interaction with
substrate
Number of photoelect rons
Quantum efficiency 100%
Number of photons
67. Quantum efficiency is a measure of the
sensitivity of a light detector. Many
photoelectric materials typically emit an
electron for every 5 to 10 incident
photons and so therefore have a
quantum efficiency of between 10 and
20%.
Typical values are 70-80%
69. Magnification
Resolution
Two points on an object will be resolved if
the images of the two points are at least
two pixels apart
charge
charge
charge
pixels
pixels
pixels
70. Digital Camera Resolution
The more pixels a
camera has, the more
detail it can capture and
Photo courtesy Morguefile the larger pictures can
The size of an image taken at
different resolutions
be without becoming
blurry or "grainy."
71. Image quality
Larger magnification = more pixels
activated = more detailed
Greater resolution = more pixels /unit
length = more detailed
72. Use of CCDs
Digital cameras
Video cameras
Telescopes
Medical imaging
Photocopiers
Barcode readers
73. Advantages over emulsion
Reusable - once an image has been captured, the CCD can then
be reset ready for the next image to be captured.
Photographic emulsion is a 'one off' process and cannot be
reused.
Greater sensitivity - modern CCDs are over 1000 million times
more sensitive than the human eye
Greater colour response - modern CCDs will respond to
electromagnetic radiation over a wider range of wavelengths
than either the human eye or photographic emulsion
Linear response - the output voltage from a CCD is
proportional to the charge collected by each pixel, which in turn
is proportional to the number of photons incident on the CCD.
74. Image retrieval
Light focussed on CCD
Photoelectric effect
Number of electrons released from each
pixel will vary
Potential change occurs
Pixel location sorted along with pd
change as a digital signal
Digital signal converted to image
