Multimedia Systems -...

Multimedia Systems

Video I

(Basics of Analog and Digital Video)

Mahdi Amiri

April 2012

Sharif University of Technology

Course Presentation

Multimedia Systems, Mahdi Amiri, Video I Page 1

Video Visual Effect of Motion

The visual effect of motion is due to biological phenomenons.

1. Persistence of vision

An object seen by the human eye remains

mapped on the eye’s retina for a brief time

after viewing (approximately 25 ms)

2. Phi phenomenon

When two light sources are close by and they are

illuminated in quick succession, what we see is not two

lights but a single light moving between the two points

(perceiving movement)

Due to the above two phenomena of our vision system, a

discrete sequence of individual pictures can be perceived as a

continuous sequence


Video Definition

A sequence of still images representing scenes in motion.

Frame Rate

Number of still images/pictures per unit time

Frames / Second (fps)

Frame rate of video ranges from 6 to 8 fps

for old mechanical cameras to 120 or

more fps for new professional cameras.

The minimum frame rate to achieve

illusion of a moving image is 15 fps.

Frame N-1

Frame 0 Time


Video Representation Video Display

In conventional TV sets or monitors, the video signal is displayed using a

CRT (Cathode Ray Tube).

An electron beam sweeps the screen from top to bottom beam carrying the

corresponding pattern information, such as intensity in a viewed scene.

Video can be interlaced or progressive ( Are described in the following slides ).


Video Display Progressive Scanning

Flicker free at around 50 frames per second (fps)

Flicker is a visible fading between

cycles displayed on video displays,

especially the refresh interval on

cathode ray tube (CRT) based

computer screens.

Flicker and Bandwidth


Video Display Interlaced Scanning

First the solid (odd) lines are traced, P to Q,

then R to S, etc., ending at T; then the even

Field starts at U and ends at V.

The jump from Q to R, etc. is called the

horizontal retrace, during which the

electronic beam in the CRT is blank.

The jump from T to U or V to P is called

the vertical retrace.

Electronic signal for

one NTSC scan line.

Flicker free at around 25 frames per second (fps)


Video Display Odd and Even Fields

Odd

Field

Even

Field

Difference

of Fields

A Video Frame

Because of interlacing, the odd and even lines are displaced in

time from each other - generally not noticeable except when

very fast action is taking place on screen, when blurring may

occur.

Initially the odd-numbered lines are scanned and then the

process is repeated for even-numbered lines - this time

starting at the second row.


Video Display Deinterlacing

When interlaced video is watched on a

progressive monitor with very poor

deinterlacing, it exhibits combing when there

is movement between two fields of one frame.

Deinterlacing is the process of converting interlaced video, such as

common analog television signals into a non-interlaced form.

Method 1: Capturing one field and combining it with the next field

Problem: "combing" effect

Method 2: Line doubler The most basic and literal way to double lines is to repeat each

scanline, though the results of this are generally very crude.

Most line doublers use digital interpolation to recreate the

missing lines in an interlaced signal, and the resulting quality

depends on the technique used. Generally a line doubler will

only interpolate within a single field, rather than merging

information from adjacent fields, to preserve the smoothness

of motion, resulting in a frame rate equal to the field rate.

Serrated image


Analog Broadcast TV Systems NTSC

NTSC (National Television System Committee)

Mostly used in North America and Japan

Aspect Ratio: 4:3

525 scan lines at 30 fps

Interlaced scanning (262.5 lines/field)

Color Space: YIQ


Analog Broadcast TV Systems YIQ Color Space

In the YIQ color system, the I axis runs from cyan to

orange, and the Q axis runs from green to violet.

Eye is most sensitive to Y, next to I, next to Q.

Bandwidth allocation for color components

4 MHz is allocated to Y,

1.5 MHz to I,

0.6 MHz to Q.

When compared to PAL in particular, NTSC color

accuracy and consistency is sometimes considered

inferior, leading to video professionals and television

engineers jokingly referring to NTSC as Never The

Same Color, Never Twice the Same Color, or No True

Skin Colors,[16] while for the more expensive PAL

system it was necessary to Pay for Additional Luxury.


Analog Broadcast TV Systems PAL

PAL (Phase Alternating Line)

Mostly used in Western Europe, China and India

Aspect Ratio: 4:3

625 scan lines at 25 fps

Interlaced scanning (312.5 lines/field)

Color Space: YUV

5.5 MHz is allocated to Y, 1.8 MHz each to U and V.


Analog Broadcast TV Systems Supplementary Materials

The horizontal sweep frequency is

525×30 ≈ 15,750 lines/sec, so that each

line is swept out in 1/15,750 sec ≈

63.5μsec.

A typical waveform of a NTSC composite video signal

Vertical retrace takes place during 20

lines reserved for control information at

the beginning of each field. Hence, the

number of active video lines per frame is

only 485.

Similarly, almost 1/6 of the raster at the

left side is blanked for horizontal retrace

and sync. The nonblanking pixels are

called active pixels.

NTSC video is an analog signal with no

fixed horizontal resolution. Therefore one

must decide how many times to sample

the signal for display: each sample

corresponds to one pixel output.

NTSC Video Signal



Spectrum of a System M television channel with NTSC color.

An NTSC television channel as transmitted occupies a

total bandwidth of 6 MHz

Modulation of NTSC

The actual video signal, which is amplitude-modulated, is

transmitted between 500 kHz and 5.45 MHz above the

lower bound of the channel. The video carrier is 1.25 MHz

above the lower bound of the channel. Like most AM

signals, the video carrier generates two sidebands, one

above the carrier and one below. The sidebands are each

4.2 MHz wide. The entire upper sideband is transmitted,

but only 1.25 MHz of the lower sideband, known as a

vestigial sideband, is transmitted.

The color subcarrier, as noted above, is 3.579545 MHz

above the video carrier, and is quadrature-amplitude-

modulated with a suppressed carrier.

The audio signal is frequency-modulated.



Chroma Modulation of NTSC

In NTSC, chrominance (C) is encoded using two

3.579545 MHz signals that are 90 degrees out of phase,

known as I (in-phase) and Q (quadrature) QAM. These

two signals are each amplitude modulated and then

added together.

In the ideal case I(t) is demodulated by multiplying the

transmitted signal with a cosine signal.

0 0cos 2 sin 2C t I t f t Q t f t

0 is the carrier frequencyf

Chroma Demodulation of NTSC

0

2

0 0 0

0 0

0 0

2cos 2

2 cos 2 2 sin 2 cos 2

1 cos 4 sin 4

cos 4 sin 4

i

i

i

i

r t C t f t

r t I t f t Q t f t f t

r t I t f t Q t f t

r t I t I t f t Q t f t

Low-pass filtering ri(t) removes the high frequency terms

(containing 4πf0t), leaving only the I(t) term.

Similarly, Q(t) can be extracted by first multiplying C(t)

by 2sin(2πf0t) and then low-pass filtering.



Spectrum of a System I television channel with PAL color.

The basics of PAL and the NTSC system are very similar;

a quadrature amplitude modulated subcarrier carrying the

chrominance information is added to the luminance video

signal to form a composite video baseband signal.

Modulation of PAL

In order to improve picture quality, chroma signals have

alternate signs (e.g., +U and -U) in successive scan lines,

hence the name “Phase Alternating Line”. This facilitates

the use of a (line rate) comb filter at the receiver —the

signals in consecutive lines are averaged so as to cancel

the chroma signals (that always carry opposite signs) for

separating Y and C (chroma) and obtaining high quality

Y signals;

However, this resulted in a comblike effect known as

Hanover bars on larger phase errors. Thus, most receivers

now use a chrominance delay line, which stores the

received color information on each line of display; an

average of the color information from the previous line

and the current line is then used to drive the picture tube. Hanover bars Cancellation of Hanover bars

through a chroma delay line



SECAM stands for Système Electronique Couleur Avec

Mémoire, the third major broadcast TV standard.

Aspect Ratio: 4:3

625 scan lines at 25 fps, Interlaced scanning

SECAM and PAL are very similar. They differ

slightly in their color coding scheme:

In SECAM, U and V signals are modulated using separate

color subcarriers at 4.25 MHz and 4.41 MHz respectively.

They are sent in alternate lines, i.e., only one of the U or V

signals will be sent on each scan line.



.

More at

http://en.wikipedia.org/wiki/Broadcast_television_systems


Analog Color Video Signal Protocols

Composite video

1956

1 wire

Color (chrominance) and luminance

signals are mixed into a single carrier wave.

Since color and intensity are wrapped

into the same signal, some interference

between the luminance and chrominance

signals is inevitable.

Composite video jacks are often grouped

with corresponding stereo audio jacks (the

composite video jack is usually yellow)



S-Video (Separated video, e.g., in S-VHS) 1979

2 wires, one for luminance and another for composite

chrominance signal

As a result, there is less crosstalk between the color information

and the crucial gray-scale information.

A compromise between component analog video and the

composite video.

A standard 4-pin S-Video cable

connector, with each signal pin (3, 4)

paired with its own ground pin (1,2)



Component video 1990

3 wires

Each primary is sent as a separate video signal. The primaries

can either be RGB or a luminance-chrominance transformation of

them (e.g., YPbPr, YIQ, YUV).

Y: Green, Blue: Pb, Red: Pr

Best color reproduction

Requires more bandwidth and good

synchronization of the three components

More about signal protocols at

http://en.wikipedia.org/wiki/List_of_display_interfaces

YPbPr is analog

form of YCbCr


Digital Video Advantages

Video can be stored on digital devices or in memory,

ready to be processed (noise removal, cut and paste, etc.),

and integrated to various multimedia applications.

Direct access is possible, which makes nonlinear video

editing achievable as a simple, rather than a complex, task.

Repeated recording does not degrade image quality.

Ease of encryption and better tolerance to channel noise.


Digital Video HDTV vs. Conventional TV

HDTV has higher resolution

1280 × 720 or 1920 × 1080.

HDTV has a much wider aspect ratio of 16:9 instead of

4:3.

16:9 is closer to aspect ratio of the human eye sight

HDTV moves toward progressive (non-interlaced) scan.

The rationale is that interlacing introduces serrated

edges to moving objects and flickers along horizontal

edges.


Digital Video HDTV Display Resolutions

720p

Referred to in marketing materials as “HD”

1280×720, progressive scan

0.9 megapixels

1080p

Referred to in marketing materials as “Full HD”

1920×1080, progressive scan

2.0 megapixels

1080i

1920×1080, interlaced scan

Aspect Ratio

for all is 16:9

(W:H)


Digital Color Video Signal Protocols

DVI (Digital Visual Interface), 1999

Analog and Digital

2560 × 1600 @ 60 fps

3840 × 2400 @ 33 fps

Max. 1.65 Gbit/sec

HDMI (High-Definition Multimedia Interface), 2003

Uses a pixel based data stream

2560 × 1600 @ 75 fps

4096 × 2160 @ 24 fps

Max. 1.65 Gbit/sec


Digital Color Video Signal Protocols

DisplayPort, 2007

Uses a packetized data protocol often used in high-speed data communications.

This provides a faster data rate over the same number of wires.

2560 × 1600 @ 75

1.6 or 2.7 Gbit/sec


Thank You

1. http://ce.sharif.edu/~m_amiri/

2. http://www.dml.ir/

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