video a multimedia component

Multimedia ElementVideo

Prepared By

Jhansirani.R Assoct.prof/ECE

Video

Video, like sound, is recorded and played an as analog signal

Analog video must be digitized in order for it to put into a multimedia file

Digital video has many advantages, but file size is important

Video

Several elements determine file size:Frame rateImage sizeColor depth

To determine file size use the following formula:Frames per second X image size X color

depth / 8 = file size

VideoVideo features in a range of multimedia

applications.

Entertainment: broadcast TV & VCR/DVD recordings

Interpersonal: video telephony & video conferencing

Interactive: windows containing short video clips.

Broadcast television Scanning sequence:To produce smooth motion, a refresh rate of 25times/s is sufficient.

In order to minimize the amount of transmission bandwidth that is required to broadcast the television signal, this characteristic of eye is exploited by transmitting the image/picture associated with each frame in two halves

Each is known as field, the 1st comprising only odd scan lines and the 2nd the even scan lines.

The 2 fields are then integrated together in the TV receiver using a technique known as interlaced scanning.

Interlaced scanning In 525 line system each field

comprises 262.5 lines240 visible

In 625 line system each field comprises 312.5 lines

288 visible

Each field is refreshed at 60/50 fields/sec, resulting frame refresh rate at 30/25 frames/sec

Colour SignalsThe three main properties of a colour source that the

eye makes use of are:Brightness: represents the amount of energy that stimulates

the eye (from black-lowest to white-highest)

Hue: Represents the actual colour of the source (each colour has a different frequency/wavelength)

Saturation: represents the strength of the colour, a pastel colour has low saturation than red.

Luminance is used to refer to the brightness of a source, and hue and saturation (concerned with its colour) are referred to as chrominance characteristics

Colour Signals

A range of colours can be produced on a TV display screen by varying the magnitude of the 3 electrical signals that energize the red, green, and blue phosphors.

Eg: 0.299R+0.587G+0.114B = White on display screen.

Hence, since the luminance of a source is only a function of the amount of white light it contains, for any colour its luminance can be determined by summing together the 3 primary components that make up the colour in this proportion

Colour SignalsYs = 0.299Rs+0.587Gs+0.144Bs

Ys is the amplitude of the luminance, since luminance is the amount of white light it contains, it is same as the signal used by a monochrome TV.

Two other signals, the blue chrominance (Cb), red chrominance (Cr), are used to represent the coloration –hue and saturation – of the source.

They are obtained from 2 colour difference signals

Colour Signals

Cb = Bs-Ys and Cr = Rs-Ys

The combination of the three signals Y ( amplitude of luminance signal), Cb (blue chrominance) , and Cr (red chrominance) contains all the necessary information to describe a colour signal, while at the same time being compatible with monochrome TV which use the luminance signal only.

Chrominance components

All color TV system use this same basic principle to represent the coloration of a source, there are some small differences between the two systems in terms of the magnitude used for the two chrominance signal.

The constraint that the BW of the transmission channel for color broadcast must be the same as that used for monochrome.

In order to fit the Y, Cb and Cr signals in the same BW, the 3 signals must be combined together for transmission.

Chrominance components The resulting signal is then known as the composite

video signal.

If the two color difference signal are transmitted at their original magnitudes, the amplitude of the luminance signal can become greater than that of the equivalent monochrome signal. This leads to a degradation in the quality of the monochrome picture and hence is unacceptable.

To overcome this effect, the magnitude of the two color difference signals are both scaled down.

Chrominance components

Scaling factors used for the 3 signals are:PAL: Y = 0.299R+0.587G+0.144B

U = 0.493 (B-Y)

V = 0.877 (R-Y)(Cb and Cr are referred to as U & V respectively)

NTSC: Y = 0.299R+0.587G+0.144B

I =0.74 (R-Y) – 0.27 (B-Y)

Q = 0.48 (R-Y) + 0.41 (B-Y)

Signal bandwidth

Band width of the transmission channel used for color broadcasts must be the same as that used for a monochrome broadcast.

The two chrominance signals must occupy the same bandwidth as that of the luminance signal.

Most of energy associated with luminance signal is in the lower frequency signals and hence the lower part of its frequency spectrum

Signal Bandwidth In order to minimize the level of interference between the

luminance and two chrominance signals, firstly, the latter are transmitted in the upper part of the luminance frequency spectrum using 2 separate sub-carriers and secondly, to restrict the bandwidth used to the upper part of the spectrum, a smaller BW is used for both chrominance signal

Both chrominance subcarriers have the same frequency, but they are 90° out of phase with each other.

By this technique, the two signals can use same portion of the luminance frequency spectrum.

Signal Bandwidth

• In NTSC the eye is more responsive to the I signal than the Q signal, hence maximizing the available bandwidth and minimizing the level of interference with the luminance signal is needed

• I signal bandwidth – 2 MHz

• Q signal – bandwidth – 1 MHz

Signal Bandwidth

• In PAL, the larger luminance bandwidth (5.5MHz to 4.2MHz)

• Allows both U and V chrominance signals have the same modulated bandwidth of 3 MHz

• The addition of the sound and video signal is called the complex baseband signal

Analogue Colour Encoding

• There are three main systems of analogue colour encoding: NTSC (used in USA), PAL (used in UK) and SECAM (used in France)

• All three systems split the colour picture into luminance and chrominance

• All three types use the colour difference signals to transmit the chrominance

• SECAM transmits the colour difference signals on alternate lines

• The other two systems NTSC and PAL transmit both chrominance components simultaneously using a technique known as Quadrature amplitude modulation (QAM)

Digital Video

• With digital television it is more usual to digitize the three component signals separately prior to their transmission to enable editing and other operations to be readily performed

• since the 3 component signals are treated separately in digital TV, it is possible to digitize the 3 RGB signals that make up the picture. Disadvantage is that same resolution must be used for all 3 signals.

•Since the eye is less sensitive for colour than it is for luminance, a significant saving in terms of resulting bit rate and BW can be achieved by using the luminance and two colour difference signals instead of the RGB directly

Digital Video

The international telecommunication union-radio communications branch (ITU-R), known as consultative committee for international radio communications (CCIR) defined a standard for digitization of video pictures known as Recommendation CCIR-601.

Digitization formats exploit the fact that the two chrominance signals can tolerate a reduced resolution relative to that used for the luminance signal

This is the digitization format used in recommendation CCIR-601 for use in TV studios

The 3 component can have bandwidths of up to 6MHz for the luminance signal & less than half this for the two chrominance signals

To digitize these signals, band limiting filters of same specification with a sampling rate of 12MHz & 6MHz used

However, line sampling rate of 13.5MHz for luminance & 6.75MHz for 2 chrominance signals was selected.

4:2:2 format

4:2:2 format

Number of samples per line chosen is 702.

In 525 line system: Total line sweep time 63.56µs, retrace 11.56µs, active line sweep time 52µs.

In 625 line system: Total line sweep time 64µs, retrace 12µs, active line sweep time 52µs.

In both case sampling rate is 13.5MHz

52 x 10-6 x 13.5 x 106 = 702 samples per line

4:2:2 format

In practice, number of samples per line is 720 resulting in small number of black samples at the beginning and end of each line for reference.

No of samples for each of 2 chrominance signals is half value 360 samples per active line

This results in 4Y samples for every 2Cb & 2Cr samples, hence 4:2:2

The number of bits per sample is 8 for all 3 signals corresponding to 256 quantization intervals

4:2:2 format

Vertical resolution for all 3 signals was also chosen to be same

480 lines (active) with 525 line system, 576 lines with 625 line system

Since, 4:2:2 is intended for TV studios, non interlaced scanning at a frame refresh rate of either 60/50 Hz for 525/625 line system respectively.

4:2:2 Format (4Y, 2Cb, 2Cr)

• Since each line is sampled at constant rate (13.5 & 6.75MHz) with a fixed number of samples per line (720 & 360), the samples for each line are in a fixed position which repeats from frame to frame.

• The samples are said to be orthogonal & sample method orthogonal sampling.

4:2:0 Format

• It is a derivative of the 4:2:2 format and is used in digital video broadcast applications (achieving good picture quality)

4:2:0 Format

It is derived by using the same set of chrominance samples for 2 consecutive lines.

Since, it is intended for broadcast applications, interlaced scanning is used

Absence of chrominance samples in alternative lines, hence 4:2:0.

This yields the same luminance resolution as the 4:2:2 format but half the chrominance resolution.

525 line system: Y= 720 x 480; Cb = Cr = 360 x 240

625 line system: Y= 720 x 576; Cb = Cr = 360 x 288

Bit rate: 13.5 x 106 x 8 + 2(3.375 x 106 x 8) = 162Mbps

HDTV formatsDigitization formats associated with HDTV. Resolution for 4/3 aspect ratio tubes 1440 x 1152 pixelsFor 16/9 wide-screen tubes is 1920 x 1152 pixels. In both cases, number of visible lines per frame is 1080

Both use either 4:2:2 format for studio applications or 4:2:0 format for broadcast applications.

Frame refresh rate is either 50/60 Hz with 4:2:2 format or 25/30 Hz with the 4:2:0 format

In 1440 x 1152 resolution, bit rates are 4 times the values derived previously and proportionally higher for wide-screen format.

SIFSource intermediate format gives a picture quality comparable

with that of VCRs It uses half the spatial resolution in both horizontal & vertical

directions as that used in 4:2:0 format a technique known as sub sampling

Uses half the refresh rate known as temporal resolutionFrame refresh rate is 30/25 Hz for 525/625 line systemNon-interlaced scanning (progressive) is used.The digitization format is 4:1:1.525 line system: Y= 360 x 240; Cb = Cr = 180 x 120

625 line system: Y= 360 x 288; Cb = Cr = 180 x 144

Bit rate: 6.75 x 106 x 8 + 2(1.6875 x 106 x 8) = 81Mbps

Sample positions for SIF and CIF

CIFCommon intermediate format used in video conferencing

applications It uses a combination of the spatial resolution used for the

SIF in the 625 line system and the temporal resolution used in 525 line system.

This yields spatial resolution of: Y = 360 x 288, Cb = Cr = 180 x 144

With temporal resolution of 30Hz using progressive scanning

Position of the sampling instants are same as SIF and hence the digitization format is 4:1:1

Bit rate also same of 81Mbps

CIFVideo conferencing applications involve linking desktop PCs

or linking set of video conferencing studios.Bit rate used is 64Kbps ISDN channel for PCsFor video conferencing studios, dedicated circuits are normally

used that comprises multiple 64Kbps channelsBit rate of these circuits is much higher, typically 4 or 16

64Kbps channels, then a higher resolution version of the basic CIF can be used to improve the quality of the video.

Eg: 4 CIF: Y = 720 x 576

Cb = Cr = 360 x 288

16 CIF: Y = 1440 x 1152

Cb = Cr = 720 x 576

QCIF

Quarter CIF format used in video telephony.Uses half the spatial resolution of CIF in both horizontal and

vertical directions.Temporal resolution is divided by 2 or 4This yields spatial resolution of: Y = 180 x 144, Cb = Cr = 90 x 72

With temporal resolution of 15 or 7.5HzBit rate: 3.375 x 106 x 8 + 2(0.84375 x 106 x 8) = 40.5 MbpsThe digitization format is 4:1:1

Sample positions for QCIF

QCIFA typical video telephony application involves a single

switched 64Kbps channel and QCIF uses such channelsLower resolution versions of QCIF are used in applications

that use lower bit rate channels such as that provided by a modem and the PSTN

These lower resolution version are known as sub QCIF or S-QCIF

Eg: Y = 128 x 96

Cb = Cr = 64 x 48

In practice, a small window of a larger screen is used for video telephony and hence the total set of samples may occupy all the pixel positions on the screen.

PC video

All digitization formats are intended for standard TV receivers.

However multimedia applications involve live video, use a window on the screen of a PC monitor for display purpose.

Eg: video telephony, video conferencing, video in a window.

For multimedia applications that involves mixing live video with other information on PC screen, the line sampling rate is modified in order to obtain the required horizontal resolution.

PC videoLine sampling rate is reduced from 13.5MHz to 12.2727MHz for

525 line monitor, and increased from 13.5MHz to 14.75MHz for 625 line monitor.

In desktop video telephony and video conferencing, the video signals from the camera are sampled at this rate prior to transmission and hence can be displayed directly on PC screen.

In digital TV broadcast a conversion is necessary before the video is displayed.

All PC monitors use progressive scanning.

Video content

The video may be generated by a computer program rather than a video camera, they are referred as computer animation or animated graphics.

Special programming languages is available for creating computer animation.

Graphical image produced by a graphics program can be represented in the form of either a high level program or a pixel image, so a computer animation can be represented in the form of either an animation program or digital video.

Video contentDigital video from of animation requires more memory

and transmission bandwidth than the high level program form.

Negative form of high level program is that the low level animation primitives that the program uses – move object, rotate object, object fill, so on…..have to be executed very fast in order to produce smooth motion on display.

Hence it is common to have additional 3D graphics accelerator processor to carry out these functions.

The End