Digital Video Digital video is basically a sequence of digital images  Processing of digital video has much in common with digital image processing First we review the basic principles of analog television

Television Fundamentals Color television cameras and television receivers use the RGB (red, green, blue) color system to create any color We have seen how raster scan devices operate Commercial television systems, however, use interlaced scanning as opposed to the progressive scanning of computer monitors

Television Fundamentals In interlaced scanning, one half of the horizontal scan lines (every other line) are transmitted and “drawn” by the receiver  Then the other half of the lines are transmitted and are drawn in between the first scan lines Each half is known as a field, and two fields together are known as a frame

Television Fundamentals Since the phosphors retain their values for longer than the time that it takes to transmit two fields, and since rate of transmission of a field is shorter than the human eye can perceive, the viewer does not perceive this interlacing If the frame rate is at least frames per second the viewer does not perceive motion in an image sequence as discrete, but as continuous

Interlaced Scanning In the figures, the first field is transmitted at time t = 0 and displayed at time t = f / 2  f is the frame rate The second field is transmitted at time t = f / 2 and displayed at time t = f Note that the display at time t = f consists of information (scan lines) from two distinct points in time

The first field of a television transmission

The second field of a television transmission

A complete frame

Interlacing Interlaced scanning is used in commercial television systems to decrease the bandwidth of the transmitted signal and to reduce the phenomenon known as large area flicker These problems had been overcome by the time bit-mapped computer monitors were being developed

Deinterlacing There are several common operations that you might want to perform on interlaced video  Producing stills  resizing the video  changing the frame etc. Performing these operations on raw, interlaced, video can produce undesirable artifacts

Interlacing Artifact

Deinterlacing Deinterlacing provides a way around these problems All deinterlacing methods involve turning the field-based image into a frame-based image by modifying one of the fields in the image Popular methods include duplication and interpolation

Deinterlacing Duplication Interpolation

Television Systems The exact frame rate depends on the system as does the number of scan lines per frame There are currently three conventional commercial television systems in use throughout the world  North America, South America and Japan use NTSC  The United Kingdom, Western Europe, Africa and Australia use PAL  France, Eastern Europe and Russia use the SECAM

Television Systems SystemScan lines/ frame Frame rate Pixels / frame Bandwidth NTSC52530 / sec 130, MhZ PAL62525 / sec 210,0006 MhZ SECAM62525 / sec 210,0006 MhZ

Digital Television Considering the bandwidth of the NTSC signal, how would digital transmission compare to today’s analog? We have: 30 frames/second x 130,000 pixels/frame x 24 bits/pixel = 93.6 Mbits/second To be competitive with analog transmission, a data compression of more than 20:1 is required  All digital television standards therefore include some form of compression. The disadvantage of digital television therefore, is the extra bandwidth required

Digital Television The real advantage may be seen by examining the signal-to-noise ratio of digital vs. analog television This figure shows the approximate ratio of error rate to signal-to-noise ratio for digital transmission

Digital Television An error rate of or one bit in 100 million bits is practically undetectable Channel error rates of still permit acceptable pictures, especially if error correction techniques are used An analog TV signal requires a channel with a signal-to-error ratio (SER) of 55dB

Digital Television If we use PCM for a digital television signal, the principal source of error is due to quantization  The error is a maximum of + or - 1/2 the least significant bit  For a quantization level of 8 bits, this is + or - 0.2%  This “fine” quantization would appear as white noise if viewed as a picture

Digital Television Theoretically, the SER with 8 bits is 59 dB and for each 1 bit reduction in quantization, the SER is reduced 6 dB The actual SER of a composite color TV signal is about 4 dB less Thus, 8-bit PCM encoding of a noise-free NTSC composite color signal yields a SER of 55 dB

Digital Television A bit error rate of is practically undetectable  From the figure above, this requires a SER of only 21 dB  If we use the rate with error correction bits added, a SER of 18 dB may be sufficient  This requires less than 1 bit/pixel The essential problem in digital TV coding is therefore to reduce the picture bandwidth at the expense of the bit error rate and retain acceptable picture quality

Aspect Ratios Each of the systems listed above has an aspect ratio (ratio of width to height) of 4:3 Cinematic films and high-definition television (HDTV) systems have aspect ratios of approximately 16:9

Aspect Ratios 4:3 aspect ratio 16:9 aspect ratio

Compatible Color TV In order to permit compatibility of color TV transmission with preexisting black and white receivers, the RGB image generated by a television camera is converted to a YIQ image by using the transform

Compatible Color TV

See book notes for more info on YIQ The bandwidth allocated to a black and white television signal is illustrated on the next slide

Compatible Color TV

In order to maintain compatibility, the color TV signal has to fit in the same bandwidth This is accomplished by first combining the I and Q signals using a method called quadrature modulation  The two signals are multiplied by a sine and cosine function, respectively, added and become a single composite signal  The second idea is to choose the color subcarrier to be an odd multiple of one half the line frequency  The resulting bandwidth allocation is illustrated on the next slide

Compatible Color TV

At the receiver, the inverse transformation given on the next slide reforms R, G, B from the received Y’, I’, Q’ signals

Compatible Color TV

Pixel Aspect Ratio When we are displaying a digital video stream on a standard television receiver, we have another parameter to consider - the pixel aspect ratio This is related to the aspect ratio of the television screen and to the sampling rate

Pixel Aspect Ratio If we have a screen with an aspect ratio of 4:3 and we have a digital image of size 711x487, then in order to maintain the 4:3 aspect ratio we must have a pixel aspect ratio p, where p can be found as follows.  3/4 = 487/711 * p  p = 0.75 * 711/487 = Computer monitors generally have pixel ratios of 1.0 (square pixels)

Aspect Ratio Conversion We are given a video sequence with an aspect ratio of 16:9 and we want to display the sequence on a device with an aspect ratio of 4:3 For example, the source image may be 640x360 and the display device may have a resolution of 480x360 We have several alternatives