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Digitization of video signal is done by an ADC driven by clock pulse locked to incoming video.


The clock frequencies used should be greater than the Nyquist rate now for the color subcarrier frequency of 3.58 MHz For NTSC or 4.43 MHz for PAL systems The sampling frequency that we get for NTSC/PAL is 10.74/13.29 MHz (3 times) or 14.32/17.72 MHz (4 times) or on the other hand 13.5 MHz can be used for both NTSC and PAL systems. Using 8 bits per sample we can achieve a resolution of 256 levels.





Type of image Pictorial / Non-pictorial
Photometric contents Monochrome / color / multispectral
Amplitude Scale Binary, Continuous tone
Type of signal processing Analog / Digital
Processing Dimensionality Line scan Processing /

Intra frame processing /

Interframe processing

In digital image processing the analog image is first digitized and then processed such that the processed signals digital rate is reduced compared to the input digitized signal .Therefore after DAC the analog bandwidth is reduced compared to a normally scanned signal.Upon playback de compression is performed in a digital manner.


One of the simplest method of reducing the bandwidth is to reduce the inherent bandwidth of requirement of an image sensor by limiting its spatial resolution and frame repetition rate.

There exists a large amount of statistical redundancy or correlation in normal image .For e.g. two points spatially close to each other tend to have the same amount of brightness and color level. Certain amount of information is irrelevant and may be eliminated without causing a loss in image quality and a large amount may be eliminated with only a slight loss in detail.

Variable spatial resolution can also be used as a mean of bandwidth reduction. Normally, in an image the object of interest lies at the center while the peripheral information is not as useful. In addition, peripheral vision resolution is degraded from foveal vision for a human observer The picture information from frame to frame changes by approx. 10% while the change in brightness is 8%

Now Image processing consists of

Image Enhancement

Image restoration

Image data compression.

Jags World 1999 This page was last updated on Tuesday, July 06, 1999 .

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