Digital TV System

DIGITAL TV SYSTEM

· Analog signals suffers degradation due to noise, crosstalk, linear and non-linear distortion. In this, analog is converted to digital by analog to digital converter ADC performs sampling, quantization and encoding.

Sampling:

The TV signal is sampled at rate of twice the maximum band limit frequency- Nyquist criteria.

Fourier transform of band limit signal at T seconds is scaled by 1/T and given by wo = 2π/T.

If input signal is not band limited or sampling frequency does not meet the requirement of sampling theorem, the various components of spectrum overlap.

 It suffers from distortion called aliasing effect and moiré patterns appear in picture.

To avoid aliasing errors, the sampling frequency must exceed 10mhz.

Quantization:

The sampled output, discrete in time in continuous in amplitude and assigned a discrete numerical values as ADC.

Since DAC is reverse process.

Encoder:

The sampled value is assign a binary code along with additional coding like parity for error detection , scrambling,etc.

Errors occurred are eliminated by redundancy in digital signals.

Because of this, delay may occurs in TV , so forward error correction (FEC) is obtained.

Block coding and convolution coding are two approaches in this direction.

3DTV

For 3D picture, we have to show the depth also along with length and breadth. ( L + B + H)

Principle:

The left and right eye of human see a different image of same object, so brain interprets these as a single composite image in 3D.

Two different images are generated by 2 cameras, like our eyes located.

For stereo sound transmitter, the left and right channel signals are FM at 5.5MHz audio carrier.

The combined audio and video signals are amplitude modulated by channel carrier using separate modulators.

USB and LSB selected and combined before final transmission.

3-D Display methods:

By using 3 set of color signals we reproduce the 3-D effect on picture tube screen.

First method:

All green color phosphor strips excited by green(G) video signal right. Similarly red color phosphor strips excited by red(R) video signal left.

Blue (B) color phosphor strips are not exited at all.

 If viewer wears, G filter at right eye and R filter at left eye, than brain combined to form various Y shades.

Second method:

Two video display units(VDU) are used, to create 3D pictures in natural colors.

A polarizing film is fitted in front of VDU and light outputs are reflected using a mirror arrangement on screen.

Third method:

This is known as ABDY 3D system.

In this, R color information are delayed by nearly 600ns and it is obtain by using delay circuits.

So, double image is formed on screen of picture tube.