Analog Signal Into A Digital Signal example essay topic

Digital Television Television is a medium for recording and transferring images and sound from one point to another. The Image portion of the system is a light-To-light system, gathering variations in light at the source via pickup device such as a camera and recreating those variations in light as a visual image via a display device such as cathode ray tube (CRT). Television was initially intended for sports, news and entertainment applications and its now finding application in telecommunications as videoconferencing, for computing as desktop video, for medicine as telemedicine and in education as distance learning, because of this rapid application, digital television gives the development of accurate and detailed picture and sound. Digital Television does not imply digital transmission or reception of some new type of TV signal. The Television set tackle in this term paper receive conventional TV signals and produce corresponding pictures and sound which is superior in conventional Television. Digital Television came into being when ITT developed five Integrated circuit (ICs) that replace virtually all of the circuits between the tuner or what we called channel selector and the picture tube / audio amplifier section found in analog or conventional television set.

The five digital ICs replace about 24 analog ICs in conventional television and this ICs are VLSI (Very Large Scale Integration) devices. The key factor to digital television is in analog to digital (A / D) and digital to analog (D / A) conversion. In the simplest terms, the analog signal which is the composite video and sound at the output of television tuner are converted to their digital equivalent by and analog to digital (A / D) process somewhat similar to that found in compact discs. The resultant digital signals are then processed to produce the corresponding video and audio in digital form. When the processing is complete, the digital signals are restored to analog form by digital to analog (D / A) converters and applied to the picture-tube and audio circuits. The analog to digital (A / D) conversion, digital processing, and Digital to analog (D / A) conversion are all take place in the five digital ICs.

This term paper are primarily the discussion of digital television system (block diagram), the basic principles in how to the conversion take place, the comparison of both analog television and digital television tackles in research and analysis section and the advantages tackles in finding section. BACKGROUND OF STUDY To understand further the comparison of both digital and analog television and there advantage listed in finding section of this term paper. Let as first tackle the digital television system, this included parts of Analog to digital and digital to analog conversion and the THE DIGITAL TELEVISION SYSTEM basic digital conversion and storage concept. FIGURE 1 DIGITAL TELEVISION SYSTEM CRT Figure 1, show the digital television system which consist of image source or a image you want to capture, a means such as a camera of converting the image into analog electronic signal, a mean of transforming the analog signal into a digital signal (analog to digital converter or A / D), some process (PD) which is applied to the digital signal, a means of re-transforming the resulting digital signal back to the analog domain (digital to analog converter or D / A) and means such as a display of reconverting the analog signal back into light for the benefit of the observer. The digital process varies according to the application. It can be a variable time delay used for frame synchronization and time base correction a video processor that correct for luminance and chrominance error, an image manipulator that permits digitally generated effects and graphics to be added to the image a data-compression scheme to reduce the amount of data or simply a transmission path.

ANTIALIASING SAMPLE & QUNTIZING ENCODER FILTER HOLD LEVEL SELECTION fs FIGURE 2 THE A / D CONVERTER INPUT DECODER SAMPLING RECONSTRUCTION REGISTER fr FILTER FIGURE 3 D / A CONVERTER The A / D converter a show in the figure 2 consist of four basic component's, a band-limiting, antialiasing filter. a sample- and-hold circuit (S&H) that samples the analog signal, the quanti zing unit (Q) that divide the range of each analog signal sample into Q distinct levels, and finally an encoder (ENC) that place a specified code on the output data line (Do-Dn) for each of the Q level. The D / A converter (see the figure 3) consist of four basic component, a digital input register (REG) in which the bits of the world to be converted are stored for one time period, a decoder (DEC) that converts the data lines into Q distinct analog levels, a resampling (Rc) circuit that correct for distortions due to the digital sample and hold process, and a band-liming, restoration or reconstruction filter (FD). BASIC DIGITAL CONVERSION AND STORAGE CONCEPTS Special effect (as the advantage listed in finding section) like preview mode, freeze mode, editing mode and finally the picture in picture mode, are made possible by converting the conventional analog video signal at the tuner output to a digital format of data bits. These data bits are stored in the memory as is done with computers. Later, the date bits are read from memory and changed back to an analog signal that produces the desired special effect pictures. FIGURE 4 BASIC DIGTAL CONVERSION / STORAGE Figure 4 A, illustrates an original analog video signal for one horizontal line.

Figure 4 B, show how the analog level of the signal is sampled many times at regular intervals. Figure 4 C, shows the video signal reconstructed by adding the analog levels at each sample time. Figure 4 D show the reproduced signal after filtering. As shown in figure 4, to convert an analog signal to a digital format, the specific level of the analog signal must be sensed at periodic intervals.

These intervals are called sampling times. The analog value at each sample time is converted to a digital code or data bits that is stored in memory. When the information level for a sample time is read from memory, the data bits are converted to the specific analog level represented by that specific sample-time code. The series of sample-time analog levels are then filtered to reproduced the original video level.

BASIC A / D Conversion Figure 5 shows the basic circuit involved for A / D conversion. Keep in mind that these circuits are general in nature and do not apply to any specific digital TV. the functions shown are performed within ICs. The first step in A / D conversion is to limit the maximum desired input frequency with low-pas filter (LPF). Although this also limits resolution, the use of a filter prevent high frequency noise and possible beats signals. The limiting effect on resolution are generally not noticeable in the picture. The next step in A / D conversion is to sample the analog signal.

The sample clock determine when the samples are taken. The capacitor holds the instantaneous sample until the next sample occurs. This familiar sample-and-hold process generates an equally familiar staircase signal. Note that the staircase signal is still in analog from, since the signal still contains discrete level for each sample. The staircase signal is applied to Quantization circuit. The input of the quantization circuit is a series of voltage comparators.

Through a voltage divider, each comparator receives a different reference voltage. The amplitude of the incoming signal determine the number of comparators that produce an output to an encoder. Typically, the output from the comparator's swing from one state to another (0 to 1, or 1 to 0, depending on design) when the input voltage is equal to (or greater than) the reference voltage. The encoder converts the number (represented by the comparator that are switch to the 1 or 0 states) to a digital word representing the level of each sample. BASIC D / A CONVERSION Figure 6 shows the basic circuit involve for D / A conversion. The digital decoder determine how many of the output inverters turn on (to 1 or 0, depending on design) for a given digital word.

The familiar ladder or parallel resistor (having different value) convert the signal sample back to a staircase analog signal. As usual, the resistor value are multiples of the powers of 2. This corresponds to the value of each bit in the digital word. A larger sample produces a lower ladder output voltage. The output inverter correct this reverse relationship and produces the correct analog signal. Any sharp. level change in the signal from the ladder inverter are smoothed by LPF.

The resultant analog output signal from the D / A converter is an exact reproduction of the input analog signal to the A / D converter. The LPF also function to eliminate unwanted harmonic produced by the sharp level change in the staircase signal. RESEARCH AND ANALYSIS: DIGITAL VERSUS CONVENTIONAL (ANALOG) TELEVISION To compare the basic functions of digital TV to a conventional TV the figure shows the basic block diagrams of both conventional and digital TV. as shown in figure 7 in digital television the central control unit (CCU) is a microprocessor that controls all operation in digital TV. There is no comparable circuit in an analog or conventional television. The detected video out put of the tuner / FIF stage supplies an analog signal to the A / D. Next, the digital Y / C circuit perform all necessary processing of the video component and generate the red, green, and blue (RGB) signals. The D / A converter supplies the processed signal to drive the cathode ray tube (CRT).

The deflection circuit in a digital television use the sync portion of the digital video signal. The digital deflection circuits perform all necessary corrections for the deflection yoke and CRT system digitally, and generate the necessary output. Once converted to analog, the signals drive the deflection yoke and flyback transformer in the normal manner (similar to a conventional or analog TV.) With digital television, sensor circuits provide feedback information from the CRT system, while memories store information for desired performance (color, tint, etc. ). The actual feedback from the sensors is compared to the memory data, and the correction signals are produced automatically to keep the CRT system producing the desired picture. Since all of these functions are under digital control in a digital TV, many of the alignment found in a conventional TV are unnecessary.

Similarly, many circuit components such as inductors and capacitors (that cannot be incorporated into ICs) are not necessary. For example, many of the delay lines, filters, and phase shifters found in conventional TV are eliminated. instead, clocking circuit count precise times for sampling or out signals in digital TV. Also, by using a clock frequency four times that of the color sub carrier, all functions are at 90 degrees intervals of the color signal. This makes for easy hue changes in the color control system. Finally, because of fewer discrete components and more automatic digital functions, direct interfacing with external digital systems is made easier (without the usual A / D or D / A conversion. For example. direct interfacing to computers is possible, as well as direct interfacing to matrix or flat-screen TV displays.

FINDINGS: The obvious finding in digital processing in the quality of the signals reproduced. As the case of compact discs, the tuner signals to be processed are sampled at high frequency, the sampled signals are stored by digital means and after processing, the restored digital signals are retrieved. If the TV circuit can tell the difference between a 1 and a 0 (the basic digital code), the video and audio signals can be applied to the picture tube / loudspeakers exactly as they appear at the tuner output. As a point of reference, the picture / sound performance of a digital TV is about the same as compact disc performance when compared to audio tapes and LPs. A digital television set can easily be adapted for the three basic TV broadcast systems. This is because the sampling clock for the A / D converter is phase locked to the broadcast colorburst frequency.

Simply by changing the frequency of the clock, the system can accommodate different TV broadcast system like NTSC (3.58 MHz) or PAL (4.43 MHz) colorburst frequencies. In many cases, there are no coupling components between circuits in digital TV. This mean that there are no capacitor, inductor, or RC circuits to break down or to distort audio / video signals. generally, all connections between ICs in digital TV are made directly through wires or wire buses. The digital signal processing is controlled by phase-locked loop (Plus), providing much more accurate control.

For example, TV sync checked against broadcast sync on every horizontal line. Since the TV picture can be stored in digital form, a digital caused by interlaced scanning and can increase the TV can reduced flicker apparent resolution of the picture. Digital TV eliminates interlace by storing all 525 lines and then displaying the complete picture on the screen all at once (instead of having only half the total scan lines on the screen for each field of video, as is the case for analog or conventional TV. Noise-reduction circuit and echo-canceling systems are included in some digital TV systems. this feature provide superior picture quality for viewers in weak-signal area, or broadcast. In addition findings to the superior Z Z areas where are excessive ghosts performance of digital television is the ability to display special effect like preview mode which display the still picture of nine channel in sequence; the Freeze mode which freeze the current picture being viewed; the editing mode which allow the user to change the still picture displayed in the strobe mode, and the picture-in Picture mode which inserts a 1/9 normal size, real time picture from an external video input in the corner of the screen. CONCLUSION: Television is arguably the most commonly used image communication system in the world today.

However, the present TV transmission standard are base on analog technology. but by the invention of digital television you can easily adapted the three TV broadcasting system (NTSC, PAL, & SECAM) with video & audio quality performance and can create special effect. Therefore digital television have more efficient and accurate over analog format.

Bibliography

1. Edward C. Jordan: Digital Television system: Reference Data For Enginners PP 35 2. B.W. beyers: Digital Television: Opportunities for change. I trans cons. Electronics vol. 38 no. 1 pp x -xiv, Feb 1992 3.
Digital television, I spectrum pp 34-80 apr 1995 Bibliography 1.
I trans cons. Digital television, I spectrum pp 34-80 apr 1995.