NAPIER. University School of Engineering. Advanced Communication Systems Module: SE Television Broadcast Signal.

Transcription

1 NAPIER. University School of Engineering Television Broadcast Signal. luminance colour channel channel distance sound signal By Klaus Jørgensen Napier No Teacher Ian Mackenzie

2 Abstract Klaus Jørgensen page 2/18 In this paper there is a brief history about the television from black and white to HDTV (High Definition Television). Some theoretical explanation of interlaced scanning and how it is used in television broadcasting, and some theoretical explanation of the bandwidth and spectrum in a European Pal system, how the luminance, colour and sound signal is combined together. The different kind of synchronisation methods, horizontal and vertical synchronisation pulses, and a small explanation of teletext, and some description of the colour signal and the use of the colour burst signal, there are also described some of the advantages and disadvantages about the different TV systems used in the world today.

3 Klaus Jørgensen page 3/18 1. Table of contents 1. Table of contents Introduction Interlaced scanning Bandwidth and Spectrum Synchronisation methods Horizontal signal and sync pulse Vertical sync pulse Teletext Colour signal Colour bar Colour burst TV systems NTSC/525 Advantages NTSC/525 Disadvantages PAL/625 Advantages PAL/625 Disadvantages SECAM/625 Advantages SECAM/625 Disadvantages Conclusion Biography Reference...18

4 Klaus Jørgensen page 4/18 2. Introduction Interlaced scanning was developed in order to save broadcast frequency bandwidth in the early part of the 20 th century and at the same time reduce image flicker. A practical non-interlaced display was not possible at the time when television was in its infancy, so a compromise picture scanning system had to be developed for the most promising display technology of the time, that being the CRT (Cathode Ray Tube). The CRT was developed long before broadcast television. In fact CRT technology is over 100 years old. It has of course undergone many refinements over the years but the basic principles of its operation are the same as they have always been. The USA was first in starting regular colour TV broadcasts, in Japan was second, starting in 1960 using the American NTSC TV broadcast system. In 1965 Europe failed to agree on one colour TV broadcast system, France went SECAM, Eastern Europe decided to use a variety of SECAM, while most of the remainder of Europe decided to use PAL. England and Germany started regular colour TV broadcasts in By 1970 regular colour TV broadcasts had started in most European countries. Digital Television is considered to be one of the most important television innovations since colour TV. Digital television technology begins a new era of convergence between home electronics and computing technology. One digital channel can carry several sub-channels, the reason for this is because digital TV standards allow a number of different formats. 480i - The picture is 704x480 pixels, sent at 60 interlaced frames per second (30 complete frames per second). 480p - The picture is 704x480 pixels, sent at 60 complete frames per second. 720p - The picture is 1280x720 pixels, sent at 60 complete frames per second. 1080i - The picture is 1920x1080 pixels, sent at 60 interlaced frames per second 1080p - The picture is 1920x1080 pixels, sent at 60 complete frames per second (30 complete frames per second). "p" stands for "progressive" and "i" stands for "interlaced." In a progressive format, the full picture updates every sixtieth of a second. In an interlaced format, half of the picture updates every sixtieth of a second.

5 Klaus Jørgensen page 5/18 3. Interlaced scanning Figure 3.1 shows a typical CRT (Cathode Ray Tube) screen from the side, which is used in any ordinary television and computer screen to day. Figure 3.1 [2] The picture in a television is made by lines as showed in figure 3.2 each line present a part off the whole picture, there is 625 lines per Picture in a PAL system and 525 lines per picture in a NTSC system, there has to be a minimum of 20 hole pictures in a second so that the eye don t respond to the flicker from the picture, in a PAL system there is 25 pictures per second and 30 in a NTSC system. Figure 3.2 [2] One picture is made by 2 sub-pictures (First and Second Field figure 3.2) this gives 50 sub-pictures per second, which gives a picture of 50Hz, in a television with 100Hz there is 4 sub-pictures per second, each line need a synchronization pulse and so dos every sub-picture, this synchronization pulses are made by using a frequency of 31250Hz which is divide by 2 to make the synchronization pulse for each line and by 625 to make the synchronization pulse for the sub-picture.

6 Klaus Jørgensen page 6/18 Figure 3.3 [1] The line synchronization pulse is made from the 312,5 lines there is in a sub-picture and the time it takes to make one sub-picture 1/50Hz this gives a line frequency of ,5 = 15625Hz One line has a time of 64µs this time comes from the line frequency of 15625Hz. 1 = 64μs 15625Hz Each line has a forward run and a reverse run, the forward run takes 52µs and in this time the line is ON and show a part of the picture, the reverse run takes 12µs in this time the line is OFF and is invisible, the forward run take place from left to right of the screen, the reveres run is then from right to left screen, as shown in figure 3.2. forward run :52μs + reverse run :12μs Totale time :64μs One sub-picture has the length of 20ms this time comes from sub-picture synchronization pulse 1 = 20ms 50Hz This time is divided in to two times a forward run and a reverse run, the forward run shows 287,5 lines and have the time of 18,4ms, and will move from the top of the screen to the bottom of the screen, as shown in figure ,5 64μ s = 18, 4ms The reverse run is made by 25 lines and takes 1,6ms, in this time the line is OFF and is invisible, and will move from the bottom of the screen to the top, to begin a new picture, as shown in figure 3.2. sub - picture forward run :18,4ms + sub - picture reverse run : 1,6ms Totale time of sub - picture: 20,0ms

7 Klaus Jørgensen page 7/18 4. Bandwidth and Spectrum The frequency spectrum that are used for television broadcast is shown in figure 4.1 and for the VHF band the channel with is 7MHz, and for the UHF band the channel with is 8MHz. VHF band I 47 68MHz Channel 2 4 VHF band III MHz Channel 5-12 UHF band IV MHz Channel UHF band V MHz Channel Figure 4.1 [1] The signal that is broadcasted for a television is made by 3 signals, a luminance signal (black and white signal), a colour signal and a sound signal this is shown in figure 4.2 and 4.2. The luminance and colour signal is Amplitude-Modulation (AM), and the sound signal is Frequency-Modulation (FM). The maximum modulation for the luminance signal is 5MHz, because is it a AM signal modulated onto a carrier, a bandwidth of 10MHz is required, however a VHF channel only has a bandwidth of 7MHz and a UHF channel only has a bandwidth of 8MHz and that must also include the FM sound signal. To get a smaller bandwidth of the luminance signal the lower sideband is reduced to 1,25MHz and the upper sideband mains intact figure 4.1 and 4.2. The colour signal has it own sub-carrier at 4,43MHz with sidebands of ±600kHz figure 4.1 and 4.2, the sound which is FM, has a carrier 5,5MHz above the luminance carrier which has sidebands of ±50kHz and the carrier of the Nicam sound is 5,85kHz above the luminance carrier. luminance colour channel channel distance sound signal Figure 4.2 [1]

8 Klaus Jørgensen page 8/18 The lower sideband is heavily reduced by a filter near the output of the transmitter, it is only necessary to transmit one sideband, the reason that some of the lower sideband is still there is because, if is was completely removed it would affect amplitude and phase of the lower frequencies in the upper sideband and the carrier frequency, hence the signal in figure 4.2 is the final structure of a TV signal. lower sideband upper sideband Couler Signal Picture carrier Sound Signal -1,25 MHz Figure 4.2 [1] Figure 4.3 shows a table of information about the different Television broadcast systems there are used in the world today. System Pal B (VHF) Pal D Pal E (VHF) Pal G (UHF) Pal I Pal M (NTSC) Area Europe East Europe France Europe UK. USA/Japan Lines per Picture Pictures per second Sub-pictures Line frequency Sub-pictures frequency Picture format 4 : 3 4 : 3 4 : 3 4 : 3 4 : 3 4 : 3 Video Bandwidth (MHz) Distance Picture/Sound +5,5 +6,5 +11,5 +5, carrier Channel Width Picture Modulation AM AM AM AM AM AM Sound Modulation FM FM AM FM FM FM Figure 4.3 [1]

9 Klaus Jørgensen page 9/18 5. Synchronisation methods The television signal carries pulses (at sub-black levels) that allow synchronisation of both the horizontal and vertical deflection circuits. These pulses are separated by the video information using a sync separator. There are two deflection generators one for horizontal and one for vertical. Both must be triggered at the right time to ensure that the picture is correctly reproduced. Therefore both time-bases need to be synchronised and there needs to be a method of triggering either one independently using the same single set of sync pulses. The method used to allow the receiver to distinguish between the two types of sync is to use different widths for the horizontal and vertical sync pulses. 5.1 Horizontal signal and sync pulse The sync and brightness information are kept separate by using different voltage levels for the sync and brightness information signal, the total range of a monochrome (black and white) signal is 1 Volt peak-to-peak. The luminance signal occupies a range of 0.7 V and the sync 0.3 V as shown in figure 5.1. The brightest parts of the picture are at a level of 1 V with the black level at 0.3 V. Line blanking occurs in the non-picture parts of the waveform when it is necessary for the electron gun of the CRT screen to be 'off' so that the retrace is not visible on the screen. A single line of a TV signal is shown in figure 5.1. The synchronising pulse in figure 5.1 is separated from the active picture information by the 'porches': the 'back' and 'front' porches. These avoid the picture information to have an affect on the accuracy of the synchronisation. The line blanking has a length of 12µs out of these 12µs the 4.7µs is the synchronising pulse which is shown in figure ,7V 0,3V Figure 5.1

10 5.2 Vertical sync pulse Klaus Jørgensen page 10/18 The vertical sync is similar to line sync but of longer duration and is used at the start of each sub-picture at the top of the TV screen. Before and after the vertical sync pulse there is 2½ lines divided up in 5 pulses of 32µs (figure 5.2 and 5.3) with a frequency of 31250Hz this pulses is called equalisation pulses the purpose with this pulses is to ensure that the lines from each sub-picture is correctly weave into each other. The vertical sync pulse has a length of 160µs and is also is divide in to 5 pulses of 32µs (figure 5.2 and 5.3) to ensure the horizontal sync pulse while the vertical sync pulse is present all this happens in the first 5 lines of each sub-picture. 5.3 Teletext The lines 8 (321) to 16 (329) contain teletext data, there is an Insertion Test Signal (ITS) on line 19 (332) and 20 (333) and the first line of the picture is 23 (336). The ITS is normally generated at a studio and measured at various points in the programme chain, including the output of a transmitter, in order to measure the amount of distortion that has been introduced. The binary signalling levels are defined on a scale where television black level is 0% and white level 100%. The binary '0' level is then 0 (±2)% and binary '1' level is 66(±6)%. The difference between these levels is the basic data amplitude. The data waveform will contain overshoots so the peak-to-peak amplitude will exceed the basic data amplitude. The basic data amplitude may vary from Data-Line to Data-Line. The bit signalling rate is Mbits/s. It is 444 times the nominal television line frequency. Figure 5.2 [3] Figure 5.3 [3]

11 6. Colour signal Klaus Jørgensen page 11/ Colour bar The colour signal is made by two signals the luminance and the chrominance signal. The luminance is a weighted sum of the three colours of light used in colour television screens which are Red, Green and Blue i.e. RGB signal, also named the Y-signal. The three colours combined have a value which is equal to the colour white which is 1 (100%) the green colour is 0.59 (59%) the red colour is 0.30 (30%) and the blue colour is 0.11 (11%). Y = ( 0.59 G) + ( 0.30 R) + ( 0.11 B) = 1 Form these three colours it is possible to make eight colours which makes a colour bar as the one shown in figure 6.1. This is often used to test a colour television with, to be secure that the television can show all the colours that are required. Figure 6.1 [4] The eight colours in the colour bar figure 6.1 is from the left to right White, Yellow, Cyan, Green, Magenta, Red, Blue and Black, each colour has its own value as shown in figure 6.2. Colour RGB signal Luminance signal (Y-signal) R G B Y=(0.30xR)+(0.59xG)+(0.11xB) Luminance value White Y=(0.30x1)+(0.59x1)+(0.11x1) 1.00 Yellow Y=(0.30x1)+(0.59x1)+(0.11x0) 0.89 Cyan Y=(0.30x0)+(0.59x1)+(0.11x1) 0.70 Green Y=(0.30x0)+(0.59x1)+(0.11x0) 0.59 Magenta Y=(0.30x1)+(0.59x0)+(0.11x1) 0.41 Red Y=(0.30x1)+(0.59x0)+(0.11x0) 0.30 Blue Y=(0.30x0)+(0.59x0)+(0.11x1) 0.11 Black Y=(0.30x0)+(0.59x0)+(0.11x0) 0.00 Figure 6.2 [1]

12 Klaus Jørgensen page 12/18 The channel bandwidth in a television is too small to transmit all eight colours and the luminance signal together therefore there is made two new signals R-Y and B-Y this signals is made by adding a luminance signal in 180º phase (-Y) to the red and blue colour signal this gives the values shown in figure 6.3 (R-Y) and figure 6.4 (B-Y), and the waveform of the signals is shown in figure 6.5 page 13. Colour RGB signal Luminance signal (Y-signal) Luminance R G B Y = 0.30R G B value R-Y White Y = Yellow Y = Cyan Y = Green Y = Magenta Y = Red Y = Blue Y = Black Y = Figure 6.3 [1] Colour RGB signal Luminance signal (Y-signal) Luminance R G B Y = 0.30R G B value B-Y White Y = Yellow Y = Cyan Y = Green Y = Magenta Y = Red Y = Blue Y = Black Y = Figure 6.4 [1]

13 Klaus Jørgensen page 13/18 Figure 15 shows the values of the RGB, luminance, luminance -180º, R-Y and B-Y signals and the relation between the signals. White Yellow Cyan Green Magenta Red Blue Black Figure 6.5 [1]

14 6.2 Colour burst Klaus Jørgensen page 14/18 The decoding process requires pure sub-carrier reference, the sub-carrier modulated with colour, but an accurate signal free from variation. The decoder, therefore, regenerates from the incoming video signal (CVBS), an accurate sub-carrier, using a colour reference attached to the signal. This is the 'colour burst'. The colour burst signal is made by 10 cycles with a frequency of 4.43MHz and is 0.3 volts peak to peak (0.3Vpp), this signal is placed after the line sync pulse as shown in figure 6.6. Colour burst Sync pulse Figure 6.6 [4] The decoder seeks out the colour burst and from it generates the pure sub-carrier to which it is able to synchronise and so recover from the chrominance, the original R-Y and B-Y signals. It is present at all times, except during sub-picture sync period. The sync detection circuits in receivers, etc. are designed to ignore the colour burst when deriving the black reference. Figure 6.6 has no picture information and is therefore often called 'colour black', or 'black & burst', to ensure that colour burst and colour information remain in very close relationship to each other, the burst is added to the signal as part of the coding process. Figure 6.7 shows the relationship between the R-Y, B-Y, chrominance, luminance and the composite (CVBS) signals. Figure 6.7 [4]

15 Klaus Jørgensen page 15/18 In practise on a oscilloscope the CVBS, chrominance and luminance signals will look like the pictures in figure 6.8. Figure 6.8 [4] 7. TV systems There is three different TV system used in the world today, Pal, NTSC and Secam (see figure 7 on page 8) and each system has its advantages and disadvantages. 7.1 NTSC/525 Advantages Higher Frame Rate - Use of 30 frames per second, reduces visible flicker. Atomic Colour Edits - With NTSC it is possible to edit at any 4 field boundary point without disturbing the colour signal. 7.2 NTSC/525 Disadvantages Lower Number of Scan Lines - Reduced clarity on large screen TVs, line structure more visible. Smaller Luminance Signal Bandwidth - Due to the placing of the colour sub-carrier at 3.58MHz, picture defects such as moiré, cross-colour, and dot interference become more pronounced. This is because of the greater likelihood of interaction with the monochrome picture signal at the lower subcarrier frequency. Lower Gamma Ratio - The gamma value for NTSC/525 is set at 2.2 as opposed to the slightly higher 2.8 defined for PAL/625. This means that PAL/625 can produce pictures of greater contrast.

16 7.3 PAL/625 Advantages Klaus Jørgensen page 16/18 Greater Number of Scan Lines - more picture detail. Wider Luminance Signal Bandwidth - The placing of the colour Sub-Carrier at 4.43MHz allows a larger bandwidth of monochrome information to be reproduced than with NTSC/525. Higher Gamma Ratio - The gamma value for PAL/625 is set at 2.8 as opposed to the lower 2.2 value of NTSC/525. This permits a higher level of contrast than on NTSC/525 signals. This is particularly noticeable when using multi-standard equipment as the contrast and brightness settings need to be changed to give a similar look to signals of the two formats. 7.4 PAL/625 Disadvantages More Flicker - Due to the lower frame rate, flicker is more noticeable on PAL/625 transmissions; particularly so for people used to viewing NTSC/525 signals. Lower Signal to Noise Ratio - The higher bandwidth requirements cause PAL/625 equipment to have slightly worse signal to noise performance than it's equivalent NTSC/525 version. Loss of Colour Editing Accuracy - Due to the alternation of the phase of the colour signal, the phase and the colour signal only reach a common point once every 8 fields/4 frames. This means that edits can only be performed to an accuracy of +/- 4 frames (8 fields). 7.5 SECAM/625 Advantages Stable Hues and Constant Saturation - SECAM shares with PAL the ability to render images with the correct hue, and goes a step further in ensuring consistent saturation of colour as well. Higher Number of Scan Lines - SECAM shares with PAL/625, the higher number of scan lines than NTSC/ SECAM/625 Disadvantages Greater Flicker - (See PAL/625) Patterning Effects - The FM sub-carrier causes patterning effects even on non-coloured objects.

17 Klaus Jørgensen page 17/18 8. Conclusion The use of interlaced scanning has its advantages and disadvantages, depending on which system there is used, in the Pal system there is 625 lines per picture this gives a more detailed picture, then in the NTSC system, where there only is used 525 lines per picture, but the NTSC system shows 30 whole pictures per second this gives a more steady picture then there Pal system which shows 25 pictures per second, the Pal system also comes to better use then the NTSC system when the television screen becomes bigger because is has 100 lines more in each picture. The Bandwidth and spectrum also has its advantages and disadvantages, depending on which system there is used, the Pal system has a bandwidth of 7 8 MHz and the NTSC system only has 6MHz this gives a disadvantages to the NTSC system, is has to transmit a smaller luminance signal that there also has to be room for the colour signal which has a sub-carrier at 3.58MHz in the 6MHz bandwidth, but the Pal system has a worse signal to noise performance then the NTSC system because of the larger bandwidth in the Pal system. The synchronisation methods in the Pal and NTSC systems are almost identical the only difference between them is the length of the vertical sync pulse length, in the Pal system the vertical sync pulse is 160µs, in the NTSC system the vertical sync pulse it 190µs. The calculations of the colours in the Pal and NTSC systems is the same, but in the Pal system there is used R-Y and B-Y signals, there is used some signals called I and Q in the NTSC system, also the sub-carrier for the colour signal in the in the Pal system is 4.43MHz with sidebands of ±600kHz, the Q signal in the NTSC system has a sub-carrier of 3.58MHz with sidebands of ±500kHz, and the I signal has a lower sideband 1.5MHz below the colour sub-carrier. The colour burst is the same for the two signals. The teletext is the same for the two systems as well. 21/ Klaus Jørgensen

18 Klaus Jørgensen page 18/18 9. Biography 1. Fjernsynsmottakeren 2 nd edition by Trond Kristoffersen ISBN: (Norwegian) 2. Modern Electronic Communication 8 th edition. by Jeffrey S. Beasley & Gary M. Miller ISBN: Reference 1. Fjernsynsmottakeren 2 nd edition by. Trond Kristoffersen ISBN: (Norwegian) HANDBOOK of Video Test & Measurement ÓHAMLET VIDEO NTERNATIONAL LTD. 2003