Basic theory of the Terrestrial Digital television
As in other scopes of the technology, the television also has been digitized by the benefits that 0 and 1 incorporate to the image, benefits that we will be seeing. On the other hand, to understand the operation of the transmission of the TDT, we must approach a series of concepts (of simplified form) so that we can assimilate better how it works
Series of Fourier
Thanks to the French mathematician, whose last name gives name to this theorem, all periodic function (for example, a square signal) can be disturbed as the sum of infinite terms with frequencies and variable coefficients.
We did not enter calculating the coefficients and the rest of sine terms because one escapes to the intention of this subject. We will see it of intuitive form but in some of the examples. For it we are going to suppose a DS that it takes values 1 and zero indefinitely.
It is the way but simple to transmit a information.
We already know that the signals of radio frequencies are sine signals and, therefore, is interesting to see the relation that exists between both.
In the image superior we have a signal to compose (almost one square wave) that is going to be composed by the sum of a series of sine signals of different amplitude and frequency.
To fix you that appears frequency F1, that main overtone is called, of the same frequency that the squared signal, but that 2º harmonic has a frequency that is 3 times the frequency base, that is to say, that f3 is of a greater frequency 3 times to the frequency of signal F1.
We enclose another image where an almost square signal and its components (it is just like imagines before, but shown in separated axes).
By Fourier, to obtain the squared signal, we must add that series of signals, give rise a
Square signal = F1 + f3 + f5 + f6 +….
If we happened of the space of the time to the space of the frequency, we have a graph of the style.
Pick up of image RVA (RGB)
When we recorded with a TV camera, three signals are received: Red, Green and Azul (RVA). In English RGB is denominated (network, green, blue)
If we sent these modulated signals, a suitable receiver would interpret to show them them with its color, brightness, etc., but this does not work thus, since, in the first development of the color TV, many in black and white existed (in the home, a color TV cost grazes) and, therefore, which became would also have merit for teles of black and white (which they only interpreted the brightness and contrasts of the signals.
Histogram of the color
Since we have indicated before, a camera receives the illumination of the scene and translates by means of three specific sensors to signals RGB.
We have a linear answer of the illumination of each color and is translated to a certain tension. But does the eye respond equal?
The idea is to measure the rank of brightness of each point of our image, to be able to form the points of light and the shade. Here the question comes from the spectral response of the eye.
The human eye linearly does not detect the brightness with the color.
In the image superior we have several colors and some pure ones as the blue one, green and the yellow. The camera offers a maximum value for that color (that digital would be the 255) and in format RGB it would be (0, 0, 255), but the eye perceives that the green one (0, 255, 0) is but the luminous ones, with the same saturation value.
The term was invented luminance, that is the fit brightness to indicate suitably what we really see.
The luminance is a value of tone of gray calculated from RGB through the formula: AND = 0.30 0.59 R + 0.11 V + A
A color RGB of (100, 150, 200) offers a luminance of (100 xs 0,3) + (150 xs 0,59) + (200 xs 0,11) = 140
Changing the values of G and B, we have a color of (100, 200, 150) and one luminance of (100 xs 0,3) + (200 xs 0,59) + (150 xs 0,11) = 164
For the same absolute values of RGB, we have different luminance.
Not to confuse the illuminance with the luminance
The illuminance is the amount of luminous flux that affects a surface a certain area.The luminance indicates the amount of light coming from an illuminated object that the human eye perceives in particular from a point of view.
This would be as the sun and the moon. The light of the sun would be illuminance and the one that we received from the moon, would be the luminance.
In the process of transmission of the signals, which becomes is to send the denominated signals of luminance and “difference of color” (To - and, R - and, Y)= -0,30 R - 0.59 V + 0.89 A We did not send the green one
1º and = 0.30 0.59 R + 0.11 V + A
2º R - and = 0.70 R - 0.59 V -0,11 A 3º To - and
We are going it to obtain by differences of three signals 1, 2 and 3.
To the color difference the flame Or and V.
U = TO - AND
V = R - AND
A television in black and white would take the luminance (brightness and contrasts, grays) having shown the images in a range from grays. The color TV, takes the signals Or, V, and to obtain the colorsRG
Level of the obtained signals of video
For the signal of “color difference” Mhz in quadrature is modulated with a subcarrier of 4.43, that is to say an oscillator modulates signal B - and, taking a phase shifter from 90 degrees to generate the other orthogonal signal to first for the modulation of the signal signal R - and.
The two signals are added vectorially (extreme of vectors in the flat x-y), obtaining a called signal Chrominance. We can see in the following image
According to the caught color a vector of chrominance with a certain angle and amplitude is had.
The angle gives to the shade of the color and the amplitude us the saturation, that is to say the mixture with the target. In order to see the chroma signals a called device Vectoscopio is used.
In the following connection - > Vectoscopio < -, we can unload a simulator program to verify the chromatic composition of several samples. To unload, to decompress and to install in the PC. Then, to make 5 or 6 samples and to obtain the values.
To verify that the obtained values agree down with the shown ones in the figure of bar of colors shown a little but
To the color difference when it has been modulated it calls Cr and Ca to him. When he is digital denominates Pr and Pa.
The chrominance adds to the luminance obtaining the signal to emit.
We can see in the following graph, as it would be the signal composed of some I sweep of colors in system the PAL, once with standard values (applying the previous factors).
In the graph of above, the resultant adjusts to a 70% of the value. For example, the value of target remains in 0,7 corresponding to 700 mV.
For the yellow, the luminance is in a 0,7 * 0,89 = 623 mV that must add or remain to the value of chroma of yellow (0,4493 * 0,7) mV, being in a margin from 306 to 937 mv.
System of interlace
Interlace is “basically”, a technique to insert. For example, for the other analogical system (NTSC) where they change some aspects, but that the base is the same, the signal of Chrominance (color information) is put in intelligently between the spaces that leaves frees the Luminance (information of grays).
We enclose an image for this system where the black lines are of luminance and the red ones of chroma.
Fh is the frequency of I scan horizontal, that is equivalent to in this system to 15734 (something different from the one of the PAL).
The lines of phantom are seen as a handful or group because the video signals are sent as lines where each line of information is modulated in a specific interval. Due to this, spaces between line and line exist, that are not used in the transmissions of black and white. When introducing the color, resorted to the interlace concept, by which, the spaces that are left free, are used to position the different components from color. To see image
For system the PAL, he is analogous. Some of the technical aspects of system the PAL are that we have 625 lines, that the vertical frequency (to show each picture) is of 50 Hertz and that the horizontal frequency is of 15625Hz.
We have a main carrier of Luminance with a series of attached and separated frequencies that are conforming the information of each line. This is something as it is in the image.
Since much phantom in the bandwidth is wasted, it is come to interlace components of the chroma in the spaces that are free of the luminance. To this INTERLACE is called.
The value of the horizontal frequency calculates multiplying I number of lines * complete images of screen, this is:
625 * 25 = 15625
They lack a pair of things to be able to understand as it is outlined the image, and have to do with the moments at which finishes a row of colors (pixel or points of color) and when a photogram to begin the other finishes completely.
This information gives it the synchronism signals.
We show one old TV so that it is seen how it was made before.
We also appreciate that it has an interlaced system (first is made uneven lines and soon the pairs).
This signals of synchronism we are going them to see in the following practice:
The practical PAL
1º Ver the videos of the La Sallian ones where a video camara is used to generate the signal of strips of colors. To consider that camera uses controller of Control unit of camera (CCU) that we do not have, but we must see it because it says 5 to us of the sections that there are to do actually
2º Tomar the generating PAL. To connect the cable of RF VNC of the generator to the TV of the tuner of the TV. To select channel 60 in Mira (the generating PAL) for the first work group. The second will do it in channel 61, so on.
3º In the TV, search the channel using the automatic search of channels. We will see our strip of colors already.
4º In the sight, we have two exits of Banana. Although it appears of synchronism, one has modified to remove to a typical signal (not standardized) composite video and sound. To enter that signal an oscilloscope to visualize the signal.
5º Visualizar the signal without color and soon to put the chroma signal. Screenshot of the two signals.
6º Visualizar the signal with and without sound. Screenshot of the two signals
|Composite video and video by components.||Quality||Images|
|Characteristics||The Best one|
|By Components. The video signal is divided in three signals, two of color and one of luminance, giving rise to the best possible image. They are RCA connectors. The blue one takes the A - and (analogical Pb call in and digitalis CB) the red one takes the R - and (analogical Pr in and Cr in digitalis). The green one takes the luminance and.||Better|
|The second way to pass signal of video is Po the S-Video, that has more quality than the composite video, but less than video by components. S-Video supports a resolution of video of standard definition that can be 480i or 576i.||Good Composite video , that usually comes in a pin RCA yellow. In a same cable, it takes to all the information of video and synchronisms, and usually one is used in recording of videos,|
|Coaxial (RF) - In this case, we can transport it signals of audio video and in a single cable. In the image we have used a VNC connector, although other Nota types can be used. In the previous connectors, the sound must go aside in another connector.||The latest|
Passage of analogical to digitalis
In this process, shown of form simplified, after a sampling of the signal, the analogical values are taken from the signal and they are codified to binary. For example, the first value (blue point) is equivalent 100, the second to 101, the third party to 110, etc.
We are taking only three bits, consequently, the signal is not going to leave very defined. If we took 8 bits, the digital definition will be far better.
In the following image we have how to turn the three signals (luminance and chroma) to digitalis, for which, we assigned in chroma 431 values and 863 for luminance.
According to the criterion of Nysquit, to be able to put together a signal previously turned to digitalis, the sampling frequency must, at least, be the double of the frequency of the analog signal.
We take the 864 possible digital values for luminance and 432 for the chroma (to see previous graph).
As the duration of the total cycle is 64 microseconds, we obtain a frequency of 64 µS/432= 0,1481 µS, which supposes a frequency of 6.75 Mhz.
For the luminance, coming in the same way, we obtain 13.5 Mhz. When adding 13.5 + 6.75 + 6.75 = 27 megamuestras per second.
Applying a codification of 8 bits, we have 27 * 106 * 8 = 217 * 10 6 Muestras per seconds, which supposes a bandwidth of 108 MHz
|So that the system is compatible with the UHF division of channels, we must happen that 108 to 8, and for this, system MPEG2 is going to help to us. MPEG-2
Redundancy means repetition, therefore the space repetition makes reference to blocks with the same and, Cb and Cr.
In the image, we observed that many lines with the same information exist (blue, blue, blue, blue).
|The idea is not to have to send to 5000 times the blue word, but to say, are 5000 followed blue colors.
To this case we fixed to time, and if we see that the image has not changed, is sent only the information that it has not changed. Often in the images only it changes a part of her but there is much part of the image that remain a time, as the case of a news presenter, where the bottom remains quiet almost all along, and only the gestures while it speaks generates information to be processed.
Types of modulations
Aside from the seen modulations, we are going to concentrate in the modulation by phase angle.
The information comes in the moduladora, that when modifying the carrier, generates the modulated one where it is appraised that the phase angle is modified, so that the phase changes when it changes the state of the signalIf >>>> happens from 0 to 1 the signal begins with a phase angle of 0º
If >>>> happens from 1 to 0 the signal begins with a 180 phase angle of º
In the left image, we have the denominated diagram of constellation, where one imagines in the flat x-y.
In this case components I-Q the symbols that are taking the digital variable.
One sees that we have two, that is to say, very distant 180 to each other out of phase symbols º, and therefore with good answer in the propagation (low error rates).
QPSK (Quadrature Phase-Shift Keying)
This digital modulation is represented in the diagram of constellation by four equidistant points of the origin of coordinates. In this case, the four phases can codify two bits by each symbol (value or point of the constellation diagram).
The gray for the allocation of bits to each symbol is used, code that only allows a bit that they change in each increase of the value, for example:
In binary the natural one, we have after the 01 we have the 10. This is not allowed in Gray. A BIT CAN ONLY BE CHANGED. This has the advantage to diminish the rate of erroneous bits.
The multiplexer is in charge to put together the signal
The representation in the time domain of how those signals are conjugated comes represented in the image inferior.
We can continue explaining other systems as the 8-PSK, 16 PSK, etc In them, increases to the number of the used symbols and bits. When being those points but close some of others, we must suppose that it can have confusion at the time of transmitting, admitting a value that is not. This treats it Seeing
TO SEE Or Bit Error Rate defines as the number of received bits of incorrect form with respect to the total of bits sent during a specified interval of time.
We are going to represent a graph with the evolution of these three systems, where one imagines Seeing and the Eb/No rate
Eb/N0 is the relation energy by bit/spectral density of noise power and it indicates measurement to us SNR (relation signal-noise) standardized, also well-known as “SNR by bit”.
The QAM or QAM (acronym of Quadrature Amplitude Modulation, consists of transports two independent signals, that amplitude and phase are modulated in, of a carrying signal.
The Digital QAM, has as entrance a binary data flow, that is divided in groups of so many bits as they are required to create the N modulation states.
For example, in 8-QAM, each three bits of entrance, that provide eight possible values (0-7), to the phase and the amplitude of the carrier are altered to derive eight unique states of modulation.
In order to represent the modulation states, the call is used constellation diagram.
The points of the “constellation” uniformly are distributed in a square grid If symbols are required but, we have the 16-QAM, 64-QAM or the 256-QAM.
We already saw that when transmitting more bits by symbol, for a same “energy average” of the constellation, the points must be nearer and are therefore more susceptible to the noise and the distortion. This takes Seeing us more discharge.
A QAM of order superior can offer more data, but less reliable than the QAM of order inferior.
1º What allows to make the series of fourier in a periodic signal?
2º Dibuja a signal squared next to its three main overtones.
3º Calcular the luminance of a value color RGB (200, 50, 100) and another one of value (100, 40, 200). That color will be seen more luminous?
4º Dibuja a graph of tension that represents a line of color bar and the same in black and white.
5º the standard tension of horizontal synchronism in system the PAL is ______
6º Tenemos a digital value RGB of value 101. To calculate the values of and, Or and V, as well as the values of chroma and compound signal
7º Explica what is the system interlaced with the help of a drawing
8º Explica that is the vertical and horizontal synchronism in system the PAL
9º What is the space Redundancy
10º Explica QPSK (Quadrature Phase-Shift Keying)
11º Tenemos a Eb/N0 of 10. To deduce what system is going to transmit but data being used the table To see, if 1º system is a 8-PSK and another 16-PSK.
12º Dibujar the diagram of constellation of a 8-QAM