DVB - Digital Video Broadcasting - Digital TV


TV broadcast involves simultaneous transfer of the picture (video) and sound signals through a communication channel. So, basically transmitted, according to their features, two low frequency signals. Low-frequency signal of image and sound (NF) has not a feature of propagation through space as propagating high frequency signal (VF). Therefore, in the process of delivery messages, VF signal is used for 'assistance' in the way that the NF-modulated signal message 'imprinted' in the HF signal in the reception process is reversed. So VF signal serves for NF signal like a 'horse' to carry it, and therefore VF signal is often called a 'carrier signal'. According to international recommendations were agreed upon schedule and high-frequency ranges for individual channels (carrying signals), and the recommendations are consistent with the way in which the HF signal is transmitted.

Transmission of TV signals for years is done in such a way that the low-frequency analog video signal amplitude modulation performs high-frequency signals, and frequency-modulated high-frequency signal close to him (mostly for 5.5 MHz more) performs the transfer of low-frequency analog sound signal. There is still incorporated color signal. All three high-frequency signals and their components form a TV signal. Frequency range that both these high frequency signals occupy (about 6-10 MHz) is called the width of the channel, and the channel of communication through which the transmitted without jeopardizing their peculiarities and timeline signal is uninterrupted. The best known systems for the transmission of analog TV signals are NTSC, PAL, and SECAM and their variants, and therefore the different channel bandwidth they use. Furthermore, systems are completely incompatible with each other, but due to market and political reasons. The frequency of high-frequency signal basically defines a communication channel, and by means of a special device in the receiver - TUNER selects which channel will be used. This principle of data transfer is FDMA. TDMA however uses the principle of using the same communication channel for the transmission of multiple contents. CDMA concept in TV systems are not used.

It is immaterial whether the distribution of TV signals carried as an electromagnetic wave space (terrestrial TV), or routed through a coaxial cable (cable TV), and the modulation is done at much higher frequencies than in the previous mention the type of communication to communicate using strongly focused parabolic antenna (satellite TV). International recommendations for these transmission modes are not the same, though they are quite similar, and therefore the different devices. By international agreement, in accordance with the frequency of high-frequency image signal distribution of a frequency area in the segments of 6 MHz (channel), and are joined by their numbers (channel 1, 2, 3, 4, 9 ...), and each country is assigned its names (HTV1, HTV2 ...). As with terrestrial signal transmission Italians do not bother Croatians? Simple, high-frequency signal does not reach large because he can not follow the curvature of the earth and circumvent obstacles.

But the development of electronics in terms of high-speed data processing low-cost integrated circuits, allows the signals instead of analog images and sound and complete their A/D conversion, and then execute code for each level of sampling according to Figure 7.1.1 and execute high-frequency modulation with digital NF signal, and then a transfer of digital television signals - DVB (Digital Video Broadcasting). Communication scheme is similar to the scheme shown in Figure 1.1.1, and a communication scheme for a single data transfer - SPTS (Single-Program Transport Stream). Schemes differ in the fact that in the following figure combines to create a source of messages, and the corresponding analog signal to her, and the impact of noise and interference on transmission route is not shown because of digital signal when transferring is much more resistant.

 DVB communication system
Figure 1. DVB communication system.

With this it is possible to perform compression and digital video signal according to MPEG (Moving Picture Experts Group) standard, and sound according to AC3 (Dolby Digital Audio Codec 3) standard, or an equivalent, which allows the sending of data in a time-sequential groups like TDMA principles and the more different DVB content can be sent through the existing analog channels, which allows the use of much of the existing infrastructure, the channel width remains the same, the data transmission more resilient to interference compared to analog systems, and transfer more digital 'subchannels' through devices have transmitted an analog signal. Devices, the transmission path is immaterial what the signal transmitted if it is within the limits of the existing channel width. The following chart presents the principle of the multiple data transfer - MPTS (Multiple Program Transport Stream).

 Multiplexed DVB communication system
Figure 2. Multiplexed DVB communication system.

Depending on the quality of the image that is transmitted with a time multiplex (MUX) can be through already existing channels transmit multiple video content, up to ten. What has significantly changed the source device in the process of transmitting and receiving circuits, the system as a whole is almost no raises, indeed TV sets have never been cheaper and more versatile for the same money. Concepts of SPTS and MPTS are illustratively called binary sequence, because you are essentially portable path propagates modulated high-frequency signal. This does not only allows you to view the contents of different TV stations, but in the collection system service and view the contents of such order. Allows the viewing of the same content from different sources (angles) or according to monitor the content interests moment, say track car racing with the possibility of monitoring the flow of the race, tracking developments in 'boxes', monitoring developments with a camera from the car and the like. Besides, the current offer is extensive and 'title' are separated from the image, which means that it can be adjusted separately (size, color ...). When all of the above into consideration, broadcast TV content is increasingly reduced to the slogan ' What you give, you get it. '. Pure market supply and demand.

In MPTS receiver is a device that allows the high-frequency demodulation of the received signal to obtain a sequence of digital pulses, which contains one or more of the TV signal. Such a receiver, STB (Set Top Box), usually has a slot to insert a module that accepts user subscription card in that is registered for that type of programming packages paid service. Generally, STB TV can deliver analog, digital or high-frequency modulated signal image and sound. CRT TV unit is usually supplied video and audio TV via SCART (Syndicat des Constructeurs d'Appareils Radiorécepteurs et Téléviseurs) port. Newer TV sets, LCD display, for example, can accept a digital signal via HDMI (High-Definition Multimedia Interface) port, for example. To avoid any confusion, cable TV was used in the time when the end user to deliver an analog signal, with which is an 'old' TV might normally used. Cable TV allows a service provider to appoint the way will be some available channels are used. For example, the cable operator in Croatia allows monitoring of analog channels as before, thus enabling further use of the 'old' analogue TV and thus saving the user to not be prompted to change existing TV sets and STB lease allows the reception of a or a large number of digital TV signals which are generally encrypted contents regarding pricing options desired service.

Newer TV sets have a combined 'tuner' who are able to receive and 'analog' and 'digital' signal with support for MPEG-2 and MPEG-4 encoding, and even a satellite signal, and some have built CI (Common Interface - PCMCI connector, type II) interface in which it is possible to insert some of the CA (Conditional Access) module (CAM) as Cryptoworks, Mascom, or Conax, which can be read about the subscription card to decrypt the content. So, be careful that the module taken in order to monitor precisely the desired content. And the subscription card is entered for that channel is enabled to decrypt the content and normal reception. used STB that has a built CI interface with Conax.

Modulation of high-frequency signal is used by the DVB differ in the characteristics of the DVB-T (terrestrial TV), DVB-C (cable), DVB-S (satellite TV), which means that the device for receiving and processing of high-frequency signals are different for each of these distribution methods TV content and mutually incompatible. DVB-T is based on the modulation of OFDM (Orthogonal Frequency Division Multiplexing), DVB-C is based on QAM modulation (Quadrature Amplitude Modulation) and DVB-S is based on the modulation of QPSK (Quadrature Phase-Shift Keying). Thus, the application of the principle is similar, the transmission of digital TV signals, but the frequency range used, bandwidth and modulation and demodulation method are not. Devices designed for one system can not 'understand' signal from another.

QPSK and QAM modulation using similar methods to those that QPSK something simpler. Not used when the modulation amplitude changes and requires greater bandwidth compared to QAM, what to work at very high frequencies of satellite transmission is not essential. QAM (Quadrature Amplitude Modulation) - a mixture of phase modulation and amplitude modulation is basically an extension of QPSK. QAM modulates the phase and amplitude of the transmission of high frequency signals. In Chapter 1.3 on signal theory states that the high frequency signal has the property as an electromagnetic wave propagation, and that any change in one of its parameters in accordance with changes in the analog signal, the analog signal can be transmitted at a distance. But no need to make modulation analog signal but can perform the digital signal as shown in Figure 3.3.26. If the above principle is used in a way that the phase change relates to binary information, the four stages of change would mean using 2 bits to transmit four different states, for example:

Quadrant Binary
1 00
2 10 90°
3 11 180°
4 01 270°

According to Figure 1.3.1 are four phasor position ' A '. If the phase change composes and amplitude changes, then resize ' A ', we get the QAM. Thus, each position and given his phasor amplitude joins one binary combination. Depending on the number of used bits determines the number of possible binary combinations (N = 2^b) and the amount of ' N ' using tags 8-QAM, 16-QAM, 32-QAM ... Means according to the binary combinations of the high frequency modulation is carried out signals.

8-QAM means that the coded with 3 bits (2^3 = 8) which uses two values for the amplitude A and a four corners of each of them, and the 16-QAM means that encodes the encoded 4 bits (2^4 = 16) which uses 4 different signal amplitude and 4 angles for each amplitude, as shown in Figures 3 and 4.

 8-QAM         16-QAM
Figure 3. 8-QAM.   Figure 4. 16-QAM.

Figure 4 shows that the choice of angles such that it is the same for every second amplitude of phasor. It contributes significantly to the correct recognition of each of the 'green dot' which joins the binary combinations. Since each binary combination represents a single character, QAM is basically coding of signs (symbols). Number of combination from Figure 4 is insufficient for high quality transmission of TV signals, and therefore use more complex methods of QAM: 16-QAM, 32-QAM, 64-QAM, 128-QAM and 256-QAM modulation for transmission of television and radio signals. To transfer a standard TV picture (SDTV) is sufficient 64-QAM modulation and demodulation system shown in the following figure.

Figure 5. 64-QAM modulation system.

First, it can be seen from the previous analyzed model is theoretically block-diagram which has a rectangular shape and each point belongs to a small rectangular portion of the surface that it is entitled in respect of its detection, and easier mathematical treatment to consideration of a data matrix. Thus, the phase and size of vectors which define a binary combination in question. Vector to any point considered as a segment on the abscissa and ordinate according to the expression:

V(t) = I(t)•cos(ω•t) + Q(t)•sin(ω•t)

Just value of ' I ' and ' Q ' are used in the process of modulation and demodulation of QAM signals, so it is often called the previous image Q diagram. The Figure shows that for each symbol to 6 bits. Transmission speed digital communications system, or the number of symbols per unit time is expressed as 'Symbolrate' (also known as the baudrate). In addition, in this case for each of the components (I / Q), then one can not use high-frequency carrier signal, for high-frequency two subsignala. If 'Symbolrate' multiplied by the quantity of bits per symbol gets 'bitrate'. The term 'symbol' can be compared with the concept of signal transmission at 1000Base-T standard.

In order to successfully convey a symbol other than sharing the time slots during which it is done sending symbols, can be divided the frequency of channel at sub-channels as shown in the next image.

 Broadcasting of 'Symbols'
Figure 6. The division of the communication channel by time and frequency.

This picture basically shows how to use the OFDM modulation. Resembles the FDMA concept in a way that one source close to the signal frequency used sub-channels in one channel. When the elements of each symbol is distributed along the frequency channel width, pulse interval after one period is added to the 'silence' - a protective interval, during which the logic demodulator does not perform any analysis because at that time may occur before some sort of reflected signals or interference. According to the above, it is now possible to perform efficient electronic circuits in the design of 'digital TV' because the technology enables the design of integrated circuits advanced and highly accurate logic regarding the successful and the interference-resistant communications, all aiming for the high quality of the TV display. Theoretically, it all comes down to the transmission of a 'symbol' along the communication channel regardless of the type of modulation applied.

High quality images, i.e. the standard high-definition television, includes image display with HD resolution of 1920x1080 pixels with a display format ratio 16:9 (HDTV). For example, the former 'ordinary' TV resolution was 768x576 pixels (SDTV) with a 4:3 display format (Europe - PAL system). When it comes to digital TV, for the best quality you need to have a screen of TV that supports HD resolution, although they have to offer manufacturers and TV sets with a 'HD Ready', which is a cheaper version of the HDTV definition, because they have a display resolution of 1366×768 pixels in picture format 16:9. So, every TV has electronic circuits (image processor) to enable scaling the image to be able to properly display the quality and contents of different resolutions. Scaling mechanism and mode of action of gamma correction, are essential elements for the assessment of quality TV.

As this is a temporary solution during the transition to HDTV resolution, the application of standards with tags DVB-T2, DVB-C2 and DVB-S2 (second generation), distributors in Croatia provide TV content as MPEG-2 coded TV signals in SDTV resolution (720×576 px / PAL 4:3 or 1024x576 px / PAL+ format 16:9), as well as HDTV MPEG-4 encoded TV signals for several TV programs for experimental purposes. SDTV is slightly less resolution than standard definition analog TV signals - 768x576 px / 4:3 format. Both supported the electronic program guide - EPG (Electronic Program Guide). Of course, the radio program is also digital. Is it worth it to buy an HDTV device? Maybe not now, unless you use optical media with HDTV content (Blu-Ray). Incidentally, the proposal was first HDTV standard is 1280×720 px / 16:9 format.

If the TV signal is monitored on a computer monitor, especially on cable TV, where the card is required for admission to decrypt, the next set of pictures showing what it should have.

 DVB Tuner
Figure*** 7. Tuner / CI interface / CA module. ( +/- )

It is important to note that the hardware device is not required MPEG-2 or MPEG-4 decoder and it would be very helpful to the client software to display the image in its window execute proper gamma correction. Modulation scheme for DVB-C range includes 16-256 QAM and modulation scheme for DVB-C2 range includes 16-4096 QAM, which is understandable because HDTV has much more of an image compared to SDTV. Incidentally, offers only four TV channels that are not encrypted, so you should be very careful if you are buying TV card installed about DVB-C, CI and CAM capabilities.

Of special note in this regard is the Internet TV (DVB-IPTV). From classic television is distinguished by a medium which implements broadcast. The program is broadcast via the Internet and are not used to high-frequency electromagnetic signal, but it is the standard TCP / IP packets and transmission of digital signals through a wired ADSL line and principles of the system and OFDM modulation. Transfer content can be carried out as usual downloading and saving to disk, you do not really belong in Internet television. Another way is to either a computer or a device for IPTV STB receives packets at the speed they are and displayed on the screen. Due to the unreliability of Internet connections, showing delayed awhile to receive, in order, in case of failure, arrived catch up, or to collect all the required packages in order to avoid an interruption in playback, while the content is in principle not saved in drive. Should be noted that the end result of the process of transmission of TV signals to a user or program support, which is used for displaying images - analog signal? Devices like the usual TV STB receiver sends an analog TV signal via one of the SCART cable to one of its A / V inputs. Thus, the A/D conversion, and then transmit a signal to the user, and the D/A conversion and delivery of analog TV signals to the user; supplement scheme in Figure 1.1.1 with two rectangles - encoder and decoder, either in the composition or the transmitter and receiver separately. Distinctiveness in relation to the previously mentioned the DVB-H (handheld) standard, which defines the view TV content on mobile phones and other small portable devices, for TV addicts :-).

In the first chapter of this textbook has already been noted that the digital signals contain information about the scheduled sequence of pulses (symbols) resistant than the analog signal because the signal shape is not of great importance, which allows receiving recognition in a sequence of pulses with various signal distortion over 50%. The quality of the received image signal and digital signal resistance compared to the analog signal is best illustrated by the following diagram.

 Analog-Digital Display quality
Figure 8. Picture quality as a function of signal strength.

As already protective interval for DVB-T transmission largely prevented distortion of digital signals for receiving reflections, pattern recognition digital signal is nearly flawless. If the received signal is weaker logic electronics is not able to determine what is "0", and the "1", falsely interpret contents of binary sequences and display quality is rapidly falling. This is a more linear decrease in quality when receiving an analog signal, image 'snow' but it's watchable, but with the digital signal in one moment everything into an incomprehensible colored 'dice'. Referred to the conclusion that in areas with a strong signal for receiving DVB sufficient ordinary home antenna, while in the more remote areas where the signal is weak needs a good directional antenna for the frequency range where the TV signal is broadcast. Thus, the slogan 'Remove antenna from roofs!' is not entirely true. Cable operators use DVB-C are at an advantage here with repeater along the transmission path may be needed to preserve the strength of its digital signal without significant distortion.

Thus, the era of the old analog TV goes. Some service providers favor the beneficiaries as, which continue through the cable system with digital and analog broadcast TV signal which is a great convenience for users. Old TV set does not have to have all kinds of connectors for connecting a variety of additional devices, such VCR (Video Cassette Recorder), DVD's and similar. For this purpose an old TV sets usually have:

  • RF (Radio Frequency) - ANT IN - RF IN - Antenna input for receiving analog TV signal is connected to a high frequency 'channel selector' (tuner) in TV device which will isolate the desired VF signal with respect to all the received signals and performs its demodulation in order to obtain the NF picture signal and the NF sound signal. ANT OUT usually merely relaying incoming signals. Until applying a SCART cable, VCR is sending a signal to the TV as the RF signal, for example at 21 channel. Antenna signal coming in through VCR ANT IN connector, and the recorded content is sent to the TV ANT IN connector via VCR ANT OUT connector. Of course, with regard to recording TV content VCR had to have its own 'tuner'. In fact, the signals of TV channels are forwarded through the VCR to the TV, and on their 'tuner' is rigged is what will be recorded and / or watch.

  • SCART (Syndicat des Constructeurs d'Appareils Radiorécepteurs et Téléviseurs) - One or more ports for transferring NF video signal (RGB, composite and / or S-video) and audio (audio left / right), the European standard, established 80 ', is designed to connect a variety of additional devices for the TV set. It is designed to be a two-way (outgoing and incoming signal) and to have easy control and ensure that the TV recognizes that something is connected to it.

  • CHINCH (RCA connector) - Connections for video signal (composite) and audio signal. A simpler definition TV had only incoming jacks, while more complex TV and VCR have the output jacks. Standard was established by 'Radio Corporation of America' back in 40'. This connection of devices was applied in Europe up to establish a SCART standard.
 TV connectors

If the old TV has a SCART connector (input) then for him to join the STB, which typically has a SCART output. If the STB and the TV have chinch connectors and they can be used. Selecting the channel can be made by the STB. If the old TV has no SCART or RCA jack connectors, should be taken STB with built in RF modulator and a RF output (RF OUT) sends a re-modulated TV signal to the old standard TV antenna input (RF IN), one of the analog channels (typically 21), which will be send to the TV determines selecting by STB. It can be used as separate device that signals from the SCART STB knows into a modulated RF signal again by the old standard, or in video and audio signals for input RCA jack connectors on the TV if TV has. Modern TV sets can have all the 'miracles' of the connections: DVB-T, DVB-C, SCART, composite, audio, s-video, VGA, DVI, HDMI, DVMI, S / PDIF ...

DVB uses the 'Transport Stream' (TS, MPEG-TS) as a standard format for transmission and storage of audio, video and data, and TV signals. TS transfer is based on transmitting packets with a mechanism for error correction and synchronization features for maintaining the integrity of the receipt of the signal to prevent its degradation. Besides TS sequence may contain several different TV programs. Available on DVD without the need for some of the above, and using them in the usual MPEG-2 format - 'Program Stream' (PS, MPEG-PS). So if software support TV cards do not perform this conversion should be about creating a DVD based on the data stored on the disk to make some conversion described for this purpose-made tools. A similar procedure will apply to HDTV and BR optical media where they basically use the MPEG-4 format, although BR units and be able to play MPEG-2 format, or DVDs. It is expected that the introduction of DVB and analogue format war between images and broadcast TV signal is finished (NTSC, SECAM, PAL) and will be used as such a worldwide standard.

Some DVB systems vary, and depending on different frequencies designated for sampling and quantization levels, shown in Figure 7.1.1, and the sampling frequency for example the picture, brightness and color signals (video signal), up to 13.5 MHz with a resolution, or the number of sampling levels of 8-10 bits (sometimes 12 bits). For audio signal used sampling frequencies up to 48 kHz with a resolution up to 16 bits. So, according to 256-1024 different shades of each color (4096 for 12 bits) and up to 65'536 volume level. With regard to auditory and visual characteristics of human organs is barely enough to get after D/A conversion would not notice the difference. Besides grouping of adjacent colors and their additive is obtained by mixing other colors (dithering), and the range of colors apparently increases, and the image is so 'clean' that all men are clean-shaven look is finished.

The most commonly used color depth of 8 bits is. The problem of low depth of color is particularly evident when playing animated content, which are fairly monochromatic, so grouping them as JPEG images regularly make patches. If one adds to sampling chrominance (Chroma subsampling), who was also selected by the criterion of saving bandwidth and not by quality criteria (usually type 4.2.0), the quality of the signal transmitted through a communication channel is 'poor'. Nevertheless, the display image quality is subjective because the contents are transferred digitally and thus are exposed to insignificant interference from an analog signal to be visibly and audibly significantly distorted transmission through the communication channel, but the selected standard coding and transmission should be stricter in terms of quality. We should not forget Figure 8, which clearly indicates what will happen if the received digital signal is too weak. If we take into account the fact that when creating a digital signal using different compression methods and content (MPEG-2, MPEG-4 ...) effect to transfer content through existing analog channels is qualitatively and quantitatively negligible. Expecting a significantly better quality of standard.

For comparison, the classic film 35 mm wide with a quality camera can record up to 20 million shades of color per picture element. Typically used ten million shades of colors as the human eye can discern the (adequate sampling of 24 bits), with a density of about 250 picture elements per 1 mm (millimeter) of line at film. DVD specifications for its not even close to that, especially not the current TV sets, and only the second generation of HDTV, with sample rates up to levels of 14-bit and will offer something acceptable in comparison to the classic film. Sampling audio with 16-bit and is somewhat acceptable. Professional equipment uses 24-bit A/D converters.

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