How to receive digital satellite tv?

The principle of satellite TV receiving system is introduced. digital satellite tv has developed rapidly in recent years. Using the earth terms "scrambling" and "encryption", the data stream is encrypted, but they are two different satellite TV receiver systems.

Brief introduction of principle

Digital satellite tv has developed rapidly in recent years. It is a form of radio and television, which uses geosynchronous satellites to transmit digitally encoded and compressed television signals to users. There are two main ways. One is to transmit digital TV signals to the front end of cable TV, and then the cable TV station converts them into analog TV and transmits them to users' homes. This form has been widely used all over the world for many years. Another way is to transmit digital TV signals directly to users' homes, that is, direct to home (DTH). ABC is the first satellite TV operator to apply this technology. Compared with the first mode, DTH mode satellite has high transmitting power and can be received by a smaller antenna, which can be used by ordinary families. At the same time, it can directly provide user authorization and encryption management, and develop advanced TV services such as digital TV, pay TV (PPV) and high-definition TV, without being restricted by intermediate links. Besides many TV services, DTH can also provide other digital information services, such as high-speed Internet download and interactive TV.

DTH has two international standards, the European standard DVB-S and the American standard DigiCipher. However, DVB standard has gradually been widely used in the world, and later the American DTH company Dish Network also adopted DVB standard.

A typical DTH system consists of six parts:

1) headend

The front-end system is mainly composed of video and audio compression encoder and multiplexer. The main task of the front-end system is to digitally encode and compress TV signals, and use statistical multiplexing technology to transmit more programs on the limited satellite transponder frequency band. DTH compresses video and audio signals according to MPEG-2 standard, and can be transmitted on 27MHz repeater through dynamic statistical multiplexing technology. 0 TV programs.

2) Transmission and uplink system (uplink)

The transmission and uplink system includes communication equipment and uplink equipment from the front end to the uplink station. There are two main transmission modes: intermediate frequency transmission and digital baseband transmission.

3) satellite

DTH system adopts high-power direct broadcast satellite or communication satellite. Due to technical and cost reasons, some DTH systems use high-power communication satellites, and DTH companies in the United States and Canada use more suitable dedicated high-power direct broadcast satellites (DBS).

4) User Management System (SMS)

User management system is the heart of DTH system, which mainly completes the following functions:

A. registering and managing user data.

B. purchasing and packaging procedures.

C. establish program charging standards and charge users.

D. market forecasting and marketing.

User management system is mainly composed of database management system of user information and program information and call center, which answers users' questions and provides various customer services.

5) Conditional Access System (CA)

Conditional access system has two main functions:

A. encrypt program data.

B. authorized procedures and users.

At present, the conditional access systems used in DTH systems in the world mainly include NDS in the United States, Irdeto in Israel, Via Access in France, Nagra Vision in Switzerland, etc.

The American Direct Television Company and Canada Star Choice Company, which adopt direct television technology, use NDS conditional access system. The Dish Network(Echostar) in the United States and Bell ExpressVu in Canada, which adopt Echostar technology, use Nagra Vission conditional access system.

6) User Receiving System (IRD)

DTH user receiving system consists of a small dish satellite receiving antenna (DISH), an integrated receiving decoder (IRD) and a smart card.

IRD is responsible for four main functions:

A. decoding program data stream and outputting it to TV.

B. Decrypt with the key in the smart card.

C. receiving and processing various user commands.

D. download and run various application software.

DTH IRD system is not a simple hardware device, but also includes an operating system and a large number of application software. At present, the more successful IRD operating system is Open TV. Dish Network Company of the United States has started to gradually upgrade users' IRD open TV system.

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What is a geosynchronous satellite

Geosynchronous satellite is an artificial satellite, which orbits the earth in a circular orbit above the equator, with a distance of 35,786 kilometers from the ground. Its angular velocity is the same as that of the earth's rotation, and its revolution direction is the same, so it is relatively static with the earth.

What is the function of feed?

Feed is also called corrugated horn. There are two main functions: one is to collect the electromagnetic wave signals received by the antenna, convert them into signal voltages and provide them to the tuner. But polarized the received electromagnetic waves.

What function does the tuner have?

Tuner is also called low noise down converter (LBN). Its internal circuit includes low noise frequency converter and down converter, which completes low noise amplification and frequency conversion functions. It not only amplifies the 4GHz signal output by the feed, but also reduces the frequency to the first intermediate frequency signal of 950-2 150MHz.

Types of satellite antennas

A satellite antenna usually consists of a parabolic reflector, a feed and a tuner placed at the concave mirror focus of the paraboloid. At present, KU channel mostly adopts feed integrated tuner. According to the relative position of feed and tuner with paraboloid, there are feedforward (also called central feed), offset feed and feedback feed. Feedforward and biased feedback are mostly used for receiving, while feedback is used for sending.

What kind of antenna is good?

The gain of satellite receiving antenna is one of the important parameters, and the gain is related to the antenna aperture. The larger the aperture, the higher the gain. The beam of the antenna is as thin as a line, and the higher the accuracy and surface smoothness of the antenna, the better. The antenna paraboloid is generally plate-shaped and mesh-shaped. Obviously, the gain of plate paraboloid is higher than that of net paraboloid, and the gain of plate integral paraboloid is higher than that of split combined paraboloid.

What is IRD?

IRD (Integrated Receiving Decoder) refers to the integrated decoding satellite receiver.

Comparison between digital IRD and analog IRD

Compared with analog IRD, digital IRD has the following advantages:

1。 The image received by digital IRD is basically the same as that received by the sender.

2。 Completely eliminate image distortion caused by color and brightness interference, differential gain and differential phase distortion;

3。 Long-distance digital transmission will not produce noise accumulation;

4。 Easy to process, store, reuse and encrypt;

5。 Save spectrum resources.

If digital IRD has shortcomings, it is that its price is slightly higher than that of analog IRD.

How to select digital satellite receiver

When purchasing a digital satellite receiver, in addition to the usual factors such as technical indicators, appearance, quality, price and after-sales service, the following issues should be carefully considered:

(1) If the threshold is low, satisfactory results can be obtained under the conditions of weak signal, small-aperture antenna reception, one tuner receiving with two satellites or multiple tuners receiving with one antenna.

(2) There are PID code addition settings and at least PID code modification methods to ensure successful viewing of PID code programs.

(3) Only by choosing the DISEQC switch can we ensure the excellent level in multi-satellite reception with one machine.

(4) Select those with complete interfaces, such as two AV outputs, S terminal, RS-232, etc. , so as to meet different needs and lay the foundation for upgrading.

(5) Only by selecting enough channels, such as more than 250 channels, can the viewing content be expanded.

(6) Selecting the card reading equipment is beneficial to searching satellite positions and satellite TV programs on different satellites in all directions.

How to test the power-off memory function of IRD?

The shutdown memory function of IRD is very important for users. The simple test method is: Connect IRD normally.

When you receive a dynamic image of a channel program, please turn off the power and turn it on again after ten minutes to see if you can still receive it on the tuned program channel. If so, IRD has power-off memory function. The animation of the program is chosen here to avoid misjudgment.

What method is used to test the polarization voltage switching function of IRD?

(1) Visual inspection method: See if you can directly watch horizontally polarized and vertically polarized satellite programs.

(2) Three-meter measurement method: use three meters to check whether the voltage provided by IRD to LNB can be changed; Required change range: 12-20V. But generally, as long as there is a switch of 14- 18V, horizontally polarized and vertically polarized satellite programs can be received.

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Whose side are you on? Demodulation threshold of landing RD

When there is no measurement condition, the demodulation threshold of IRD can be judged by comparison method. The method is:

(1) Connect the detected IRD and the detected IRD to the power divider under the same antenna, and both of them are transferred to the same set of satellite programs (with moving pictures and sound), and their working conditions are normal.

(2) slowly change the antenna orientation (that is, change the C/N) and observe the two groups! Whether there is block effect (mosaic) in the picture solved by RD and whether there is distortion or interruption in the sound, we can judge the quality of their demodulation threshold by comparing the error codes of two IRDs.

How to calculate the focal length of satellite receiving antenna

The focal length of the satellite receiving antenna refers to the distance from the central vertex of the parabolic antenna to the reflection and convergence focus of the parallel electromagnetic wave signal. The focal length is expressed by f, and its calculation formula is:

F=R*R/4H (m)

Where: r is the surface radius (m) of the parabolic antenna, and h is the depth (m) of the parabolic antenna.

For the feed-forward parabolic antenna, the focal length is determined by the feed of three support rods and corrugated slots fixed on the antenna. This formula can be used to verify the advantages and disadvantages of the product and installation technology.

How to calculate the azimuth angle, elevation angle and polarization angle of satellite receiving antenna tuner

It is known that E0 is the longitude of the satellite ground station, N0 is the latitude of the satellite ground station, E 1 is the longitude of the satellite fixed-point orbit position, FW is the azimuth angle of the receiving antenna, YJ is the elevation angle of the receiving antenna, and JH is the polarization angle of the tuner, then

FW = TG- 1[{ cos(e 1-E0)×cos(N0)-0. 15 127 }/SQR { 1-(cos(e 1-E0)×cos(N0))×(cos(e 1-E0)×cos(N0))}]

YJ = TG- 1 { TG(e 1-E0)/sin(N)}

JH = TG- 1 { SIN(e 1-E0)/TG(N0)}

If FW=0, it means that the satellite is located in the south direction; Fw < 0, indicating that the satellite is located in the east direction due south; Fw > 0 indicates that the satellite is in the direction of due south and due west.

What is the reason for the large clutter when the simulator receives satellite programs?

Satellite program reception clutter is large, and the common reasons are as follows:

(1) The receiving antenna is not aimed at the satellite, and the signal is too weak. We need to adjust left and right to find the best and least cluttered position of the image, and then move up and down to fix it in a cluttered position.

(2) The frequency drift of the tuner causes the IF signal to shift, and the amplification factor is greatly reduced. The local oscillation frequency should be adjusted to make the clutter disappear.

(3) In heavy rain, heavy snow and foggy weather, the signal (especially Ku band) is attenuated. It returned to normal after the rain and snow.

In addition, if the antenna aperture is small, the received signal will be weakened, which will also cause clutter. Consider the power of the satellite transponder when purchasing. If the power is small, you should use a larger aperture and leave a proper margin. You can also choose a high-quality tuner with low noise and high gain.

How to judge the cause of failure by noise

When receiving analog satellite signals, if there is a lot of noise in the received image, the cause of the failure can be judged according to the noise situation. Specifically, when the picture is full of black noise, it means that the receiver frequency is high and should be lowered; When the picture is full of white noise, it means that the receiver frequency is low and should be raised; If there is a lot of black and white noise on the picture, it may be that the installation, focusing, polarization, azimuth and elevation of the tuner are improperly adjusted, or there are obstacles such as buildings and trees in the antenna direction, which should be solved.

What is the reason for the damage of LNB?

LNB is an active electronic component that works outdoors for a long time. The causes of failure are chronic, such as rain corrosion, and transient, such as lightning strike and surge (voltage and current) impact.

Rain corrosion: LNB is exposed to the sun and rain for a long time. If the sealing box has poor sealing performance, it is easy to leak water, resulting in poor contact and even damage. Therefore, you can't just disassemble it. Better add a protective cover.

Lightning damage: this is a common phenomenon, especially in thunderstorm areas and rainy seasons, and lightning protection measures must be taken for the antenna feeder system.

Surge voltage and current impact: In areas with large fluctuation of power supply voltage, when the quality and layout of AC voltage stabilizer and power supply incoming line installed indoors are problematic, surge impact damage often occurs. This can be judged by measuring the positive and negative resistance of LNB output interface with multimeter.

Why is there no satellite signal on the receiver?

According to the analysis of the receiver structure principle, there is a problem that there is no satellite to receive signals, mainly in the following situations:

1. The coaxial cable between the tuner of the receiving antenna and the receiver is in poor contact, resulting in signal interruption.

2. The frequency converter on the satellite antenna tuner needs an external power supply to work, which is generally provided by the satellite receiver (for example, after the general receiver is powered on, its signal input population has a voltage of 18V, which can be used as the working voltage of the frequency converter). When a receiving antenna is connected to several satellite receivers at the same time with a power distributor, and the power distributor only has one port to feed the population, it is necessary to ensure that the receiving satellites connected to this feed port must work for a long time, otherwise satellite programs will not be received.

3. The power supply circuit of the tuner inside the receiver is out of order.

What is the difference between analog system and digital system when receiving weak signals?

When the analog weak signal is received, black or white noise is displayed on the image. The weaker the signal, the weaker and more unstable the image.

Yes, there is no image, only noise and murmur. But when a digital weak signal is received and is lower than the threshold of the digital receiver.

Value, the screen displays no images or only mosaic images.

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What is the reason for the poor reception quality of satellite programs?

When watching satellite TV programs, the signal is unstable, the picture is "mosaic" and the sound is intermittent.

Phenomenon, the common reasons are:

(1) Because the signal strength is in the critical receiving state, you can readjust the antenna direction to enhance the signal, and at the same time, you must

Accurately adjust the polarization angle to improve the receiving effect.

(2) After the receiver works for a period of time, it is overheated due to poor heat dissipation conditions, resulting in errors and black screens or mosaics.

As long as there is enough cooling space, or using air conditioners and fans to cool down, it can return to normal.

Why do satellite programs have good images and hoarse voices?

The main reasons for receiving satellite programs with good images and hoarseness are: frequency drift of sound demodulator or 6.5MHz subcarrier shift of RF modulator. For the former, the audio and RF outputs are abnormal; In the latter case, the audio output is normal and the RF output is distorted. The corresponding frequency needs to be readjusted to the normal state.

Why is it worse to receive KU signals in rainy days?

Ku signal is attenuated by rain (snow and fog), which is a common problem when receiving satellite TV programs. The greater the rainfall, the worse the reception effect. Generally speaking, moderate rain (below 3- 1.5 mm/h) will cause image interference, and in severe cases, mosaic images will appear; Heavy rain (15-60mm/h) or rainstorm (over 60mm/h) will interrupt reception. After repeated tests and comparisons, it is found that the main reason for the attenuation of Ku signal is the accumulation of rain on the reflector and feed port of the antenna, especially after condensation into water droplets, which strongly scatters Ku signal, attenuates it and makes the receiving effect worse. In contrast, it has little effect on C-band signals.

What are the simple methods to reduce the rain attenuation of Ku signal?

1, the choice of antenna aperture can increase the limit aperture by about 40% when watching a program in rainy places.

To reduce the impact of rain attenuation.

2. The antenna should be placed in a place where it is not easy to get wet.

3. Appropriate waterproof measures should be taken for the antenna, such as adding a plastic waterproof sheath on the tuner. Outdoor antennas below 1 m should be covered with cartons and plastic bags without shielding, so as to prevent rain attenuation and corrosion.

What is a conditional access system?

CAS(Conditional Access System) refers to a system that controls the whole process of program scrambling and descrambling by sending an appropriate control word CW to the descrambler. Only when the user is authorized to use the program, the descrambling control word is sent to a specific user. Scrambling and authorization management are inseparable parts of a complete management system (that is, conditional access system).

What is authorization management?

Authorization management means that authorized users who have paid the viewing fee according to regulations can watch the corresponding TV programs, while unauthorized users can't watch them normally, especially to prevent illegal production decoders and unauthorized personnel from decrypting the descrambled information and stealing it illegally.

What are the ways of conditional acceptance?

Manual charging mode (passive).

Automatic charging method (activation):

1. Encryption/descrambling mode:

1. No addressing (decryption stick); Baseband processing | digital compression

2. Addressing (Authorization) Analog | Amplitude Processing Digital | Random Signal

3. Smart card and IC card (authorized by the front-end center) | Time base processing | Password mode

Second, no scrambling mode:

1. Addressing is off. A: Some channels are closed. B. Turn off all channels.

2. Addressing end scrambling (end center authorization).

Is scrambling and encryption the same thing?

The terms "scrambling" and "encryption" are both used to encrypt data streams, but they are two different concepts and should be distinguished.

Scrambling is to change the characteristics of standard TV signals and prevent unauthorized people from receiving clear images and sounds. This change should be handled according to the regulations of the sender under the control of the scrambling and descrambling system.

Encryption means that "information related to descrambling" is encrypted at the sending end of the scrambling and descrambling system and then transmitted to prevent unauthorized people from directly using the information for descrambling.

Is descrambling and decryption the same thing?

Just like scrambling and encryption, the corresponding descrambling and decryption are two different concepts.

Descrambling is to restore scrambled TV signals to standard TV signals. Under the control of the scrambling and descrambling system, this recovery is handled according to the regulations of the receiving end.

Decryption is to restore the "information related to descrambling" to the original state at the receiving end of the scrambling and descrambling system for descrambling.

Is scrambling and descrambling the same as encryption and decryption?

The terms "scrambling and descrambling" and "encryption and decryption" are both technologies for encrypting data streams, and they are important components of CAS. They are closely related and have technical similarities. But in CAS standard, they are two independent parts and two different concepts, which should be distinguished.

Scrambling-descrambling is to change or control some characteristics of the transmitted service (program) under the control of the sending terminal CAS, so that unauthorized users cannot obtain the benefits of the service.

Encryption-decryption is to provide an encrypted message at the sender so that an authorized descrambler can decrypt the data. This information is controlled by CAS and configured in the transport stream information in encrypted form to prevent unauthorized users from directly using this information for descrambling. Different cas manage and transmit this information in different ways.

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We often see some abbreviations in articles about satellite TV and receiver instructions, and we don't quite understand them. Let me explain here.

DVB-S refers to satellite digital video broadcasting;

DVB-T refers to terrestrial digital video broadcasting;

DVB-C refers to cable digital video broadcasting.

CA machine refers to the receiver directly inserted into the viewing card, so it can't convert the encryption format, and it is only suitable for one encryption system. For example, Batty 90 1, Yum! 3900 and Tongzhou 2000E.

CI machine refers to the receiver that converts the encryption format through the module (CAM) and then inserts the viewing card, which is suitable for all kinds of encryption systems. Such as Strong4355, Digaton6 1 1s series, etc.

AllCAM is a module used in various encryption systems, which is directly connected with the machine motherboard and externally connected with a card reader. Mostly used in old machines, such as the popular 9500S module.

MagicCAM and FreeCam are also modules used in various encryption systems, which are connected to the machine through slots and are mostly used in CI machines.

Because AllCam, MagicCam and other modules can be compatible with multiple systems, the movie card used must also be able to support multiple systems. The common FunXin 1 file is the card writing file of 85 15 card, while DS9 is the card writing file of 876, which usually consists of two files, a system file and a data file.

Analog analog signal: It is a continuously variable signal, such as human voice, music, TV images, etc. The early satellite communication system was basically analog signal transmission.

Apogee: The point farthest from the Earth in the elliptical orbit of a satellite. When a geostationary satellite orbiting the earth in a circular orbit is launched, it is first sent to an apogee of 35,888 kilometers in an elliptical orbit, and then a small booster rocket on the satellite is ignited. With the thrust of this rocket, the satellite entered and kept running in a circular orbit of 35,888 kilometers.

ATM (Asynchronous Transfer Mode): Asynchronous transfer mode, used in broadband digital networks, is a way to transfer information between devices in units of cells. Any type of information (such as video, voice, images and other multimedia data) can be transmitted in cellular carriers and can run at high speed. A connection is established between a source device and a destination device through an ATM switch. After the connection is established, devices can communicate with each other.

Attenuation: the process of attenuating and lowering the input signal level to avoid overload of the receiver. Attenuator is a passive device, usually placed between satellite receiver and coaxial cable. In the differential TV system, users who are close to the difference substation often use attenuators to reduce the excessive signal level.

Azimuth (AZ): Azimuth, the angle that the parabolic antenna of the satellite ground station must rotate in the horizontal direction when tracking the geostationary satellite. For any ground station, as long as the longitude of the tracked synchronous satellite is known, the azimuth angle at which its antenna should rotate can be determined.

BB (Baseband): Baseband, a signal with a bandwidth of 6MHz output by a television camera, a satellite television receiver or a video recorder. Only the display can display baseband signals.

Beta format: Beta system. Beta system is a home video recorder system developed by Sony. This system is not compatible with VHS system.

Bird Sat: a typical communication satellite, weighing thousands of pounds, with an average service life of 7 years. It usually "stops" in a circular orbit 35,888 kilometers from the Earth. Communication satellite seems to be like an electronic mirror, relaying telephone, television and data signals sent by various ground communication networks and ground stations, and transmitting these signals to the corresponding satellite ground stations.

Bit rate: Bit rate, the bit rate/code rate of the compressed code stream transmitted from the channel to the input of the decoder.

Blanking frame interval In traditional TV signals, 25 still images or 25 images are transmitted every second. Inter-frame time refers to the time interval between the end of one frame image and the appearance of the next frame image. Using this interval, some data signals can be transmitted, which are not received by ordinary TV sets.

BNC connector: BNC connector standardized small bayonet coaxial cable connector.

C/N (carrier/noise) The ratio of satellite signal power to noise power (expressed in dB). The greater the ratio, the better the TV image quality. When the C/N is lower than 7dB, the TV image quality is poor, and when the C/N value is higher than 1 1dB, the image quality is excellent.

Carrier carrier The center frequency of the signal transmitted by a radio or television transmitter. The carrier wave is usually amplitude modulated or frequency modulated. In analog satellite TV, the carrier is frequency modulated to transmit image signals and sounds.

Carrier frequency The operating frequency of a radio station, television station or microwave transmitter. The working frequency of AM broadcasting is 535 to 1600 kHz. The frequency band of FM broadcasting is 88 to 108 MHz. The transmission frequency band of terrestrial TV station is 54-890MHz. The working frequency band of microwave and satellite communication system transmitter is 1 to 14 GHz.

Cassegrain antenna is a kind of antenna commonly used in satellite TV reception. The unique secondary reflection structure of the antenna not only saves the huge feeder bracket, but also retains the advantages of long focal length and high gain.

CATV Converter Cable TV Channel Preselector In the cable TV system, a special device connected between the TV set and the cable replaces the tuner of the TV set, so that users can choose TV programs of various channels transmitted by the cable at will.

C-band is a band with a frequency of 3.7-4.2GHz, which is used as the frequency band for downlink transmission signals of communication satellites.

Cdtv (Traditional HDTV) is a term used to represent the analog NTSC TV system proposed by ITU-R470.

Channel The frequency band in which a channel transmits a specific signal.

Color information of chroma video signal

Circularly polarized international communication satellites use circularly polarized antennas to transmit signals to the ground in a spiral form. Some communication satellites transmit two different signals at the same frequency according to the left and right spirals, thus doubling the channel capacity of the satellites.