Recently, the problem of choosing a receiver for receiving programs transmitted in digital format has become more relevant.
In this transmission, the original analog TV signal is digitized, compressed, and packaged into a transport stream for transmission. In one stream, one TV channel or a package of several channels can be transmitted. Streams of the first type are called SCPC*1, and those of the second type are called MCPC*. On the receiving side, the digital stream is decoded and the analog form of the TV signal is restored.
The advent of packet transmission of channels has changed the organization of their search by the receiver. The analog satellite channel always occupies the entire transponder. Therefore, the carrier parameters can be unambiguously assigned to the channel name. During digital transmission, up to several tens of channels can be transmitted in a packet. Their detection by the receiver occurs in two stages. First, the parameters of the carrier digital transponders are stored in memory - SR *, FEC *, frequency and polarization. As a rule, the parameters of the carrier digital transponders of some satellites are stored in memory already at the factory. When pointing at such a satellite, the receiver successively tries to decode packets and single streams. As a result, separate lists of detected channels are formed in memory.
Digital receivers have a number of new features to look out for. Some of them are related to the parameters of the digital stream. One of the most important characteristics is the range of speeds accepted by the receiver. The transmission rates of digital television satellite streams range from 1.2 Msymbol/s to 30.5 Msymbol/s.
Most often there are problems with low-speed streams. They are especially typical of receivers designed to receive a specific packet. The lower limit of accepted speeds is, as a rule, 18-22 Msymbol/s. Such receivers do not allow receiving either single channels or low-speed packets. Most receivers that do not have a narrow orientation
tion, the lower limit is 2-5 Msymbols / s, and only a few have 1 Msymbols / s. Therefore, before buying a receiver, you should definitely find out the transfer rates of the streams of interest.
Another feature related to bitstream parameters is the ability to manually enter the PID*. It allows you to change the audio language of the program, of course, if there are several audio streams in the channel. This is true for those receivers in which the language replacement function is not included in the menu. In addition, there are rare channels using non-standard elementary stream addresses that cannot be received at all without manually entering the PID.
The digital format, to a greater extent than the analog one, provides convenient opportunities for transmitting a variety of related, including service, information. These features are widely used by operators to transfer pages of teletext*, and electronic guide*. Please note that digital receivers with teletext support can use two options for its processing. In the first case, teletext is decoded, stored in the receiver's memory and can be transmitted as a normal television signal. Teletext mode is selected by pressing a special button on the receiver's remote control. This method is similar to that used in D/D2 MAC decoders. It does not require a teletext decoder on the TV and is preferred for private reception. The second method involves the restoration of teletext in the interval of the vertical quenching pulse (CHI), in which it was in the original analog signal. In this case, the teletext must be re-decoded by the TV's built-in decoder. Receivers that restore teletext in the THD interval are convenient to use for collective reception, as they allow each of the connected subscribers to switch to teletext mode independently of each other.
Features unique to digital receivers include the ability to automatically search for channels from network information transmitted in the digital stream*. Software is expected in the near future to allow receivers to start searching without any background information. However, so far (at the beginning of September) we do not know of any model with such capabilities.
The advent of digital broadcasting has led to a sharp increase in the number of transmitted channels. This circumstance is taken into account when developing digital receivers. The amount of memory allocated in them for channel lists, in most cases, allows you to store up to 1000-3000 TV and up to 500-1500 radio channels. The problem of limited memory, inherent in some analog receivers, is practically eliminated here.
A large number of channels and an abundance of service functions, characteristic of digital transmission, have led to a complication of the menu structure. Therefore, the receiver requires a convenient and logical organization of the menu and the ability to arrange channels in a convenient way.
It is rather difficult to concretize this requirement in a few phrases. The menu structure has improved from version to version, and today most of the main manufacturers offer fairly similar options, which they came up with in search of the optimum.
The receiver should be easy to upgrade. In most cases, a new version of the software is transferred from the computer, so the receiver must have a port for connecting it. As a rule, this is RS-232.
There are other options for using the PC. Often the use of computer editors can greatly facilitate the editing of transponder and channel lists. And for some receivers, programs for computer diagnostics of malfunctions have been developed.
When creating a reception system from several orbital positions, the receiver may need to support the DiSEqC protocol. In a system with two or more antennas, it can control DiSEqC switches, which have recently become widespread. If a rotary antenna is installed in the system, then it will be convenient to use a modern DiSEqC positioner. DiSEqC switches are often used when organizing collective reception systems with distribution of a satellite signal at an intermediate frequency.
The DiSEqC protocol is supported by almost all digital models. However, you should pay attention to the compatibility of the DiSEqC commands of the receiver and external devices. The type of supported DiSEqC protocol is usually specified rather arbitrarily. Therefore, it is necessary to make sure that the command set is compatible. Usually, external devices offer a choice between several such sets and you can choose one compatible with the receiver's commands. Control signals 13/18V, 22 kHz are also widely used. Since they are needed to control universal converters, they are formed by all receivers without exception. For some switches and commutators, they can be used as an alternative to DiSEqC.
Due to the specifics of digital streams, some important characteristics of analog receivers for digital receivers are irrelevant. This primarily applies to the bandwidth and noise reduction threshold. The IF bandwidth of a digital signal is directly dependent on the bit rate and can vary over a very wide range. Therefore, in digital receivers, the bandwidth is automatically adjusted according to the IF bandwidth of the received digital stream. In addition, the digital stream is not the TV signal itself, but the code of this signal compressed and protected by noise-immune encodings. As for the noise reduction procedure, practiced in analog reception for detuning from interference, it comes down to cutting the edges of the IF band of the received signal. At the same time, due to the loss of some information about small colored details, it is possible to increase the ratio of the useful signal level to the noise level. The value of this ratio is decisive for the possibility of receiving the signal. A digital signal has a smoother spectrum than an analog signal, so cutting off the edges will not increase the signal-to-noise ratio noticeably. In addition, it is not the TV signal itself that is transmitted in the digital stream, but its code, and cutting off the edges of the band can lead to the loss of a very significant part of the information. For these reasons, noise reduction is not used in digital reception.
It should be said that for a digital TV signal there are no gradations of the quality of the reproduced image. If the distortions received during transmission can be eliminated due to the restoring properties of protective encodings, then the TV signal is restored almost in its original form. Image quality is determined by the circuits that generate the analog signal in the receiver and the quality of the television receiver. If the protection coding depth is insufficient, then the signal is not restored at all. In the borderline state, horizontal synchronization breaks occur, the frame stops, or the image crumbles into separate cubes. Such an approach is by no means acceptable. The presence of a sharp border between a high-quality image and its complete absence makes it impossible to visually assess the stock "by quality". Therefore, many digital receivers are equipped with signal level and quality indicators. The level is understood as the absolute level of the signal, and the quality is determined by the number of errors in the stream before noise-correcting decoding.
To receive programs closed with a certain encoding, an access module (decoder) for this encoding is required. An individual card with information about the conditions of subscription to services must be additionally installed in the module slot. The access module can be either built into the receiver or external. Built-in modules are equipped with receivers focused on receiving certain paid packages. External decoders are not connected via SCART, as in analog receivers, but via a standard interface (O) - PCMCIA.
To date, external modules with CI exist for all major digital TV signal encodings - Viaccess, Irdeto, Seca / Mediaguard, Cryptoworcs, Conax, Nagravision. The exception is the PowerVu encoding, which is implemented with certain deviations from the DVB recommendations. Some receivers have 2 or even 4 O slots, which allows you not to worry about changing the decoder when switching to receiving programs covered by a different encoding. In the future, CI slots are expected to be used to connect a wide variety of functional modules. This will allow you to flexibly change the functionality and service capabilities of the base unit.
Often a digital receiver is bought in addition to an analog one. If digital and analog signals are received from the same antenna, then it is wise to choose a digital device with an input signal through to an additional F-connector (loop output *), to which an analog receiver can be connected. If the system is completed again, then for the joint reception of digital and analog programs it is more convenient to purchase a combined digital-to-analog receiver.
And if the system uses a rotary antenna, then you can choose a digital-to-analog receiver with a positioner. Recently, many companies have begun to produce a series of receivers, including models with a different set of functional modules. An analog tuner, CI interfaces, access modules and a positioner can be added to the basic set of modules in the simplest receiver in more complex models in various combinations. This allows you to make the best choice, taking into account the requirements for the apparatus and material capabilities.
The significance of certain characteristics of the receiver largely depends on the tasks that it has to solve. Therefore, before proceeding with the selection, it is necessary to clearly define the range of these tasks.
In conclusion, it should be said that there are many important characteristics of the practical operation of the receiver that are not reflected in its passport. So, for fans of channel "surfing", the speed of switching from channel to channel will be essential, when receiving weak channels, the sensitivity of the tuner is decisive, and when installing the device in a rack, excessive heating of the case can become critical.
Therefore, those who wish to seriously approach the purchase of a receiver can be recommended to first get acquainted with the reviews on the practical work of the device of interest. Such information can be found on the Internet, as well as in a series of test articles placed in this guide.
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May 2, 2024 01:04:17 +0300 GMT
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