Television is one of the most popular types of information services among the widest range of users. Therefore, the violation of the quality of image reception on some, and even worse on all available channels is perceived by households extremely painfully and per chapter families with more or less intensity a flurry of demands to remedy the situation falls, the intensity of which will only increase over time.
The first impulsive action in this situation is to blame the weak signal at the TV input for all sins and correct this state of affairs. the purchase of an antenna amplifier (an example of such a device is shown in Figure 1), many models of which are offered in specialized stores. And not all cases will have a positive effect. The fact is that poor image quality can be the result of a large number of reasons, and only some of them are corrected by an additional amplifier. What are these reasons?
Requirements for television signal
The TV, antenna cable and antenna form the system shown in Figure 2, which is normal functions only when the signal voltage at the input of the TV (point B in Figure 2) in certain limits. For reasons of convenience in performing measurements and calculations, the characteristics of this system are normalized not by voltage, but by its logarithm, i.e. by level.
According to GOST R 52023 as amended in 2003 the input signal level should be within 60 - 80 dBμV. Violation of this limitation can potentially lead to playback quality problems.
Insufficient signal power at the antenna output
It is not enough to achieve the standardized GOST level value at the input B of the TV. The fact is that high-quality reception of a television signal is possible only with a certain signal-to-noise ratio. For various reasons (long distance to the telecentre, poor antenna design, its malfunction, incorrect alignment) the signal from the antenna may be too weak, i.e. signal level at point A is less than 60 dBμV. Without measuring devices, you can verify this if you cannot achieve a normal image by directly connecting the TV to the antenna output.
In such a situation, turning on the amplifier is useless, because it amplifies the incoming signal along with the noise and is unable to provide a normal signal-to-noise ratio.
The signal level at the TV input is too high
When the input level at point B exceeds 80 dBμV, the input circuits of the TV are overloaded, which is accompanied by strong signal distortions and it becomes impossible to obtain a normal image. A similar effect can potentially occur with the amplifier itself, which is mistakenly turned on too close to the antenna output, Figure 3.
Unlike the previous case, amplifier overload can be detected (and eliminated):
- moving it closer to the TV (dotted line in Figure 3);
- switching on the splitter at the input (splitter, Figure 4);
- additional decrease in the input signal level with the standard regulator.
When using a splitter, keep in mind that it reduces the signal level by a little more than 10 lgN, where N is the number of outputs.
High branching ratio
The splitters mentioned above are used to connect multiple TVs to one antenna. If the branching ratio is too high, the wrong choice or the wrong connection of the splitter, the signal at point B is too weak. In this situation, which is shown schematically in Fig. 5, the use of an amplifier is justified in that it compensates for the losses in the splitter without losing the signal-to-noise ratio.
The choice of where to install the amplifier (at the input of the splitter or its output) depends on many factors. It is determined by a specialist or selected empirically by image quality. In the second case, the amplifier improves the reception quality of only one TV set (in this case, TV 1).
The functional analogue of the amplifier-output splitter combination is an amplifier with a built-in divider, Figure 6.
Choosing a location for the amplifier
As you can see, an antenna amplifier in the presence of a sufficiently powerful antenna signal can bring a positive effect.
This, however, only happens when there is no overload:
- input circuits of the TV;
- the amplifier itself.
When this condition is met, care should be taken to increase the image quality by maximizing the signal-to-noise ratio. To do this, it is advisable to install the amplifier as close to the antenna as possible, but so that it does not overload.