Progress is sweeping the planet, but, unfortunately, not as fast as we would like. Therefore, at present, millions of TVs are not able to receive a digital signal without crutches, and a provider that cares about the convenience of the subscriber must give a TV signal, including in analog form.
Contents of the article series
- Part 3: Analog component of the signal
- Part 4: Digital component of the signal
- Part 5: Coaxial Distribution Network
- Part 6: RF Amplifiers
- Part 7: Optical Receivers
- Part 8: Optical backbone network
- Part 9: Headend
- Part 10: Troubleshooting on the cable TV network
State plan to cut off analogue broadcasting of TV channels
Although this does not fully relate to the topic, it is simply impossible to ignore such a burning issue now.
And so: all these conversations concern exclusively on-air broadcasting. That is, the signal that propagates through the air from the nearest TV tower. Only the state is responsible for this signal in Russia, and only two (in some regions three) multiplexes will remain in it. The analog component of cable broadcasting depends solely on providers and most likely will not go anywhere. So if your TV is not connected to an antenna on the roof of the house or on the windowsill, then this disconnection will almost certainly not affect you. Why do I say "almost" and "most likely"? The fact is that some cable operators have already announced the upcoming termination of the provision of an analog signal to subscribers. It is difficult to understand the motivation, because as it is clear from part 1 of my articles, this cannot bring significant savings on equipment: only a few expansion cards in a common chassis are responsible for this. The release of carrier frequencies is also a dubious motivation: there is no need in the market for so many digital channels that can fit in to replace the disabled analog ones. Here, except that you can get hold of by selling set-top boxes to subscribers, but let's leave it on the conscience of the operators.
Analog signal parameters
An analog television signal is the sum of three signals: amplitude modulated brightness and color, and frequency modulated sound. But to assess the quantity and quality, it is enough to take this signal as a whole, although we have all observed more than once that with a terrible image, the sound from the TV comes out good. This is due to better FM noise immunity. To measure the parameters of an analog signal, Deviser DS2400T provides the appropriate mode:
In this mode, you can use the buttons to switch analog channels (digital channels will be automatically skipped) as on a TV. Only instead of advertising and news, we will see something like this:
On it, we can see the main parameters of the signal: this is the level in dBuV and the ratio of the signal level to noise (or rather, carrier / noise). Since channels at different frequencies are subject to different phenomena during transmission, it is necessary to make measurements on several channels (at least on the two extreme ones in the frequency range).
In accordance with the requirements of GOST, the signal level at the input to the receiver should be in the range from 60 to 80 dB. In order to ensure these values, the provider must give the subscriber at the connection point (usually a low-current shield on the landing), ideally 70-75 dB. The fact is that anything can happen on the territory of the subscriber: poor-quality or damaged cable, incorrectly selected dividers, a TV with a poor sensitivity threshold. All this will eventually lead to signal attenuation. But an excessively high signal level is also bad: a good TV with the right circuitry, including high-quality AGC, is able to painlessly process a signal of more than 100dB, but most inexpensive TVs simply cannot cope with such a signal.
An indispensable companion of any signal is noise. It is introduced by active equipment at the stage of signal formation, then amplifiers amplify it along with the signal, and even add a little of themselves. For an analog signal, this is very critical: all that snow, stripes and other distortions are noise that needs to be measured and it is desirable, of course, to reduce. To evaluate the quality of an analog signal, the useful signal-to-noise ratio is used, that is, the larger the value, the better. GOST defines a minimum value of 43 dB, in fact, the subscriber receives of course more, but for the same reasons as attenuation, this parameter may deteriorate on the way from the shield to the TV. Although it is believed that passive wiring cannot introduce noise, it can take on interference from a nearby electrical cable, for example, or take on a powerful terrestrial signal from a repeater. In addition, low-quality or aged dividers can do their job - you should pay attention to this.
In practice, the final image quality depends to a large extent on the TV itself. Of course, the analog signal does not have redundancy for noise protection, but filters in high-quality receivers, as well as built-in amplifiers, can work wonders, but, of course, the provider should not rely on this.
Source: habr.com