Cable TV networks for the smallest. Part 3: Analog Signal Component
Progress is moving on 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.
Content of a series of articles
State plan for disabling analog broadcasting of television channels
Although this does not fully relate to the topic, it is simply impossible to ignore such a burning issue now.
So: all these conversations relate exclusively to broadcasting. That is, that signal that spreads through the air from the nearest TV tower. Only for this signal in Russia is the state responsible and only in it will remain only two (in some regions three) multiplexes. The analog component of cable broadcasting depends solely on providers and is likely to go nowhere. So if your TV is not connected to an antenna standing on the roof of the house or window sill, then almost certainly this disconnection will 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’s hard to understand the motivation, because it is clear from part 1 of my articles that this can not bring significant savings on equipment: Only a few expansion cards in the 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 place of disconnected analog ones. It’s possible to get hold of selling consoles to subscribers, but let’s leave it to the conscience of the operators.
Analog Signal Parameters
An analog television signal is the sum of three signals: amplitude-modulated brightness, and frequency-modulated color and sound (for SECAM, used in the CIS). But to assess the quantity and quality, it is enough to take this signal as a whole, although we have all seen more than once that with a terrible image, the sound from the TV is good. This is due to the best noise immunity of the FM . To measure the parameters of the analog signal, the Deviser DS2400T provides the appropriate mode:
In this mode, you can use the buttons to switch the analog channels (digital 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 dBµV and the ratio of signal level to noise (or rather, carrier / noise). Since channels at different frequencies are subject to different phenomena during transmission, it is necessary to take measurements on several channels (at least 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 should give the subscriber at the connection point (as a rule this is a low-current flap on the landing) ideally 70-75 dB. The fact is that anything can happen on the subscriber’s territory: poor-quality or damaged cable, improperly selected dividers, a TV with a poor sensitivity threshold. All this will ultimately lead to signal attenuation. But an excessively high signal level is also bad: a good TV with the correct circuitry, including a high-quality AGC, can easily process a signal and more than 100dB, but most inexpensive TVs simply can not cope with such a signal.
An indispensable companion of any signal is noise. Active equipment introduces it even at the stage of signal formation, then the amplifiers amplify it along with the signal, and even add a little bit from 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 it. To assess the quality of the analog signal, the ratio of the useful signal to noise is used, that is, the larger the value, the better. GOST defines the minimum value of 43 dB, in fact the subscriber receives of course more, but for the same reasons as attenuation, this parameter can worsen on the way from the back to the TV. Although it is believed that passive wiring cannot make noise, it can take on the interference from a nearby electrical cable, for example, or take on a powerful broadcast signal from a repeater. In addition, low-quality or aged dividers can do their job - this is worth paying attention to.
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 the provider, of course, is not worth it.