Digital signal levels
Digital signals have 2 states.
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perfect pictures
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non existent pictures
Actually it can be said that there is another whereby pictures work intermittently, i.e. pixel-ate and break up. This usually occurs where there is high action or detail.
This break up of picture is caused by aerial signal errors creeping in, and, as it gets worse, the tuner cannot correct the errors quick enough and then totally freezes and disappears.
This is where you see the red dot, or circle, which mostly indicates a too high or too low signal.
These errors in the signal can be caused by a number of ways.
-
the received aerial signal is too low
-
the received aerial signal is too high
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the C/N ratio of the signal is not good enough. This noise causes more errors added to the equation.
- There is high action simultaneously on several programs in the same multiplex (much rarer!)
For carrier noise ratios for UHF click here
signal levels (digital)
- minimum level 45dB
- recommended minimum level 50dB
- Maximum level 65dB
The reason that the minimum recommended signal is 50dB is that at the lower end of the signal parameters, there is more risk from impulse noise, such as a fridge or central heating thermostat.
It is therefore far more important to be accurate for digital signal levels, i.e.. 50-60 dB, a 10dB range, than it ever was for analogue signals.
Digital signals are normally 20dB lower than the digital signals transmitted from the same transmitter. (or 1/100th of the power).
The other very important point to be borne in mind, is that the Carrier Noise Ratio, or C/N ratio outlined on another page.
