A lot of this comes from the magic of error correcting codes (ECC, also called forward error correction FEC).
the idea is that you add a small amount (or possibly a large amount) of carefully generated, redundant data to the transmission. then you can lower the power and allow more bit-errors to occur at the receiver. eg. if you have a 1MB transmission and a 1 in a million chance (per bit) of error, you would expect 8 errors in transmission. if you added an ECC scheme, you might be able to correct up to 10 errors at the cost of adding an additional 100kB to the transmission. so now there is a good chance that the receiver will see 0 bit errors. If 8 bit errors is subjectively ok, then the power could be lowered until 18 bit errors would be expected.
The use of compression also plays a big role. if you reduce the data required by a factor of 10, it allows you to add a lot more ECC (or conserve bandwidth).
You are right, I was thinking on MPE-FEC as the only FEC protocol and which was implemented in DVB-h. Reed-Solomon FEC is implemented in DVB-t.
Lot of good guesses. It is very easy to get reliable answers at DVB - Digital Video Broadcasting - Home. Most of their technical papers are free for download.
A error free decoding of DVB-t requires 20-22 dB s/n. The variation in s/n is due to what coding scheme that is used.
Compare this with a decoded PAL signal that requires 40 dB s/n to be accepted as a good picture quality. So when a PAL transmitter is replaced with a DVB-t transmitter is it in most cases a factor 100 in difference in TX power for same coverage and that is the ratio most TV-transmitters are using when replacing system. However is the cost almost the same for these new transmitters even if they are a factor 100 smaller, due to much higher requirement for low phase errors and such.
FEC is not used in DVB-t. DVB-h, was the first DVB standard that used FEC. DVB-t2, which in many areas now is replacing DVB-t, is using FEC which allows for a less demanding c/n then DVB-t. It will in most cases not result in that less TX power can be used but have other advantages.
But if to take into account your logic, there is no sence in reducing the power of digital signal compare with analog signal only for the sake of the same covering zone. I think the reason in the difference of the digital and analog power of the same transmitter lies in different types of modulations. AM in analog and COFDM in digital (DVB-T)
Maintaining similar coverage as the system it replaces is in most cases a requirement to avoid interference in same channel or adjacent channels from other existing transmitters. It is regulated in international agreements. To maintain same coverage have DVB specified what field strength that is required (both max and min) and what kind of sensitivity and selectivity a receiver must fulfill to be accepted as a DVB-receiver. This do in general result in a max allowed TX power 20 dB less then similar PAL/Secam transmitter.
Final RF amplifier have not a linear gain when it is driven close to its peak values, as most amplifiers. Can be described as a compression.
It is in this case compensated by a predefined gain correction in base band signal, before 2:nd mixer.
In a AM amplifier circuit can this compensation be done by conventional feedback. Guess that solution is avoided for improved phase stability.
Final RF amplifier have not a linear gain when it is driven close to its peak values, as most amplifiers. Can be described as a compression.
It is in this case compensated by a predefined gain correction in base band signal, before 2:nd mixer.
In a AM amplifier circuit can this compensation be done by conventional feedback. Guess that solution is avoided for improved phase stability.
In analog TV,the image varies but in digital,the image is constant.
2. If usable peak level can increase with 3 dB is it much worth for average power level efficiency, resulting in less total power consumption for a given amount of output power, less heat, cheaper transistors....
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