🚀 go-pugleaf

Ian S. Salisbury <isalisb@nospam.gmail.com> wrote:
> Ron T. wrote:

>> The station engineer, stating that the digital signal could "theoretically" 
>> go farther, caught me off guard. If you have, say, a 180 mhz signal at 10kw, 
>> it doesn't matter if it's carrying an analog or digital content, it's going 
>> to be the same damn signal strength under whatever conditions at whatever 
>> distance.
>> 
>> The analog signal, however, will be usuable (though degraded) far beyond a 
>> digital signal which I believe drops usability somewhere around 25-30%. 
>> 
> I'm just guessing, but you may be able to increase the distance of a 
> usable digital signal by increasing the number of parity bits.  Then it 
> becomes a matter of how big of a buffer the receiver has, and how much 
> of a delay is acceptable.


I think the station engineer poorly worded his answer, 

"could theoretically go farther"

sort of depends which side of the fence you are standing on.

I think what he meant was, the signal is usable at a certain point where
before it wasn't. Keep in mind the old ntsc signal is a rather complex
combination of fm signals mixed together. The further you move away from the
transmitter, the weaker the signal gets. The picture starts getting snowy,
sound starts to hiss, the color may of pop in and out.

The digital channel is just carrying bits of data, no complex fm waveforms
to try to recover. If the tv/antenna is seeing the data stream, it more or
less "just works". 

With the ntsc/analog system, I beleive the minimum voltage you wanted down
from the antenna was 1000 microvolts on any channel. That figure was high
enough for any tv to pick up an acceptable and usable signal. The point to
make here is, if on any particular channel you were getting 1000uV or
10,000uV, there wasn't any difference in the picture/sound. As long as
1000uV was available, you got a good picture and clear sound.

I think with the digital broadcasting you just need to slide that scale down
to like 100uV and 1000uV. I'm not saying that is correct and it's really 10x
better or more sensitive, but because it's only data bits to deal with, you
just don't need the same level of power to make it work.

Looking at it from the transmitter side, if a station was permitted to cover
a radius of 50 miles, with analog they probably need 10 million watts to get
the 1000uV out at the 50 mile point. With a digital only signal to get 100uV
(again guessing on that), you simply won't need the same amount of power.

The plain english explanation is simply compare it to the old analog cell
phones and current digital ones. The analogs were much bigger, batteries
didn't last as long because they were basically a analog radio transmitter
and receiver. The current digital ones don't broadcast a fm signal to get
picked up, it's a spray of bits on a given frequency at flea power compared
to the old analog ones. The batteries can last for days.

Again, all this is a WAG but seems to fit the reason why the digital
transmitters are at a much lower power output than the analog counterparts
but still cover the same range in reality. They simply don't need to
broadcast at those higher levels.

-bruce
bje@ripco.com