[mythtv-users] Digital technicalities (Was: Obama ...

Brian Wood beww at beww.org
Sat Jan 10 01:07:14 UTC 2009


On Friday 09 January 2009 17:38:54 David Brodbeck wrote:
> On Fri, January 9, 2009 12:25 pm, Brian Wood wrote:
> > A "repeater" uses two frequencies, one for receive and one for transmit.
>
> Yes, but so does a translator -- it's receiving on the input channel and
> transmitting on the output channel.  Granted, it's a continuous
> transmission from one fixed source, instead of short transmissions from a
> bunch of mobile sources, but it's conceptually the same thing as a
> repeater.

Vey similar, but designed for different purposes. Some TV "translators" 
actually do not receive the original RF TV transmission, but get their input 
from a microwave or fiber-optic receiver output at baseband (some even use 
the IF out of a microwave receiver to hetrodyne to the final RF out frequency 
(this is almost never done anymore as it requires an AM microwave system. 
Hughes is the only outfit I know of that ever made those).

>
> Just to confuse the issue further, the same type of device is called a
> "transponder" when it's on a satellite. ;)

Yup, although "transponders" usually hetrodyne the incoming signal against a 
local oscillator, then amplify the resultant, while repeaters and translators 
employ separate receivers and transmitters. The output of a transponder will 
drop to zero if the input signal vanishes, this is not true of translators. 
The output power of a transponder will also vary with the input signal 
strength (to some degree), this is not true of translators, they have 
constant output power. Repeaters are keyed by an input signal, but do not 
depend on that input signal in order to produce an output (so you can 
have "hang time" as an example). Often two-way "repeaters" are not truly 
repeaters, they are really remote transmitters, and are often fed by "voting" 
receivers in multiple locations.

Transponders convert the frequency of a signal to a new frequency, repeaters 
and translators generate a new signal, which is modulated by a receiver of 
some sort. For example, a frequency error on the input of a transponder will 
result in a similar error on the output side, not true of repeaters and 
translators.

So the difference is not so much what ther thing does, but how it does it, and 
for what purpose. 

I guess I just worked with these things long enough that the names make sense 
to me, although I agree they are somewhat confusing, now that you point that 
out.

-- 
beww
beww at beww.org


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