[NUT-devel] Broadcasting nuts [PATCH]

[Måns Rullgård]

Rich Felker <dalias at aerifal.cx> writes:

> On Wed, Feb 06, 2008 at 06:50:05PM +0000, Måns Rullgård wrote:
>> Rich Felker <dalias at aerifal.cx> writes:
>> 
>> > On Wed, Feb 06, 2008 at 07:02:11PM +0100, Michael Niedermayer wrote:
>> >> > A clock this bad would drift by 1/4 an hour per day. I doubt anyone
>> >> > would buy such a clock...
>> >> 
>> >> No, but people buy various embeded devices to watch videos and their clocks
>> >> are supposed to be cheap. And while i dont think they would be off by
>> >> 15min/day i doubt you can expect more than 1min/day accuracy from them.
>> >
>> > FWIW, this matches the estimates in my other email of 0.1% error.
>> >
>> > Also, maybe this is getting OT, but truely "broadcast" (in the sense
>> > of airwaves being involved) applications have a matched-to-sender
>> > clock pulse already via the PLL or whatever similar circuits they use
>> > in the tuner equipment.
>> 
>> There is no correlation between the carrier and the data.  The carrier
>> has nothing whatsoever to do with sender/receiver synchronisation.
>
> Not in existing devices, but there's no fundamental reason it can't be
> used for this purpose.

The carrier is rarely, if ever, generated at the signal source.  A
satellite broadcast always has at least two links (earth to satellite
and satellite to earth).  On each leg, the sender generates its own
carrier (this is true whatever the medium of the link), and attempting
to synchronise them would be madness.  The carrier frequencies on
different links can differ by orders of magnitude (from 10GHz
satellite to 500MHz terrestrial).  There need not even be a carrier at
all.

>> > Incidentally, at least as I understand it, PLL itself works on a
>> > concept similar to what I described, consisting of local clock
>> > (oscillator) plus adjustments from the input signal, and operating
>> > without a separate reference clock source.
>> 
>> The input signal *is* the reference clock.  You are suggesting we
>> remove the input signal.
>
> Depending on the application, it can be, but it can also be a signal
> modulated on a carrier, and the circuit can synchronize to the
> carrier without having a separate pure reference signal. I agree
> that argument by analogy is not a good argument method, but here I'm
> using it as a suggestion of a line of thinking based on something
> familiar. Anyway it's all just an aside, and if you don't like the
> analogy, drop it, because its truth is not in any way tied to the
> truth of the important claims.

Judging by the above nonsense, you are in no way familiar with
modulation, PLLs, or anything else related to signal transmission.

-- 
Måns Rullgård
mans at mansr.com