[ntp:questions] Can NTP sync within 1ms

jimp at specsol.spam.sux.com jimp at specsol.spam.sux.com
Sun Apr 27 19:40:24 UTC 2014


Rob <nomail at example.com> wrote:
> jimp at specsol.spam.sux.com <jimp at specsol.spam.sux.com> wrote:
>> Rob <nomail at example.com> wrote:
>>> jimp at specsol.spam.sux.com <jimp at specsol.spam.sux.com> wrote:
>>>> Rob <nomail at example.com> wrote:
>>>>> jimp at specsol.spam.sux.com <jimp at specsol.spam.sux.com> wrote:
>>>>>>> The listeners should enjoy a smooth reception while driving around.
>>>>>>> So of course there should be no time lag between the modulation signals
>>>>>>> of the different transmitters.  Experts in the field tell us we should
>>>>>>> be within 12us.
>>>>>>
>>>>>> Unless I fat fingered the calculator, that means the difference in distance
>>>>>> between transmitters relative to the receiver can be no more than 3.6 km.
>>>>>>
>>>>>> 300 meters per microsecond; it is the law...
>>>>> 
>>>>> The goal is not to have 12us difference in arrival time, but to be
>>>>> within 12us for transmission time.
>>>>
>>>> What good does that do you?
>>>>
>>>> And regarding "smooth reception while driving around", have you ever heard
>>>> of multipath or the capture effect?
>>> 
>>> Well, smooth reception must be explained as "communication is possible",
>>> not with the HIFI quality expected from a broadcast station.
>>> 
>>> What I mean is that you can drive around and receive the signal all over
>>> the place, even when you drive out of range of one transmitter into the
>>> range of the next.  It works perfectly when the capture effect results
>>> in reception of one transmitter, and in the area where two transmitters
>>> are equal in strength it requires suitable synchronization to still have
>>> good communication.
>>
>> Picket fencing makes it irrelevant.
> 
> This precisely makes it a requirement to synchronize the audio.
> When this is not done, there will be extreme jitter in the audio which
> makes it unintelligible.
> What surprises me is that the audio has to be synchronized so well.
> But we are just going to do that, or approach it as closely as possible.

Nope, it is precisely what makes synchronized audio irrelevant.

If you have picket fencing, you have no intelligible audio no matter
what you do at the transmitters.

>>> I did not come up with the 12us figure, I would have guessed it a bit
>>> higher.  The figure comes from different experts in the field.  The
>>> people that designed and deployed such systems in the world of emergency
>>> services etc.
>>
>> Are you sure that figure is NOT for data communications?
>>
>> Most "emergency services" have voice and data these days.
> 
> Emergency services don't use NBFM systems anymore.  At least not here.
> The networks I am talking about were deployed between the seventies and
> nineties of last century.

Then what the hell are you talking about?

>>> We can now try it in amateur radio because the advances in technology
>>> have made things like GPSDOs and fast network connections affordable.
>>
>> I don't see where that is relevant to anything here.
> 
> I am ending this discussion, I only answered the question "what are you
> trying to do" and I don't need to account for our experiment to you or
> anyone else on this group.

No, you don't.

You can ignore any comments by people with over half a century of experience
and go off and waste your time tilting at windmills.



-- 
Jim Pennino



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