[ntp:questions] Can NTP sync within 12us for NBFM

William Unruh unruh at invalid.ca
Tue Apr 29 15:38:03 UTC 2014

On 2014-04-29, Brian Inglis <Brian.Inglis at Shaw.ca> wrote:
> On 2014-04-29 00:47, David Woolley wrote:
>> On 29/04/14 03:59, Brian Inglis wrote:
>>> I understand that the NBFM
>>> transmitter starts with phase modulation then integrates, multiplies,
>>> and filters
>>> the result to the FM carrier frequency
>> You are confusing carrier phase and audio phase.
> I omitted the preemphasis because that is applied to the audio before
> phase modulation of the IF, and I don't believe the 12PPMphase error
> would make any difference to the audio, but may affect the IF or carrier.

I cannot see how. The FM does not really care what the signal is. If the
signal is late by 7 hours, never mind 12us, it does not care.
And also we are talking about the time difference in the audio signal between two different
transmitters. Now if both signals are of equal strength at the receiver,
then the phase difference between the two audio signals might be important,
But again, not 12us. Maybe 200us might start to show some effects. 

>> (FM modulation is mathematically the same as frequency modulation except
>> for the frequency response of the audio.  Both direct frequency modulation
>> and phase modulation can be used.  I believe NBFM is pre-emphasised such that,
>> for most of the passband, it is actually more accurately described as phase
>> modulation.)
> ITYM FM ... PM, and the preemphasis flattens the audio spectrum to
> increase the resolution of the low level high frequency voice
> components and make better use of the available bandwidth.

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