Re: [ntp:questions] Can NTP sync within 12us for NBFM
On 2014-04-28 13:20, William Unruh wrote: On 2014-04-28, Rob wrote: William Unruh wrote: All in all it is funny to read all the "that cannot be done"-like comments by several persons on a ntp newsgroup while systems like this have been in use since the seventies, and in fact have already been build by amateurs and are in operation today. So I prefer to go by the experience of the people who built those networks, rather than the armchair experts. Not, "it cannnot be done", but "it is silly to try". I simply have a really really hard time figuring out why you want to do that. You apparently are not familiar with the radio amateur hobby. Most people have a hard time figuring out why you would want to construct a repeater to talk to someone else driving around in a car, when you could simply dial his number on the mobile phone. That was not what I had trouble figuring out. If it really is 12us I would be really interested in knowing why. I might even learn something. I already said that I was suprised about this as well, but I have heard it from two different persons who had not been working together and it was explicitly asked if if would not be 12ms instead of 12us. And they did not explain themselves!? Speculatively and IANA RE (asking in the hope that an RE or Time Nuts mailing list member of this group may provide the correct answer) I understand that the NBFM transmitter starts with phase modulation then integrates, multiplies, and filters the result to the FM carrier frequency, could a phase error more than 12us/s (PPM) possibly generate either a significant signal distortion, and/or unwanted harmonics within the filter passband? -- Take care. Thanks, Brian Inglis ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Can NTP sync within 12us for NBFM
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. (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.) ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Can NTP sync within 12us for NBFM
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. (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. -- Take care. Thanks, Brian Inglis ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Can NTP sync within 12us for NBFM
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. (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. -- Take care. Thanks, Brian Inglis ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
Re: [ntp:questions] Can NTP sync within 12us for NBFM
On 2014-04-29, Brian Inglis 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. > ___ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
