[time-nuts] PC clock generator without 14.318MHz

Magnus Danielson magnus at rubidium.dyndns.org
Tue Oct 18 20:10:23 EDT 2016


On 10/19/2016 01:25 AM, Attila Kinali wrote:
> On Tue, 18 Oct 2016 23:36:55 +0200
> Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
>> To this day, the 1.001 factor haunts us in all modern hardware and
>> timing designs, including the latest synchronization standards.
>> It's fascinating that we have been simulating several centuries into the
>> future to ensure that the algorithms for the synchronization will make
>> the 1.001 factor keep working as intended. I wrote one such simulator
>> being used.
> So, you are doing the inverse of a programmer archeologist[1]?
> SCNR :-)
> On a more more serious note: what exactly have you been simulating?
> And how does this frequency make a difference?

When you use GPS/GNSS as source for your TV-signals, you define a 
suitable epoch, such as the PTP Epoch. At the Epoch, all sample values 
where in phase, and from then on they progress with SI-second, so with a 
TAI-approximation you can calculate it. The time of day, date etc. you 
derive from the UTC-approximation.

The 1.001 factor then requires a 1001 s long cycle before it aligns up.
Considering some other factors, this can be as long as 4004 s for 
everything to aline up. So, TAI-time modulus 4004 s and from there it is 
relatively simple for all the phases.

You would find the details in SMPTE 2059-1 and SMPTE 2059-2.

All the traditional formats can be expressed in this time. In order to 
achieve it fully, the PTP server needs to extend with additional info, 
which among other things tells us how the TV station is setup and when 
the common daily jamming occurs.

Now, as we do full-HD signal, it is 2,97 Gb/s for European TV or 
2,97/1,001 Gb/s for US TV. So we naturally need a way to get these 
values in low-jitter form, which is a bit interesting. The older BT.656 
SDI format, also known as SMPTE 259 had a nice and round common number 
of 270 Mb/s for both formats, but somebody didn't think it through for 
the HDTV variant and the number-magic broke up.

We keep inherit this into 4k video and 8k I guess.

Lovely, isn't it?


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