[ntp:questions] Re: Clock accuracy & auto setting : digital television does a crap job of providing time services...
David L. Mills
mills at udel.edu
Sun May 7 01:00:30 UTC 2006
The 128-bit NTP date spans 584 billion years with a resolution of .05
attosecond. The 64-bit NTP timestamp spans 136 years with a resolution
of 232 picoseconds. Is there something that needs better than 232
picoseconds? A 4-GHz CPU comes close.
As for broadcasters chiming good time, I do know that PBS/NPR
distributes progams via digital satellite and IP network with NTP.
However, I am told the gadgetry that actually brands the signal doesn't
use NTP, is often banished to the bottom of the rack and nobody knows
how to adjust it. As for the broadcast program start time, so far as I
can tell it's withing 30 ms of UTC. That's the best I can do with
eyeball and ear.
When I worked for an NBC affiliate in Detroit we had two clocks, one
synchronized to the power grid and the other a Western Union gadget that
synchronized via wire. The WU clock was purposely set to run slow by a
second or two per hour. Once per hour WU sent a pulse over the wire
causing the clock to do a hands-up jerk. I quickly learned to use that
clock only to calibrate the few seconds offset of the grid at the
hands-up signal. Broadcast networks once sent an audible ping at
hands-up to do the same tning, but that was (ahem) 50 yuears ago..
Alan Larson wrote:
> In article <e2p993$qim$2 at gnus01.u.washington.edu> "Max Power" <mikehack at u.washington.edu> writes:
>>In North America, the FM RDS time service is of very low quality.
>>In Europe (I understand) this is not the case with RDS.
>>Stations running RDS should be mandated by law to provide a quality
>>service -- based on transmitter power and coverage area.
>>Over time RDS's time service should be uniform.
>>DRM (on MW and SW) time service is of a lower quality than RDS -- but could
>>be upgraded with a specialized "80 bit" NTP-UNIX time packet.
>>ATSC and DVB-T (& DVB-H/M) need a uniform ~"80 bit"...~"128 bit" time packet
>>service that is well thought out.
>>Futureproofing is important, so probably 128 bits or more is preferable.
> 128 bits? What do you want - to specify the time of the heat death of the
> universe (long after our sun dies) to nanosecond resolution, then be able to
> tell the time of the death of the next universe (if there is one)?
> 64 bit NTP is probably quite adequate, and definitely enough bits if
> one doesn't insist on the NTP nanosecond precision.
> However, the time information doesn't need to be NTP, or IP based.
> By the way, NTP is not a part of unix.
> Presently, the analog TV stations transmitting time don't even seem
> to consider it worth keeping the clocks set accurately -- some use a
> PC's clock, with no external source to deal with its drift.
> The time sent with ATSC seems to be random as well. Some stations
> seem to have gone ahead with DST, but the only way to get the program
> data to be correct is to manually force (and set) the receiver to use
> local standard time.
> Of course, not using UTC is astoundingly stupid -- folks who live next
> to a time zone boundary are out of luck if some stations are on each side.
> However, counting on broadcasters to get the time right is a fantasy.
> They cannot even get their program guide information correct in the data
More information about the questions