[tz] CCTF survey on Time and Frequency Metrology - Telecom

[Brian Inglis]

On 2021-01-27 12:58, Fred Gleason wrote:
> On Jan 26, 2021, at 19:03, Brian Inglis <Brian.Inglis at SystematicSw.ab.ca 
> <mailto:Brian.Inglis at SystematicSw.ab.ca>> wrote:
> 
>> Digital processing mainly compression/decompression delays in the common 
>> modern broadcast chain results in time announcements, transitions of minutes, 
>> audio "pips", clock tower chimes or bells, varying by eyeball and wristwatch 
>> measurable amounts from accurate time sources.
>> Some articles talk about timing broadcast sources to determine which earlier 
>> "pip" signals the correct time.
>> Apparently coordinating multiple regional sources in e.g. commentaries, 
>> discussions, or now virtual talk shows, requires time code calibration and 
>> compensation, so that discussions can appear natural; more so than many remote 
>> news reports which can show very pronounced delays, probably due to cellular 
>> mobile or wifi sources and internet routes.
>> Satellite links add only up to 250ms each round trip.
>> Many articles lack date stamps so it is difficult to know even if the BBC 
>> still transmits legacy analogue AM or FM with accurate "pips" or if stations 
>> are mainly DAB in the EU and UK.
> 
> It’s not just in the EU or UK. Digital modulation methods in common use on 
> broadcast air chains in the US (both audio and video) can easily add up to 
> several *seconds* of latency per hop. This can be the case even when the final 
> delivery link to the receiver is ‘analog’ —e.g. amplitude modulation (‘AM’) via 
> medium wave. Effectively, all audience-facing time marks on such broadcasts 
> should be treated as being precise only to the nearest minute.

https://en.wikipedia.org/wiki/Greenwich_Time_Signal#Similar_time_signals_elsewhere

I'm unaware of broadcast network time signals and their accuracy in NA other 
than CA CBC Radio One at 13:00 ET, and NR have disclaimers about those and 
telephone time and talking clocks possibly having one or two ~0.25s satellite 
hops added, which makes them useless:

https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/cbc-radio-daily-time-broadcast

https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/telephone-talking-clock

The telephone and broadcast time data require obsolete Bell 103 modem decoders 
and suffer from unknown telco or shortwave propagation or ionospheric delays and 
conditions:

https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/computer-time-date#dtime

https://nrc.canada.ca/en/certifications-evaluations-standards/canadas-official-time/nrc-shortwave-station-broadcasts-chu

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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