tvb · HackerTrans

Many pendulum clocks are capable of well under 1 ppm accuracy, which is better than many quartz oscillators. Here is a tour of clock precision by orders of magnitude:

http://leapsecond.com/ten/

Here is a world record setting pendulum clock from a few years ago:

http://leapsecond.com/pend/clockb/

Precision pendulum clocks are a very deep rabbit hole.

tvb

4년 전discuss

That's correct. On NT/Alpha context switching was done in ntos\ke\alpha\ctxsw.s, which was regular kernel mode assembler code. A PAL call was made but it simply set the new Teb and Pdr; two or three lines of code. I've never seen the PALcode for VMS or OSF/Tru64 but the NT PAL for swpctx was designed so that alpha assembler code in NT looked as much like the mips code that davec wrote. It made the port easier that way.

Interesting Raymond continues to mention NT/Alpha now and then. If you run into him mention I have a collection of NT/Alpha artifacts and anecdotes he might be interested in.

tvb

4년 전discuss

The original volt-nuts mailing list pretty much faded away as the members use the eevblog forum now. See the Metrology section.

The time-nuts mailing list is still very active, now in its 23rd year. More info at the group page:

http://leapsecond.com/time-nuts.htm

tvb

4년 전discuss

That's the price -- when new. But most "time nuts" buy their atomic clocks well used on eBay or local surplus sales at a massive discount. For example the most recent H-maser on the hobbyist market went for just a few thousand. Granted it was in need of repair, but that journey is part of the fun of an atomic timekeeping at home hobby.

tvb

4년 전discuss

Author of the web site (leapsecond.com) here if you have any questions. I don't know how the title of the HN post was chosen. The actual title of the web page is "GPS, UTC, and TAI Clocks".

The page is a javascript animation of "GPS system time", UTC, and TAI showing how they all tick together but are offset from each other by an integer number of seconds. It's a fixed integer in the case of TAI and GPS and a variable integer in the case of UTC.

tvb

4년 전discuss

> Blockbuster had a chance to buy Netflix in the early 2000's for $50,000,000.

Ah, an example of the Butterfly Effect: then we'd have a billion references to FAABG on the web instead of FAANG.

tvb

4년 전discuss

Previously (and posted 64000/89 days ago):

https://news.ycombinator.com/item?id=24929726

tvb

4년 전discuss

TAI and UTC are based on the SI second. The SI second is specifically defined in terms of cesium atoms at no temperature, no velocity, and no elevation (aka, on the geoid). Consequently TAI and UTC are immune from relativistic effects, by definition.

On the other hand, the physical clocks in the laboratory are not immune and that's why they have to be corrected for a dozen factors, the largest of which is usually gravitational redshift. To appreciate the complexity and precision of this correction see this NIST paper:

https://tf.nist.gov/general/pdf/2883.pdf

tvb

4년 전discuss

The original pi/spigot work was done by Rabinowitz & Wagon [1]. See also the very readable works by Jeremy Gibbons [2] [3].

[1] https://en.wikipedia.org/wiki/Spigot_algorithm

[2] https://www.cs.ox.ac.uk/people/jeremy.gibbons/publications/s...

[3] https://seminar-materials.iijlab.net/iijlab-seminar/iijlab-s...

tvb

4년 전discuss

You can grep the web for Dik's original version. The 3-line homage version I linked contains both his full name and the name of the algorithm used: "spigot" (as in spew, faucet), which itself contains the letters PI. In addition, this version outputs exactly 31416 digits of pi. So it's triple self-referential. Happy 2022.03.14 pi day to all.

tvb

4년 전discuss

With a nod to fellow pi-hound Dik T Winter, three equal lines of C give 32k digits of pi:

http://leapsecond.com/tools/pi3.c

tvb

4년 전discuss

> LF is line feed, thus new line.

  LF is line feed, thus next line.
                                   The CR
                                          is to
  readjust the write head
  back to the
  starting position.

A line feed alone is not a new line because some or most of the line is already used up. The B '*n' and C '\n' introduced the notion of new line, which outputs both a next line (LF) and back to the starting position (CR).

tvb

5년 전discuss

Only a positive leap second is “over in a second”. A negative leap second is essentially “over before it starts”. This asymmetry is an opportunity for code to implement UTC incorrectly.

Another issue is that a positive leap second cannot be represented using time_t-like representations. But at least every time_t value maps to a UTC time.

By contrast a negative leap second, should one occur, will cause a permanent illegal time_t value that has no existence in UTC. Checking for that illegal value in every piece of code that uses time_t values will not be fun.

tvb

5년 전discuss

DCF77 has a leap second warning bit. PTB says it is up to the user to know if it positive or negative. If a clock blindly assumes it is a positive leap that’s an error in the design of the clock. This is not ideal.

Likewise WWVB has a leap second warning bit. But it includes a UT1 sign bit so the user can know if the leap will be positive or negative. This design is fine and any WWVB clock or software can properly handle positive or negative leap seconds.

This is true for both the amplitude and the phase modulation formats.

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