4 ms·

A lot of systems use the 60hz (or 50hz depending on location) signal from AC power as a way of keeping time. It's used enough that power companies will actually

by dlgeek 10y ago

A lot of systems use the 60hz (or 50hz depending on location) signal from AC power as a way of keeping time. It's used enough that power companies will actually adjust the frequency minutely at night in order to make up for any mistakes made during the day. (Which in turn makes it so reliable that more systems use it to track time, and so on...) I guess grandparent post is saying power companies go ahead and feed in extra cycles for the leap second. It's not something I'd heard of, but it wouldn't surprise me.

metafunctor 10y ago
I remember reading that the frequency at any given time is usually somewhere between 49.9Hz and 50.1Hz (and similarly around 60Hz I'd imagine). So for a 50Hz clock, it's not very accurate at all. However, I just found that the _average_ frequency over a length of time should be exactly 50Hz, so it is, in fact, good for keeping time. More here: https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html
kalleboo 10y ago
I remember hearing that Analog TV stations were also used for timekeeping since they had a very stable clock for the blanking interval
- Animats 10y ago
  US analog TV (all three classic networks) used to be synchronized to an atomic clock in NYC. This allowed switching from one source to another without frame tearing. Separate from that, there was a scheme for sending time of day in the vertical retrace interval, to set clocks and VCRs. Few devices received that, although I once encountered a hotel TV with a built-in analog clock that did. (The clock was wrong; although it could set itself automatically, it had to be told manually which channel provided a time signal. That's probably why this feature never caught on. I was bored enough to figure out how to set the thing.) US telephony used to be synchronized to an atomic clock in New Jersey. The TDMA multiplexing was kept in sync so that frames could be transferred synchronously without buffering. If the clock connection was lost, T1 to T3 lines would get slightly out of sync, and occasionally drop or add an 8-bit frame. This is unnoticeable in audio, but an error for modems. But neither of these was a time count. It was just a square wave.
  - kalleboo 10y ago
    Interesting to hear about how it worked in the US. I think I heard about the TV frame syncing from an Australian YouTube video. I'm from Europe where every TV and VCR would easily get a clock from TeleText (we also had fancy stuff like PDC to make sure your video recordings of TV shows didn't get cut off)
Angostura 10y ago
Hmmm? In the UK I'm sure the frequency changes with load on the grid and is actually a way of measuring grid load.
- anexprogrammer 10y ago
  Indeed, and they have to remain within +-1%. They have a realtime graph: http://www2.nationalgrid.com/uk/Industry-information/electricity-transmission-operational-data/ http://www2.nationalgrid.com/uk/Industry-information/electri... Before quartz clocks were around, synchronous mains clocks were common, and accurate.
- dfox 10y ago
  The idea is that while the instantaneous frequency varies with load, the power grid is actively regulated such that frequency is constant on average over longer periods (typically such that from midnight to midnight there are 24 * 3600 * 50 or 60 cycles).
vidarh 10y ago
As an example of reliance on this, the C64 had a (rarely used) realtime clock in the 6256 CIA's (IO chips) that relied on the AC 50/60Hz signal to keep the time of day. It also used it to control the cassette motor. For those reasons and a couple others, it took both 9V AC and 5V DC in. (The machine would actually mostly run fine off just a 5V DC input, or even off a 4.5V battery most of the time if you didnt need any of the things that required AC...)
noonespecial 10y ago
Your power company: The first NTP server.
- Animats 10y ago
  Yes. "Electrically checked by your electric utility company to correspond with official Naval Observatory time." - General Electric clock ad, 1948.[1] [1] https://books.google.com/books?id=W0oEAAAAMBAJ&lpg=PA9&ots=4mKEd8Sc74&dq=%22observatory%20time%22%20clock%20GE&pg=PA9#v=onepage&q=%22observatory%20time%22%20clock%20GE&f=false https://books.google.com/books?id=W0oEAAAAMBAJ&lpg=PA9&ots=4...

by dlgeek 10y ago

metafunctor 10y ago

I remember reading that the frequency at any given time is usually somewhere between 49.9Hz and 50.1Hz (and similarly around 60Hz I'd imagine). So for a 50Hz clock, it's not very accurate at all. However, I just found that the _average_ frequency over a length of time should be exactly 50Hz, so it is, in fact, good for keeping time. More here: https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html

kalleboo 10y ago

I remember hearing that Analog TV stations were also used for timekeeping since they had a very stable clock for the blanking interval

Animats 10y ago
US analog TV (all three classic networks) used to be synchronized to an atomic clock in NYC. This allowed switching from one source to another without frame tearing. Separate from that, there was a scheme for sending time of day in the vertical retrace interval, to set clocks and VCRs. Few devices received that, although I once encountered a hotel TV with a built-in analog clock that did. (The clock was wrong; although it could set itself automatically, it had to be told manually which channel provided a time signal. That's probably why this feature never caught on. I was bored enough to figure out how to set the thing.) US telephony used to be synchronized to an atomic clock in New Jersey. The TDMA multiplexing was kept in sync so that frames could be transferred synchronously without buffering. If the clock connection was lost, T1 to T3 lines would get slightly out of sync, and occasionally drop or add an 8-bit frame. This is unnoticeable in audio, but an error for modems. But neither of these was a time count. It was just a square wave.
- kalleboo 10y ago
  Interesting to hear about how it worked in the US. I think I heard about the TV frame syncing from an Australian YouTube video. I'm from Europe where every TV and VCR would easily get a clock from TeleText (we also had fancy stuff like PDC to make sure your video recordings of TV shows didn't get cut off)

Angostura 10y ago

Hmmm? In the UK I'm sure the frequency changes with load on the grid and is actually a way of measuring grid load.

anexprogrammer 10y ago
Indeed, and they have to remain within +-1%. They have a realtime graph: http://www2.nationalgrid.com/uk/Industry-information/electricity-transmission-operational-data/ http://www2.nationalgrid.com/uk/Industry-information/electri... Before quartz clocks were around, synchronous mains clocks were common, and accurate.
dfox 10y ago
The idea is that while the instantaneous frequency varies with load, the power grid is actively regulated such that frequency is constant on average over longer periods (typically such that from midnight to midnight there are 24 * 3600 * 50 or 60 cycles).

vidarh 10y ago

As an example of reliance on this, the C64 had a (rarely used) realtime clock in the 6256 CIA's (IO chips) that relied on the AC 50/60Hz signal to keep the time of day. It also used it to control the cassette motor. For those reasons and a couple others, it took both 9V AC and 5V DC in. (The machine would actually mostly run fine off just a 5V DC input, or even off a 4.5V battery most of the time if you didnt need any of the things that required AC...)

noonespecial 10y ago

Your power company: The first NTP server.

Animats 10y ago
Yes. "Electrically checked by your electric utility company to correspond with official Naval Observatory time." - General Electric clock ad, 1948.[1] [1] https://books.google.com/books?id=W0oEAAAAMBAJ&lpg=PA9&ots=4mKEd8Sc74&dq=%22observatory%20time%22%20clock%20GE&pg=PA9#v=onepage&q=%22observatory%20time%22%20clock%20GE&f=false https://books.google.com/books?id=W0oEAAAAMBAJ&lpg=PA9&ots=4...