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  a newsletter by |_| j. b. crawford                       home subscribe rss

>>> 2021-04-12 seeing time

Something I have long been interested in is time. Not some wacky philosophical or physical detail of time, but rather the simple logistics of the measurement and dissemination of time. How do we know what time it is? I mean, how do we really know?

There are two basic problems in my proprietary model of time logistics: first is the measurement of time. This is a complicated field because "time," when examined closely, means different things to different people. These competing needs for timekeeping often conflict in basic ways, which results in a number of different precise definitions of time that vary from each other. The simplest of these examples would be to note the competing needs of astronomy and kinematics: astronomers care about definitions of time that are directly related to the orientation of Earth compared to other objects, while kinematic measurements care about time that advances at a fixed rate, allowing for comparison of intervals.

These two needs directly conflict. And on top of this, most practical astronomy also requires working with intervals, which has the inevitable result that most astronomical software must convert between multiple definitions of time, e.g. sidereal and monotonic. Think about that next time you are irked by time zones.

The second problem is the dissemination of time. Keeping an extremely accurate measurement of time in one place (historically generally by use of astronomical means like a transit telescope) is only so useful. Time is far more valuable when multiple people in multiple places can agree. This can obviously be achieved by setting one clock to the correct time and then moving it, perhaps using it to set other clocks. The problem is that the accuracy of clocks is actually fairly poor [1], and so without regular synchronization they will drift away from each other.

Today, I am going to talk about just a small portion of that problem: time dissemination within a large building or campus. There is, of course, so much more to this topic that I plan to discuss in the future, but we need to make a beachhead, and this is one that is currently on my mind [2].

There are three spaces where the problem of campus-scale time dissemination is clear: schools, hospitals, and airports. Schools often operate on fairly precise schedules (the start and end of periods), and so any significant disagreement of clocks could lead to many classes starting late. Hospitals rely on fairly accurate time in keeping medical records, and disagreement of clocks in different rooms could create inconsistencies in patient charts. And in airports, well, frankly it is astounding how many US airports lack a sufficient number of clearly visible, synchronized clocks, but at least some have figured out that people on the edge of making a flight care about consistent clock readings [3].

It is no surprise, then, that these types of buildings and campuses are three major applications of central clock systems.

In a central, master, or primary clock system, there is one clock which authoritatively establishes the correct time. Elsewhere, generally throughout a building, are devices variously referred to as slave clocks, synchronized clocks, secondary clocks, or repeater clocks. I will use the term secondary clock just to be consistent.

A secondary clock should always indicate the exact same time as the central clock. The methods of achieving this provide a sort of cross section of electrical communications technologies, and at various eras have been typical of the methods used in other communications systems as well. Let's take a look.

The earliest central clocks to achieve widespread use were manufactured by a variety of companies (many around today, such as Simplex and GE) and varied in details, but there are enough common ideas between them that it is possible to talk about them generally. Just know that any given system likely varies a bit in the details from what I'm about to describe.

Introduced at the turn of the 20th century, the typical pulse-synchronized clock system was based on a primary clock, which was a fairly large case clock using a pendulum as this was the most accurate movement available at the time. The primary clock was specially equipped so that, at the top of each minute, a switch momentarily closed. Paired with a transformer, this allowed for the production of a control voltage pulse, which was typically 24 volts either DC or AC.

In the simplest systems, the secondary clocks then consisted of a clock with a much simplified movement. Each pulse actuated a solenoid, which advanced the movement by one minute exactly, usually using an escapement mechanism to ensure accurate positioning on each minute.

This system met the basic need: left running, the secondary clocks would advance at the same rate as the master clock and thus could remain perfectly in sync. However, only synchronization was ensured, not accuracy. This meant that installation of a new system and then every power outage (or DST adjustment) required a careful process of correctly setting each clock before the next minute pulse. The system provided synchronization, but not automatic setting of clocks.

The next advancement made on this system was the hour pulse. A different pulse, of a different polarity in DC systems or on AC systems often using a separate wire, was sent at the top of each hour. In the secondary clocks, this pulse energized a solenoid which "pulled" the minute hand directly to the 00 position. Thus, any accumulated minute error should be corrected at the top of the hour. The clocks still needed to be manually set to the correct hour, but the minutes could usually take care of themselves. This was an especially important innovation, because it could "cover up" the most common failure mode of secondary clocks, which was a gummed up mechanism that caused some minute pulses to fail to advance the minute hand.

Some of these systems offered semi-automatic DST handling by either stopping pulses for one hour or pulsing at double rate for one hour, as appropriate. This mechanism was of course somewhat error prone.

The next obvious innovation was a similar mechanism to correct the hour hand, and indeed later generations of these systems added a 12-hour pulse which used a similar mechanism to the hour pulse to reset the hour hand to the 12 position twice each day. This, in theory, allowed any error in a clock to be completely corrected at midnight and noon [4].

Of course, in practice, the hour and 12-hour solenoids could only pull a hand (or really gear) so far, and so both mechanisms were usually only able to correct an error within a certain range. This kept slightly broken clocks on track but allowed severely de-synchronized clocks to stay that way, often behaving erratically at the top of the hour and at noon and midnight as the correction pulses froze up the mechanisms.

One of the problems with this mechanism is that the delivery of minute, hour, and 12-hour pulses required at least three wires generally (minute and hour can use polarity reversal), and potentially four (in the case of an AC system, minute, hour, 12-hour, and neutral). These multiple wires increased the installation cost of new systems and made it difficult to upgrade old two-wire systems to perform corrections.

A further innovation addressed this problem by using a simple form of frequency modulation. Such "frequency-synchronized" clocks had a primary clock which emitted a continuous tone of a fixed frequency which was used to drive the clock mechanism to advance the minute hand. For hour and 12-hour corrections, the tone was varied. The secondary clocks detected the different frequency and triggered correction solenoids.

Of course, this basically required electronics in the primary and secondary clocks. In earlier versions these were tube-based, and that came with its own set of maintenance challenges. However, installation was cheaper and it provided an upgrade path.

These systems, pulse-synchronized and frequency-synchronized, were widely installed in institutional buildings from around 1900 to 1980. Simplex systems are especially common in schools, and many middle school legends of haunted clocks can be attributed to Simplex secondary clocks with damaged mechanisms that ran forwards and backwards at odd speeds at each correction pulse. Many of these systems remain in service today, usually upgraded with a solid-state primary clock. Reliability is generally very good if the secondary clocks are well-maintained, but given the facilities budgets of school districts they are unfortunately often in poor condition and cause a good deal of headache.

As a further enhancement, a lot of secondary clocks gained a second hand. The second hand was usually driven by an independent and fairly conventional clock mechanism, and could either be completely free-running (e.g. had no particular relation to the minute hand, which was acceptable since the second hand is typically used only for interval measurements) or corrected by the minute pulse. In frequency-synchronized systems, the second-hand could be driven by the same mechanism running at the operating frequency, which was a simple design that produced an accurate second hand at the cost of the second hand sometimes having odd behavior during correction pulses.

The use of 24 volt control circuits was very common throughout the 20th century and is still widespread today. For example, thermostats and doorbells typically operate at 24 vac. A 24 vac control application that is not usually seen today are low-voltage light switches which actuate a central relay rack to turn building lighting on and off. These were somewhat popular around the mid-century because the 24vac control wiring could be small gauge and thus very inexpensive, but are rare today outside of commercial systems (which are more often digital anyway).

Another interesting but less common pre-digital central clock technology relied on higher frequency, low voltage signals superimposed on the building electrical wiring, either on the hot or neutral. Tube-based circuits could detect these tones and activate correction solenoids or motors. The advantage not running dedicated clock wiring was appealing, but these are not widely seen... perhaps because of the more complex installation and code implications of connecting the primary clock to the building mains.

Finally, something which is not quite a central clock system but has some of the flavor is the AC-synchronized clock. These clocks, which were very common in the mid-century, use a synchronous AC motor instead of an escapement. They rely on the consistent 60Hz or 50Hz of the electrical supply to keep time. These are no longer particularly common, probably because the decreasing cost of quartz crystal oscillators made it cheaper to keep the whole clock mechanism DC powered and electronically controlled. They can be somewhat frustrating today because they often date to an era when the US was not yet universally on 60Hz, and so like the present situation in Japan, they may not run correctly if they were originally made for a 50Hz market. Still, they're desirable in my mind because many flip clocks were made this way, and flip clocks are beautiful.

Semiconductors offered great opportunities for central clock systems. While systems conveying digital signals over wires did exist, they quickly gave way to wireless systems. These wireless systems usually use some sort of fairly simple digital modulation which sends a complete timestamp over some time period. The period can be relatively long since these more modern secondary clocks were universally equipped with a local oscillator that drove the clock, so they could be left to their own devices for as much as a day at a time before a correction was applied. In practice, a complete timestamp every minute is common, perhaps both because it is a nice round period and because it matches WWVB (a nationwide time correction radio service which I am considering out of scope for these purposes, and which is not often used for commercial clock synchronization because indoor reception is inconsistent).

A typical example would be the Primex system, in which a controller transmits a synchronization signal at around 72 MHz and 1 watt of power. The signal contains a BPSK encoded timestamp. When Primex clocks are turned on, they search for a transmitter and correct themselves as soon as they find one---and then at intervals (such as once a day) from then on [5].

More in line with the 21st century, central clock systems can operate over IP. In the simplest case, a secondary clock can just operate as an NTP client to apply corrections periodically. These systems do certainly exist, but seem to be relatively unpopular. I suspect the major problem is the need to run Ethernet or deal with WiFi and the high energy cost and complexity of a network stack and NTP client.

Today, secondary clocks are generally available with both digital and analog displays. This can be amusing. Digital displays manufactured as retrofit for pulse-synchronized systems must essentially simulate a mechanical clock mechanism in order to observe the correct time. Analog displays manufactured for digital systems use position switches or specialized escapements to establish a known position for the hands (homing) and then use a stepper motor or encoder and servo to advance them to show the time, thus simulating a mechanical clock mechanism in their own way.

In the latter half of the 20th century and continuing today, central clock systems are often integrated with PA or digital signage systems. Schools built today, for example, are likely to have secondary clocks which are just a feature of the PA system and may just be LCD displays with an embedded computer. The PA system and a tone generator or audio playback by computer often substitute for bells, as well, which had previously usually been activated by the central clock---sometimes using the same 24vac wiring as the clocks.

Going forward, there are many promising technologies for time dissemination within structures. LoRa, for example, seems to have obvious applications for centralized clocks. However, the development of new central clock systems seems fairly slow. It's likely that the ubiquity of cellphones has reduced the demand for accurate wall clocks, and in general widespread computers make the spread of accurate time a lot less impressive than it once was... even as the mechanisms used by computers for this purpose are quite a bit more complicated.

Time synchronization within milliseconds is now something we basically take for granted, and in a future post I will talk a bit about how that is conventionally achieved today in both commercial IT environments and in more specialized scientific and engineering applications. The keyword is PNT, or Position, Navigation, Time, as multilateration-based systems such as GPS rely on a fundamental relationship of correct location and correct time, and thus can be used to determine either given the other... or to determine both using an awkward bootstrapping process which is thankfully both automated and fast in modern GPS receivers (although only because they cheat).

[1] This seems like a somewhat bold statement to make so generally, considering the low cost of fairly precise quartz oscillators today, but consider this: as clocks have become more accurate, so too have the measurements made with them. It seems like a safe assumption that we will never reach a point where the accuracy of clocks is no longer a problem, because the precision of other measurements will continue to increase, maintaining the clock as a meaningful source of error.

[2] Because, due to a winding path from an idea I had months ago, I recently bought some IP managed, NTP synchronized LED wall clocks off of eBay. They are unreasonably large for my living space and I love them.

[3] This is all the more true in train stations, which generally operate on tighter and more exact schedules, and train stations are indeed another major application of central clock systems. The thing is that I live in the Western United States, where we have read about passenger trains in books but seldom seen them. Certainly we have not known them to keep to a timetable, Amtrak.

[4] This was not exactly true in practice, for example, Simplex systems performed the hour and 12-hour pulses a bit early because it simplified the design of the secondary clock mechanism. A clock behaving erratically right around the 58th minute of the hour is characteristic of pulse-synchronized Simplex systems applying hour and 12-hour corrections.

[5] Because the relatively low frequency of the 72MHz commercial band penetrates building materials well, it is often used for paging systems in hospitals. The FCC essentially considers Primex clocks to be a paging system, and indeed newer iterations allow the controller to send out textual alerts that clocks can display.