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Does 59.94 Hz = 60 Hz?  

post #1 of 15
Thread Starter 
I've been doing a bit of reading to understand some of the technical details of video monitors. Here's a question that's been bugging me:

In general, when people refer to 60 Hz, is this just shorthand for 59.94 Hz, or do they really mean a rate of 60.00 Hz that is distinct from 59.94 Hz?
post #2 of 15
Usually (but not completely always) it means 59.94.

- Tom
post #3 of 15
I sure hope so.... ;)
post #4 of 15
My Powerstrip setting have always given me 59.94. So I always assumed this was either 'good enough' or was short hand for 60 Hz.

This is odd, though, since our AC electricity system is based on EXACTLY 60Hz. And the refresh rate was done to be the same as this. So why it is 59.94 is beyond me.

post #5 of 15
So why it is 59.94 is beyond me.
The Line Frequency (f line) and Field Frequencies (f field)
for NTSC were defined in the original monochrome systems
with respect to power line frequency. Historically, 525
lines were displayed at a 30 Hz rate (1/2 AC line frequency)
or horizontal line frequency (f line) of 15.75 kHz.

Each TV frequency channel is mandated as 6MHz wide to conserve
the spectrum. The monochrome picture carrier is offset
1.25MHz. The audio carrier is separated from the monochrome
carrier by 4.5MHz (5.75MHz offset). The introduction of
color required a monochrome compatible system while
preventing interference between audio and video carriers
and maintaining a 6MHz bandwidth. To accomplish this and
maintain 4.5Mhz audio, and 525 lines, the line frequency
was changed to an odd multiple of half line rate to ensure:

1) a color burst phase reversal between alternate lines
2) the beat frequency between color subcarrier and average
audio level is approximately 920kHz.

To accomplish this the following definitions were adopted.

a.Line frequency is defined as:

f line = 4.5MHz/286 = 15.73426kHz

b.Field frequency is defined as:

f line/(525/2)= 59.94Hz.

c.Color burst frequency (fsc) is defined as:

fsc = (455/2)*(4.5MHz/286) = 3.579545MHz

post #6 of 15
Damn! I never cease to be amazed at the depth of knowledge available on this forum! It is a beautiful thing.

Ron, not only technically perfect, but also very well explained.

Now, if anyone has any good ideas on the Grand Unification Theory, please post here.....
post #7 of 15
Man, I was just about to post all that and Ron beat me to it.. dang!

hehe, yah right! :)
post #8 of 15
Maybee this is common knowledge, but FYI, the power grid is rarely running at exactly 60.0 Hz. It sways depending on the generation/load balance. They attempt to keep the frequency at 60 Hz, but there is usually some variation. Unfortunately I don't remember what the maximum variation typically is.
post #9 of 15
OK, now we are getting way off topic, but what the heck. I thought that 60Hz was critical and was always maintained no matter what. Voltage varies quite a bit, but frequency cannot. All the generators must be carefully sync'd to this frequency. Any deviation causes wave interference and is damaging to the grid and the generators themselves (creates feedback, in essence).

post #10 of 15
These days, I couldn't imagine that the refresh rate is still actually driven by the incoming power line frequency, right? In which case, it wouldn't make much difference anymore whether it varied. I would have a hard time believing in this digital age that the clock for the display refresh was being derived from the power line.

That would be basically PCM clock judder writ large, wouldn't it?

Or, for that matter, was it ever actually *driven* by the power line frequency. Was the correspondence just to prevent interference with the retrace by the oldy moldy circuitry they had back then?
post #11 of 15
I'm sorry for throwing the thread off topic. Just a couple points of clarification:

I don't believe that the power line frequency has anything to do with any timings in a computer, except for maybe driving fans and the power supply. I only brought that up because of recent hum bar problems I've been having due to differences in NTSC and power grid frequencies.

I used to work at ComEd's bulk power operations facility in Chicago. All generators run at the same frequency because they are magnetically coupled through the power grid. The frequency does vary. bpearse, you are right that the generators can be damaged if they frequency varies too much, but it does vary. In fact, if you have a clock that runs of the power grid it runs fast or slow depending on the current frequency. Of course you usually don't notice this because the variation is small. Occasionally the power companies will have to over-generate or under-generate to correct for time errors, really!
post #12 of 15
Power frequency is not accurately maintained, BUT the number of cycles per day is always the same. The reason for this is that otherwise those old electric clocks would not work (they would be fast or slow depending on the line frequency since they all used a power frequency syncronous motor). By keeping track of the number of cycles, and adjusting the line frequency at the end of the day to make sure they have the correct number of cycles, all those clocks keep good time. This is much cheaper that trying to keep the frequency exactly constant through varing loads.
post #13 of 15
Or, for that matter, was it ever actually *driven*
by the power line frequency. Was the correspondence just
to prevent interference with the retrace by the oldy moldy
circuitry they had back then?
No, a television of any vintage locks to the incoming
signal. Matching the CRT refresh rate to the power grid
is done to avoid annoying visual beat notes (flicker)
between the CRT and the room lighting (with fluorescent
lights being the worst).

This is why 25 fps PAL looks so horrible when played in a
country with a 60 Hz power grid. I've had european folks
at work tell me they never noticed the PAL flicker until
they saw it here in the USA.

post #14 of 15
But it's not the TV that's driving the gun, it's the source device pushing the data at it, right? Does the DVD player lock onto the power line and sync the output it sends to the set or to a projector so that the target display device syncs to the power line output? And what would be the point in any HT, since there ain't usually any lights on anyway.
post #15 of 15
In the computer domain, 60 Hz usually means 60 Hz. Of course there is overlap between the video domain and the computer domain.

Video equipment always syncs to the source, not the powerline frequency.

The reason that 60 Hz was chosen was _not_ to look nice under 60 Hz lights. (you should convince yourself of that by noting that your computer monitor looks just fine under fluorescent lights even when the refresh rate is set to something besides 60 Hz)

Early TVs had poor isolation in the power supplies and other components. The powerline frequency inadvertently got into signals throughout the TV. By choosing a refresh rate the same or almost the same as the powerline frequency, the hum bars you'd see on the screen would be stationary or nearly stationary, and thus much less objectionable than moving hum bars.

It's a non-issue for today's electronics.
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