Frame rate and MHz

[Backwoods]

I've seen NTSC being 60MHz and PAL being 50MHz if I'm not mistaken and I was curious as to what the MHz pertains to.

Let's say something is listed as 40MHz 27.5 fps at 1280x1024, full resolution. What does the MHz entail?

[Wilbert]

There is no "40MHz 27.5 fps". 40 MHz just means 40 fields per second are shown.

[stephanV]

shouldnt be that 40 Hz instead of 40 MegaHz?

[ultimatebilly]

If I remember correctly, the number of fields that are shown per second have been chosen in accordance to the electricity-supply, which is 60hz in the usa and 50hz in europe.
It is hz, because mhz were 50000 fields per second, guaranteed flicker-free ;)
I don't know, but maybe they chose that value to make use of the electric properties for timing or something like that...
Therefor you have 50 fields (half-frames) per second, which is 25 full frames in europe, and 59,94 fields per second and 29,97 fps in the us.
If I remember correctly, the deviance in NTSC (american system - abbreviation stands for Never Twice the Same Color ;)) is due to the chroma-channel somehow, but I don't know why...
I once knew all that stuff, but since my job is so boring and I haven't used my brain for two years now and drank too much booze, its almost all gone... :(
Just for those who ask themselves why the picture is split in fields, this is due to the fact that the motion in the TV would be fluent even with 25 fps, but it would flicker...
For a picture not to flicker, you need at least 48 "refreshs" per second.
The same is done in the cinema, there 24 different slides are shown per second, but a "umlaufblende" (sorry, no clue what this is called in english) is used to show each picture two times in 1/24 seconds...

[Wilbert]

shouldnt be that 40 Hz instead of 40 MegaHz?
Of course :D

[Backwoods]

http://optics.sumix.com/products/cameras/smx-150c/specification.html

It says MHZ on there, guess it's a mistake.

So there is a chance of flicker with this camera then?

[stephanV]

thats a sample rate i think. something different as a frame or field rate.

[ultimatebilly]

Yes, that seems to be the sample rate of the signal, not the rate with which fields or frames are refreshed.
I misunderstood that because of 50 and 60 Mhz, because both values are used with hz in TV standards...
I don't know, but maybe this helps to clarify things:
http://forum.doom9.org/showthread.php?threadid=55727
I didn't really have a clue what I was talking about in this thread, but you might want to get the ITU-documents if you want to know more about that.
There are other interesting links in the thread.

[unmei]

Yes, that seems to be the sample rate of the signal, not the rate with which fields or frames are refreshed.

i'd go with that.
1280x1024x27.5 == ~36 mio pixel (samples?) / second
the difference to 40MHz is probably due to the chip sampling a bit more than what is actually output afterwards, maybe to get info for focus/brightness etc adjustments ..or they just "rounded" the value.

[theReal]

If I remember correctly, the deviance in NTSC (american system - abbreviation stands for Never Twice the Same Color ) is due to the chroma-channel somehow, but I don't know why...

NTSC, in contrast to PAL ("Phase Alternation Line") doesn't submit phase alternated color information. PAL submits chroma values positive in one field, negative in the other. That way electrical influences can be made up for and the color stays what it should be. It's the same principle that makes professional XLR audio not pick up sounds from nearby electrical fields: It works with 2 phase alternated carrier waves.
The way it works is pretty easy: Let's say we want to send a value of 1V through a cable and we do this with only one carrier wave. Now there's an electrical field around that changes the value to 1.3V in the line and so we get the wrong information at the end. If we send 1V and at the same time -1V, then the electrical field also adds 0.3V to both voltages which makes them 1.3V and -0.7V. Now we can add 1.3V to -(-0.7V), divide the result by 2 and we're back at 1V!

btw. mostly the terms "PAL" and "NTSC" are also used to distinguish between the 60Hz and 50Hz frequency in television standards, but that's not right, PAL is only an optimized way to submit chroma information (I think somewhere in South America they have 60Hz PAL)