Refresh rate, progressive scan & electrical current - AVS Forum
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post #1 of 12 Old 07-25-2012, 03:27 AM - Thread Starter
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1) When you're viewing progressive footage on a TV that's locked to a 60hz refresh rate, you're seeing the frames refresh multiple times correct?

2) Figuring out 30fps at a 60hz refresh rate is easy - it's just doubling each frame. Does that mean we see 24p frames 2.5 times each though?

3) Why do some TVs have both a 60hz and 59.94hz option?

4) I'm hearing that the 60hz rate on some TVs is actually 59.94hz, and that they round it to 60 only in text to not confuse you. I believe we backed the refresh rate down slightly due to NTSC color and this has got me confused. In the oldschool B&W NTSC days we had 30fps and 60hz - perfectly divided. If there's now a refresh rate of 59.94, wouldn't there be some extremely slight sync issue since 59.94 isn't locked exactly to our 60hz AC electrical current supply?

5) 29.97 frames per second are broadcasted. But is a Refresh Rate 'broadcasted' too, or is it only your TV refreshing the frames itself?

6) They say progressive footage requires double the bandwidth. I thought that only your *TV* refreshes those frames in a video, like from a DVD. Does this mean that in a progressive DVD or bluray, multiple copies of the frames are in the vob files? Or does your TV handle that? Or maybe your DVD player employs a special telecine process and brings it up to the 60hz level?

7) I can understand 'double the bandwidth' in a progressive broadcast - that would mean broadcasting 60 images per second instead of fields, if it does indeed work where it has to broadcast the frames plus its repeated copies. But why not figure out a way to broadcast only single frames and let everyone's TV do the refreshing work? I'm curious as to how DVDs and blurays hande this.

PS: I wonder what it's like seeing *actual* 24 frames per second, like directly from a film projector without the shutter. We're used to seeing it displayed differently on our TVs and monitors.
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post #2 of 12 Old 07-25-2012, 05:45 AM
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But why not figure out a way to broadcast only single frames and let everyone's TV do the refreshing work?

they did, it's called MPEG.
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If there's now a refresh rate of 59.94, wouldn't there be some extremely slight sync issue since 59.94 isn't locked exactly to our 60hz AC electrical current supply?

It was never locked to the electrical supply, that's a myth.
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post #3 of 12 Old 07-25-2012, 06:09 AM - Thread Starter
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But mpeg has only been around for a little under 30 years. I read that they used a refresh rate of 60hz which is the same alternating current that electricity uses so we wouldn't see strobing in our TVs.

I also read that they backed the refresh rate down to 59.94 with the introduction of NTSC color. I don't know if they mean they back down the refresh rate on TVs, or if they backed down the refresh rate on the broadcast... I don't know if a refresh rate is 'broadcasted' itself, if you get my drift.
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post #4 of 12 Old 07-25-2012, 08:01 AM
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But mpeg has only been around for a little under 30 years.

You asked about figuring out a way to do this, it was figured out, it's called MPEG.
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I read that they used a refresh rate of 60hz which is the same alternating current that electricity uses so we wouldn't see strobing in our TVs.

It was to deal with poor power supply regulation in early TV's. This was resolved before colour was invented. The oscillators were never locked to the power line, they lock to the incoming sync.
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I also read that they backed the refresh rate down to 59.94 with the introduction of NTSC color.

To accomidate the colour subcarrier, yes.
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I don't know if they mean they back down the refresh rate on TVs, or if they backed down the refresh rate on the broadcast

Not sure what you mean here, but the TV syncs itself to the broadcast signal...that's what Horizontal and Vertical sync are for.
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I don't know if a refresh rate is 'broadcasted' itself, if you get my drift.

Yes, it is. It's part of the Composite Video, Blanking and Sync.
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post #5 of 12 Old 07-25-2012, 05:38 PM
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Originally Posted by thared33 View Post

1) When you're viewing progressive footage on a TV that's locked to a 60hz refresh rate, you're seeing the frames refresh multiple times correct?

Except if the footage is 60fps progressive, like video games and some camcorders.
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2) Figuring out 30fps at a 60hz refresh rate is easy - it's just doubling each frame. Does that mean we see 24p frames 2.5 times each though?

If you think about it, that would be physically impossible: it would mean the refresh rate is double its own precision. The normal way of showing 24p at 60Hz is 3:2 pulldown. Show a frame 3 times, the next frame 2 times, the next 3 times, etc.
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6) They say progressive footage requires double the bandwidth.

...of the equivalent interlaced rate, but only if we're talking about uncompressed content. The EBU's research indicates that 50p only needs the same bandwidth as 50i when both are compressed with the same software AVC encoder (they're European so they didn't test 60, but there's every reason to assume the results carry).
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post #6 of 12 Old 07-26-2012, 05:00 AM - Thread Starter
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So when a frame is sent to your TV, the sync signal that is also sent, tells your TV to refresh the frames however many times. Right? That must mean the 'SYNC' signal that they sent over the air was backed down to 59.94hz. Please correct me if I'm wrong.

Regarding the comment about mpeg letting your TV do the refreshing work, are you saying that there's no sync signal sent and that that's only for broadcast? I wonder how it worked for VHS.

And regarding the pulldown/telecine, my Canon DSLR shoots at native 24p (23.976 to be exact). I haven't hooked it to my TV yet but I don't think the camera introduces the pulldown. Maybe it does that on my TV or only on the video outs on the camera? It looks fine on my PC monitor too, unless my software introduces a pulldown or something. Not sure how this works.

By the way, if your TV is locked to the sync... how come there are options for 60hz, 59.94 etc?

Thanks for the help.
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post #7 of 12 Old 07-26-2012, 06:31 AM
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So when a frame is sent to your TV, the sync signal that is also sent, tells your TV to refresh the frames however many times. Right?

Sync is a part of the "composite video, blanking and sync" that is part of an NTSC broadcast. Vsync occurs every field, H sync occurs every line.
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That must mean the 'SYNC' signal that they sent over the air was backed down to 59.94hz.

Yes
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Regarding the comment about mpeg letting your TV do the refreshing work, are you saying that there's no sync signal sent and that that's only for broadcast? I wonder how it worked for VHS.

MPEG is not recorded onto VHS, you're mixing apples and oranges.
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By the way, if your TV is locked to the sync... how come there are options for 60hz, 59.94 etc?

If it's a TV, you have no choice.
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post #8 of 12 Old 07-26-2012, 09:30 PM
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Originally Posted by thared33 View Post

And regarding the pulldown/telecine, my Canon DSLR shoots at native 24p (23.976 to be exact). I haven't hooked it to my TV yet but I don't think the camera introduces the pulldown. Maybe it does that on my TV or only on the video outs on the camera?

If the camera sends 60p to your TV then the camera is doing the pulldown as part of the video output. If it send 24p to a 60Hz TV then the TV is doing it.

No different than what happens with a Blu-ray player. My TV only does 60Hz, so whether I set my PS3 to output 1080p24 or 1080p60 I'm getting frames repeated in a given pattern, either by the player or the display system.
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It looks fine on my PC monitor too, unless my software introduces a pulldown or something. Not sure how this works.

Computers can play any arbitrary framerate; the renderer duplicates frames as necessary to match what the display driver is pumping out to the monitor.
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post #9 of 12 Old 07-26-2012, 10:01 PM
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Back in the early black and white days the vertical scan frequency was indeed 60 Hz exactly to avoid having power line hum or noise bars moving in the picture. Early TV equipment could be locked to power line frequency. When NTSC color was invented the vertical and horizontal scan frequency was changed slightly because the frequencies are derived from and locked to the color subcarrier frequency. The color subcarrier frequency was picked to reduce the visibility of intermodulation with the audio transmitter intercarrier frequency which was kept at 4.5 MHz above the visual transmitter frequency. NTSC color subcarrier frequency is nominally 3.579545 MHz. The Horizontal scanning frequency is subcarrier frequency divided by 227.5. Thus there are 227.5 cycles of subcarrier per H line. That means that subcarrier alternates 180 degrees in phase from line to line which greatly reduces the visibility of color subcarrier on black and white receivers. Vertical scanning frequency is two times horizontal scanning frequency divided by 525. This comes out to 59.940052... As analog TV goes away there is no reason to keep the 59.94 vertical frequency but since much HDTV broadcast equipment is installed in what was and NTSC plant and video must often be converted back and forth between formats it makes sense to stay at 59.94 for the moment.

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post #10 of 12 Old 07-27-2012, 02:59 AM
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Originally Posted by Rory Boyce View Post

Back in the early black and white days the vertical scan frequency was indeed 60 Hz exactly to avoid having power line hum or noise bars moving in the picture. Early TV equipment could be locked to power line frequency. When NTSC color was invented the vertical and horizontal scan frequency was changed slightly because the frequencies are derived from and locked to the color subcarrier frequency. The color subcarrier frequency was picked to reduce the visibility of intermodulation with the audio transmitter intercarrier frequency which was kept at 4.5 MHz above the visual transmitter frequency. NTSC color subcarrier frequency is nominally 3.579545 MHz. The Horizontal scanning frequency is subcarrier frequency divided by 227.5. Thus there are 227.5 cycles of subcarrier per H line. That means that subcarrier alternates 180 degrees in phase from line to line which greatly reduces the visibility of color subcarrier on black and white receivers. Vertical scanning frequency is two times horizontal scanning frequency divided by 525. This comes out to 59.940052... As analog TV goes away there is no reason to keep the 59.94 vertical frequency but since much HDTV broadcast equipment is installed in what was and NTSC plant and video must often be converted back and forth between formats it makes sense to stay at 59.94 for the moment.
Great post, but a couple of minor math errors. The exact frequencies are:

field rate = (60 * 1000) / 1001 = 59.9400599400599400...

line frequency = ((30 * 1000) / 1001) * 525 = 15734.265734265734265734... Hz

subcarrier frequency = ((30 * 1000) / 1001) * 525 * 227.5 = 3579545.45454545... Hz

Also, the subcarrier frequency being a half integer multiple of the line frequency causes the luma and chroma energy to be interleaved in the frequency domain (which makes comb filters possible).

Ron

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post #11 of 12 Old 07-27-2012, 04:23 AM
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That means that subcarrier alternates 180 degrees in phase from line to line which greatly reduces the visibility of color subcarrier on black and white receivers.

Phase Alternates per Line?
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post #12 of 12 Old 07-27-2012, 10:03 PM
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dr1394

I suspect that the people that invented NTSC did not think of the possibilty of comb filters because the required line delays were not possible then. Have you ever seen what NTSC color video looks like on a black and white monitor without the extra half subcarrier cycle per line?

SAM64

The NTSC subcarrier phase alternates 180 degrees or one half cycle every other line. In other words the physical location on the screen of a positive peak of a subcarrier cycle will be a negative peak on the next line down which is actually two lines down on an interlaced display. (On the screen the next line down is the other field) The subcarrier phase also alternates from frame to frame because 227.5 times 525 lines is 119437.5 subcarrier cycles per frame. All of this assumes that the picture content color hue is the same from line to line and frame to frame which is true most of the time.

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