Frame Rate General Discussion

[Ron Evans]

Originally Posted by Steve Game

James,

..The only side issue as mentioned by Ron Evans is that 50fps cameras will normally use shutter speeds at least twice that of 25fps ones,.. ..

I didn't say that. I was questioning why one HAD to do that. With film that is the case as the shutter speed at 1/25 or 1/24 would potentially give a lot of motion blur since the judder at those low frame rates is bad enough. I always try to shoot at the same shutter speed as frame rate since I never shoot at the lower frame rates. Used to be 60i now mostly 60P for my cameras that can do that most of my shoots are in the theatre so shutter speed is 1/60 for a 60i or 60P camera. Only time the shutter speed is more is if I am outside with the little Sony's in auto where they will use shutter speed to compensate for the fact they do not have an ND. For my NX5U I use the ND's

Ron Evans

[Steve Game]

Ron,
The point that I was trying to make was that if rendering 50p source footage down to 25p for distribution, the source shutter speed will likely have a maximum duration of 20ms (i.e. a 360 degree shutter) whereas it will be 180 degrees when part of a 40ms frame period unless the NLE allows some motion blur that would emulate a slower shutter.
Strangely enough, rendering down to 1080i/25 would either produce a psf cadence or a halved shutter angle depending on whether the NLE recognised the temporal difference between the 50p frames or just grabbed every other one.

[Ron Evans]

Steve, my point is that a 1/50sec shutter speed is the same whether the frame rate is 25 or 50. And will have the corresponding blur of a 1/50 exposure. I realize the mindset is different coming from a mechanical film camera where the shutter is keyed to the film pulldown. Not so for an electronic system. It just represents the time allowed for the sensor to charge before readout ( starts ). If that is 1/50 it doesn't matter what the frame rate is the motion blur will be for that time period identical for 25p or 50p. Taking every other frame of the 50p will result in the same judder as if shot at 25p with 1/50 sec shutter speed. The motion blur comes from the motion while the shutter is open and the judder from the motion differences occurring between exposures. There would be a difference if the shutter speed was 1/25 which would result in more motion blur of course. So with the same shutter speed above 1/50sec , 25p output from a 50p source will mainly introduce the judder of the slow frame rate.

Ron Evans

[Steve Game]

Ron,
I think that we are both coming from the same direction. I have recently bought a Sony CX730E which takes a good 50p. The native footage plays back with smooth cadence, i.e. limited judder and little motion blur as the image samples are frequent enough for each to cover little movement in the scene. When I put this footage on the same Vegas timeline as some 25p material with the intention of creating a 25p output, I apply some motion blur with a gaussian profile. This means that the original frames are combined with adjacent ones to create new blurred frames that effectively show as smooth movement that would otherwise be shown as judder where the shutter time was much shorter than the frame repeat rate.

[James Hollingsworth]

Thanks for all the advice. With regard to 50p, do you all shoot at the 108050 / 60p PS Mode or does anyone shoot at 720 50 / 60p FX or FH. My Mac isn't that fast and I can't afford to upgrade at the moment and am getting a bit fed up of all the rendering time required even at 1080/50i FX at 24Mbps. I think 50p at 28Mbps would definitely be too much for my machine to handle and am curious about how 720p in FH at 17Mbps might look? Nothing is for broadcast as far as weddings are concerned so not bothered about that - I think that has to be 50Mbps now anyhow. My own documentary stuff, I might rather shoot at the top spec though. Look forward to hearing your thoughts.

[James Manford]

James

Definitely shoot at 720p 50p/60p

If your mac is struggling with 50i, then record at a lower rate.

720p is still high definition and looks glorious on TV, especially if you deliver DVDs to your clients...

The only reason why I bother with 1080p is if I ever need to slightly zoom into my video, I can go down to 720p without any noticeable quality loss. Other than that I would of shot 720p all the time! luckily so far my PC seems to be able to handle it rendering 1080p 50p

[James Hollingsworth]

Thanks James. So we have established that 720p is good, how about the recording mode FX(24 Mbps) or FH(17 Mbps)?

[James Manford]

Well for starters you've downgraded from 1080p to 720p. That should give your PC a good enough boost in terms of performance.

Secondly, I would see how it goes with filming at 24mbps, because it is a higher quality 720p. Before trying the 17mbps (which again, will give you more of a boost in performance).

Take your camera to your garden, or somewhere with a lot of detail.

Record in 1080p PS, Record in 720p 24mbps and then 17mbps.

See how much detail you see missing in the 720p files.

This will give you an idea ...

The best is 1080p PS, but we all have to compromise ... your not in a position to go spend £1500 replacing your MAC, so 720p will have to do. Which isn't a significant difference if your providing clients with DVD's anyway.

[James Manford]

In all honesty, i've seen 1080p PS look terrible if the lighting conditions are poor ...

720p is High Definition, where as 1080p is FULL HD.

The difference can not be seen by a un-trained eye. Just make sure everything else is perfect, you film in adequate lighting conditions etc and you'll be fine filming in 720p 17mbps.

[James Hollingsworth]

Thanks James. Will do my next project in 720 and 17 and see how we go.

[James Hollingsworth]

Following on from our discussions, I have been experimenting with 50p. It seems that the VG30 doesn't shoot 720P so have gone and shot some 108050p footage around the house. But having trouble converting the footage to Pros Res LT in FCP using log and transfer. Anyone help me on how they get their 50p footage into FCP 7?

[Craig Marshall]

Although I'm in 'PAL' land, I'm considering switching over to shoot 50i @ 50th or 100th shutter, rather than 50P. The reason for this is to reduce 'temporal' aliasing, ie: the slight jerkiness between 50p/60p frames. Although the absolute resolution of each 50i/60i frame may be slightly less than a 50p/60p frame, you may achieve a smoother pan and here's why: The best way to explain this technically, is to quote from an engineering friend of mine:

"This (ie: 50p/60p) is what gives it that tiny bit of jerkiness. It is tied up with shutter speeds and is apparent in all moderately priced electronic cameras. The higher end and thus more expensive ARRI Alexa and RED cameras apply an electronic shutter filter to the image. This has the effect of dissolving, if you will, between the original and the filtered image, resulting in a more smoother transition between the on/off shutter or exposure. This is done I believe around the 12Hz region.

One of the advantages of 50i/60i material is the fact that any movement of the frame through say panning is shared across 2 fields albeit half vertical resolution per field but the movement is smoother so I think you will get far better results using 50i/60i as your record format and less encoding artefacts during BD production. Temporal aliasing is not anywhere as noticeable using 50i/60i as well which is why broadcasters prefer it. Full resolution Progressive frames or Progressive Sequential Field recordings both exhibit the problem but interlacing results in a halving of the movement within a frame over 2 fields.

"...high frequency details (trees and leaves particularly) really pushes the ability of the sensor to charge and discharge at 50p/60p whereas halving this high frequency with 50i/60i recordings, minimises the aliasing requirements. 50P/60P recordings are wonderful, don't get me wrong but they really shine as static images rather than pans or where there is fast action contained within the frame. This is why the higher end cameras use shutter filtering circuits."

[Chris Harding]

Just for interest too I shoot everything at 50i ...I know if I need to grab stills off the video the result will not be as good (then again I carry my Nikon outfit with me anyway) It's a matter of choice but I just find that 50i is so forgiving and with a lot of run 'n gun stuff including weddings that gives me the best overall result and doing non-repeatable stuff like weddings you can't have any screwups !

Just for interest I did shoot a wedding in 25P on my Panasonic HMC82's last year and for some totally unknown reason I found that , especially the cheek area of people's faces, tended to pixellate quite badly ..I tried a few static shots at home and also found the image centre showed some alarming pixellation ...I have no idea why that occurred but it did prompt me to go back to 50i and stay there.

To be honest I have not used either of my EA-50's in progressive mode (25P or 50P) and stick to 50i ...sort of the principle "if it works don't mess with it!"

Chris

[Ron Evans]

Originally Posted by Craig Marshall

.. Temporal aliasing is not anywhere as noticeable using 50i/60i as well which is why broadcasters prefer it..."

I think the only reason we have interlace is cost. Originally it cost too much for the technology of the time to make the hardware and it cost too much for the bandwidth to transmit the signals. Standards were then written around this fact and stay with us today. So camcorders were made interlace . The issue of cost is still with us but technology doesn't stand still. Processors get faster, smaller and lower cost so that complex encoders can be made. 16x9 TV are native 50/60P so that interlaced input has to be de interlaced. It is much easier to input a native frame rate and gives the consumer better quality so once the processors were fast enough camcorders became progressive and the consumer can connect and see wonderful home videos. The software that comes with these camcorders can then make much better stills from the video etc etc. Nothing to do with broadcast in my mind but all consumer driven, bigger numbers and the word progressive.

I think camcorder sensors have been full progressive for a long time its just that the video encoders were not fast enough to process the full output from the sensor so just processed fields. Technically 60i and 60p have exactly the same temporal motion. One records fields the other the full frames. The same shutter speed effects both the same way. Output from the sensor goes into a buffer memory at the same rate for both 60i and 60p so they should look the same one being half the vertical resolution of the other. However big difference can occur in how these signals are encoded. Do they get encoded to AVCHD 1.0 or 2.0 and whose encoder. Is the signal encoded to a 100Mbps external recorder ? Can the on board encoder create this output to the HDMI or HDSDi for the external recorder. I think there are likely big differences between a $400 stills camera shooting 60p and a $10000 camera shooting 60p !!!

Next we have to look at the display for 60i /60p. Unless the display is a full 1920x1080 display the full HD signal will have to be scaled say to a 720 display. So in this case the display has to not only de interlace but scale as well. Most of the time our impression of quality is really how good the de interlacing and scaling algorithms in the display we watch work. Moving to record at the same size and refresh rate as the display removes all these issues. On my Sony 240hz interpolating display I see no difference between video shot at 60i or 60p from my CX700, NX30U or my HX30V. Are any of them as good as an external recorder then I think not. They are however consistently better than most cable TV channels !!!! But the data rate difference between 24Mbps and 28Mbps is enough to encode the difference between 60i and 60p and to my eyes the same picture quality and I am sure that the engineers figured this out too. 60P at 28Mbps is a lot better than 60i at 9Mbps this difference is easy to see. Is AVCHD 2.0 as good as 50Mbps encoding or more in some format, probably not.

Ron Evans

[James Hollingsworth]

Nevertheless, any thoughts on how to get the 108050P into final cut in the first place?