50p and 25p use similar memory??

[MorleyUK]

I expect this one will have been covered in the past, but it's one that i've been meaning to ask for a while now...i hope i don't sound stupid!

Regardless of whether i shoot in 50p, or 25p, i can still fit 48 mins on an 8gb card (ie they use the same in memory)?? How come this is so when you would think that 50p would take twice the memory.

Can someone explain what's happening there then? Is there some sort of trade-off with using 50p?

Chris.

 

[Barry_Green]

Welcome to the world of the long-GoP codec. You'd think 50p would take twice the memory, and in an intraframe codec it would (and does)... but with a long-GoP codec, you set an established desired bitrate, and then the codec does whatever it needs to do to fit the images within that bitrate.

Is there a tradeoff? Not really. Because the long-GoP codec doesn't compress every frame individually; it only compresses the CHANGES from frame to frame. In most circumstances very little changes from one frame to the next; at 50p you're only allowing 1/50th of a second to elapse between the frames, and how much can change in 1/50th of a second? Not a whole lot. So there's very little change between frames, and therefore the images compress very very efficiently.

Whereas with 25p, twice as much time elapses between frames, meaning that the amount of change between frames is more. So it takes more work to compress that. But fortunately there are fewer frames per second to deal with. So it ends up netting about the same.

 

[Exact]

....and there is more to it than just 50p vs 25p.

The same goes for 1080/25p vs 720/25p. The HMC will simply compress the 1080p more than the 720p to fit the same compression bandwidth. I'm finding a lot less noise in the 720p files than I get with the 1080p files - which (I am sure Barry will correct me if I am wrong) could easily be caused by the extra compression artifacts resulting from (effectively) lower bandwidth availability in relation to the data required for 1080 vs 720.

 

[Barry_Green]

Yes, 1080/25p and 720/25p use the exact same bandwidth. On an HMC150 I didn't find that much difference between the two, seeing as an HMC150 puts out about 1440x810 worth of total resolution, so yes the 1080p frame would store that additional detail, but it's not that much to make that big of a difference.

But that raises an interesting question -- is there that much of a difference on an HMC40? Because on an HMC40, there's a whole lot more detail in the 1080p frame than there is in the 720p frame, so in that case it's possible that if avoiding compression artifacts is a paramount concern, you might indeed be better off with 720/24p vs. 1080/24p on an HMC40. That's theory, I haven't tested it, but it seems reasonable.

 

[MorleyUK]

Thanks for the replies, much appreciated.

So, stepping back into the world of practical application: When i film a wedding I may aswell keep my framerate at 50p (720) at all times, so that i can pick and choose the clips that i want to apply a bit of slow mo to, and don't have to worry about fiddling around in the menus during the day. The only downside of doing this is that during filming i won't be seeing the footage as it will ultimately look at 25p. (I've been shooting in progressive for years now so i know all about strobing and how to avoid it - mainly thanks to the DVX Book!).

Is this right?

 

[Exact]

Thanks for the replies, much appreciated.

So, stepping back into the world of practical application: When i film a wedding I may aswell keep my framerate at 50p (720) at all times, so that i can pick and choose the clips that i want to apply a bit of slow mo to, and don't have to worry about fiddling around in the menus during the day. The only downside of doing this is that during filming i won't be seeing the footage as it will ultimately look at 25p. (I've been shooting in progressive for years now so i know all about strobing and how to avoid it - mainly thanks to the DVX Book!).

Is this right?

Yes and no. Played at normal speed (i.e. 50p on a 25p timeline, not slo-mo) you are not going to see a significant difference in the way the footage looks. However, I do sometimes change to 720/25p and a shutter speed of 1/25 at dark receptions (e.g. first dance etc) because of the obvious increase in light capture @ 1/25 over 50p @ 1/50. The footage still looks great but of course it's much harder to slo-mo at this capture rate and still have it look good.