Recording bitrate, is there a definitive answer?

[DeJay]

My Canon HF G30, set to MP4 mode, will record 25p at (up to) 24mbps and 50p at 35mbps, the 35mbps bitrate is not separately user selectable. I have no regular use for 50p, so normally shoot 25p. The question is, if I shoot 50p for the higher bitrate, do I lose some of the original bitrate by converting the footage to 25p in post? I have seen several opinions, but I am hoping someone will know.

Failing that, is there any way I can find out for myself? Somehow the logical 35/2 = 17.5 seems too simple.

[ChapmanDolly]

DeJay, just wondering why you prefer 25p. I shoot MP4 with a G40 always at 50p 35Mbps and keep these frame and bitrates all the way through post and delivery. The only comparison I have done is between AVCHD at 28Mbps and MP4 at 35Mbps. MP4 wins in the quality stakes. The only way is to do some test footage and compare the results.

[Bernix]

Hi,
probably shooting at 50p needs less than twice bitrate. Because there is smallest difference between frames. So compression is better. This question comes to my mind because of TV recording. And I dont know real answer. Just guessing.
To your question, it is better to keep it 50p. Otherwise you are loosing 1/2 information. And not bitrate / 2. But you should use something like CRF (dont know your codec nor resolution).

Bernix

[DeJay]

ChapmanDolly To my eye 50p offers no advantage over 25p, so I happily shoot 25p. I did multiple tests when I first got the camera a few years ago and found I prefer 25p MP4. I always transcode to an intermediate codec for editing and this is why the question came up in the first place, I used to use DNxHD and to start with didn't immediately realise it was transcoding from 50p to 25p and when I did, I wanted to know what I had lost in bitrate, if anything. I changed DNxHD to transcode to 50p as a test, editing and outputting 50p as a test, but wasn't overly impressed. I now have problems with some recent footage and increasing the recording bitrate to prevent it happening again is a possible answer, so the question arises again.

Bernix Yes, shooting at twice the framerate must need less that twice the bitrate otherwise the MP4 50p bitrate would need to be 48mbps, the best 25p being 24mbps. What I'm trying to find out is whether discarding half the information also lowers the bitrate or whether, as I read somewhere, it stays at 35mbps.

[Bernix]

Hi,
limited to 35Mb/s can be due something other than just same quality 25p and 50p. It can be Level restriction, bandwidth of data in your camera and so on. Maybe therefore it looks for you better 25p.
Try to re-encode to h265 or h264 (by CPU) using Handbrake, Vidcoder, Avidemux or Hybrid, or any other similar program. And use for start default settings but try CRF compression. Remember that for quality in h265 crf for similar quality is 6 lower than for h264. So you are supposed to get same quality with crf 26 in h265 as crf 20 in h264. If I were you I try to re encode the video to h264 at 18-22 CRF because h264(x264) is more supported nowadays TV's and other devices. If you are not satisfied with result, go to lower CRF or switch to h265(x265) which is o.k. to watch on PC, and some newest devices, that supports DVB-T2 will probably play it also. As container I suggest MP4 or MKV. It depends on device you want to playback it.

Note, that CRF gives you different output size for different video (when time is same) Because it depends on scenes complexity of video. It is faster than 2 pass and if you find ideal CRF settings for your eyes, so you will not waste bits or other way round, no bits will be added uselessly.

Bernix

[DeJay]

Hi Bernix

I appreciate your help, but all this re-encoding will not answer my question. I already transcode my footage to an intermediate codec for editing and until this recent footage, of which just a few clips have a problem, have always been happy with the results.

I had changed the shooting settings to Cinema Mode Standard because it gives a flatter image and more "wiggle room" in post. It tested OK, I shot a project in this country with it and the results were quite good, but it just didn't like the Cretan clear blue skies, producing banding. Yes, it's baked into the footage, it's almost certainly a limitation of the 4:2:0 colour space and before I decide to not use it regularly I would like to see if a higher bitrate will help. What I don't want to do is shoot at 50p 35mbps and finish up with the equivalent of a lower bitrate when reduced to 25p. I haven't had this banding problem since I stopped using Cinestyle, for this precise reason, in my EOS 60D.

[_Al_]

My Canon HF G30, set to MP4 mode, will record 25p at (up to) 24mbps and 50p at 35mbps, the 35mbps bitrate is not separately user selectable. I have no regular use for 50p, so normally shoot 25p. The question is, if I shoot 50p for the higher bitrate, do I lose some of the original bitrate by converting the footage to 25p in post?

Most likely yes, encoding it with CRF 18 or something. Your intermediate - it depends, you encode it to bitrate I guess but that is not the question I think,

BUT that is not important,

Bigger problem is reducing frame rate would introduce video to stutter, because every other frame is taken out. It was recorded most likely with shorter shutterspeed as oppose you'd recorded it directly 25p. You can kind of blend frames to 25p, but that introduces all kinds of ghosts, blurs and artifacts.

[Cornucopia]

1. You are getting banding because you are processing your footage (and in this context, re-encoding the edited output constitutes processing) and you have too few steps for the range of bits you are utilizing AND you are forgetting to dither.
Dither is recommended anytime work is done on media to where the output needs to have its bitdepth reduced (and quite often, this happens if a certain process does some of its math in floating point or creates float results). Best to upconvert to a 10 or 12bit intermediate, then do all your post work, then save again as 10/12, then do ONE dither during the distribution copy encoding steps. Should fix things right up.

2. 50p use depends on the subject and your skill and your intentions for the "look". If you (camera) or the subject/background is moving faster than 1/2 the framerate, you will experience motion blur or aliasing which could be good or bad, depending, but it would be noticeable.
If none of your subjects move fast, it probably wouldn't be worth 50p.

Scott

[DeJay]

Thanks chaps. The banding is present in the raw camera files, the first thing I checked was whether it was processing or not, which suggests that Cinema Mode Standard is not suitable for this use. It's almost identical to the effect I got with Cinestyle on my 60D, the effect of trying to cram too much into an 8-bit 4:2:0 colour space. I transcode to ProRes Normal (profile 2) which is 10-bit 4:2:2. But this is all fluff, taking the thread off topic.

The only reason I would shoot 50p (other than if I was shooting for slowmo) is for the higher bitrate, which brings me neatly back to my original question: If I shoot 50p 35mbps and reduce it to 25p in post, is it still 35mbps or not.

[Cornucopia]

If you shoot 50p at some bitrate, then you process it to 25p, at that point the processing has decoded it to full uncompressed stream. To return to an AVC-coded MP4, you'd re-encode it. So...
When re-encoding, you can set the bitrate to be just about any darn amount you want. 35Mbps or not.
But at that point, the new 35Mbps may not give you the exact same level of quality as an original 35Mbps file, simply because it has
1. Been processed (and the "shutter speed"/motion sampling of downsampled 50-25 is rarely the same as original 25, depending on algorithm)
2. Been re-encoded (with likely lossy encoding) one generation down
3. Encoding app/algorithm/parameters/settings (other than just simple bitrate) may very likely be different

The new 35 has little in common with the original 35 (it is now based off a different quality "master"). And in general, BITRATE ALONE DOES NOT SOLELY DETERMINE QUALITY.

We have been trying to tell you this all along, but in bits and pieces, which didn't seemed to take.

Scott

[DeJay]

BITRATE ALONE DOES NOT SOLELY DETERMINE QUALITY.

Where did I suggest it does?

I appreciate you are trying to help, but you are answering questions I didn't ask. When I mentioned banding (and how I wish I hadn't!), it obviously wasn't clear enough that it was shot at 25p (I said earlier I don't usually shoot 50p) and I was pondering on the idea that the extra bitrate of 50p could possibly have helped. That wasn't a question, I foolishly gave it as the reason I asked my original question, which is still unanswered. As I said in my last post, the banding is present in a few of the raw camera files, no post processing of any kind has been applied to them.

Can we please forget that and come back to my original question.

[poisondeathray]

Can we please forget that and come back to my original question.

The only reason I would shoot 50p (other than if I was shooting for slowmo) is for the higher bitrate, which brings me neatly back to my original question: If I shoot 50p 35mbps and reduce it to 25p in post, is it still 35mbps or not.

The answer is it can be 35Mb/s or it can be anything like 1Mb/s or 100Mb/s . You can set it to whatever you want.

The problem is, the way you've phrased the question - it doesn't make any technical sense.

When you do anything in post, you no longer have the original recording or compression scheme. You cannot losslessly drop every 2nd frame while keeping the original long GOP compression scheme. It gets decoded/decompressed to uncompressed then recompressed. What you use for recompression partially determines the outcome. Obviously the 100Mbps version will look better than the 1Mbps version, if every thing else was the same. 35Mb/s using "bad" AVC encoder will look worse than 35Mb/s using a "good" AVC encoder. But it will always look worse than original 50p 35Mbps version, unless you use uncompressed or lossless codec

Your real question is probably which is better 25p 24Mb/s vs. 50p 35Mb/s . It depends on a zillion factors and how it's going to be used. It depends on which encoder you use

You don't lose any quality while in post, if you stay there. If you drop 1/2 the frames, while in your editor, each individual frame quality is still the same because internally it's dealing with uncompressed frames. You lose quality when you export re-compress with a lossy codec. Even prores is lossy

50p will normally have a faster shutter than 25p. So it won't look correct unless you process it correctly with motion blur, but that technically degrades the image too

[DeJay]

The problem is, the way you've phrased the question - it doesn't make any technical sense.

Ah, yes, I see your point, an unfortunate re-phrase indeed. The original question could have been put better too, for that I apologise.