Recording bitrate is there a definitive answer?

[Dave Baker]

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.

[Mark Watson]

Convert a file, then check its video bit rate. MediaInfo is a free program for that sort of thing.

https://mediaarea.net/en/MediaInfo

[Dave Baker]

Thanks, but if it was that easy I would have done it. Every transcoder/converter I have used requires the output bitrate to be set, somehow defeating the object. Mediainfo does not convert, only gives the info. I use it often.

[Noa Put]

Don't convert, just put your 50p files in a 25p project, I do that all the time.

[Rainer Listing]

Dave, I suspect that's about the quality of the advice you're going to get. If you're delivering 25p from 50p at a CBR it's that simple. If you're delivering 25i (or as the marketers now call it, 50i) you get better quality capturing 50p at 35Mbps. Summary, for best quality per progressive frame and more latitude in post, shoot 25p 24Mbps and 50p 35Mbps for BluRay. If you're not worried about post and aiming at Youtube, that only recommends 8Mbps uploads at 1080p so 17Mbps is more than adequate.

[Noa Put]

What's wrong with my advice Rainer? Dave wanted to know if he shot 50p for the higher bitrate would he lose some of the original bitrate by converting the footage to 25p in post?

That's why I said not to convert and just place the 50p file in a 25 project because in that way you don't loose the original bitrate.

[Andrew Smith]

I'd be inclined to go with what Noa says as it will not only reduce handling steps but also the opportunities for generational loss due to re-encoding. Keep it simple.

Andrew

[Roger Gunkel]

I also agree with Noa, I often use a variety of bitrates from different cameras, and just import the different clips into the project .

Roger

[Dave Baker]

Dave, I suspect that's about the quality of the advice you're going to get.

I've looked around the web and generally it is as you say, I was just hoping for an answer rather than opinions. Then along comes Noa, the first person who I have seen say anything tangible on this subject. I have seen it said that you do lose bitrate and also that you don't, but Noa has qualified his advice by saying in what instance you don't lose bitrate.

Noa, it goes against the grain a bit because I have always used an intermediate codec for editing, but I have seen your work and the quality is outstanding, so maybe I should follow your example. I'll certainly give it a go! Thanks.

[Noa Put]

You will loose bitrate if you render it out to a low bit rate file but if you place the native file on your timeline nothing changes that file, the change occurs once you render it out, you will loose half of the frames if you render a 50p file out in a 25p project but the bitrate is something you can set yourself when you render the file.

[Dave Baker]

Thanks Noa, I would render it as ProRes Normal, target bitrate 147mbps, so no loss there. I do have some 50p footage, so I am about to test your method, I shall try it with some 25p as well.

As I said in my original post, I normally don't shoot 50p, but it's good to know that if I do I can keep the higher bitrate.

[Mark Watson]

Originally Posted by Dave Baker

Thanks, but if it was that easy I would have done it. Every transcoder/converter I have used requires the output bitrate to be set, somehow defeating the object. Mediainfo does not convert, only gives the info. I use it often.

Isn't image quality what you really care about? If that's the case, you should do the timeline conversion and render at a given bit rate and compare the image quality visually to a straight up 25P recording, rendered with the same settings. Maybe so some color correction or other effects to stress it a bit.

[Gary Huff]

Originally Posted by Dave Baker

As I said in my original post, I normally don't shoot 50p, but it's good to know that if I do I can keep the higher bitrate.

Why do you not normally shoot 50p? Is it because of the look of the footage at 50p? Shooting 50p and then dropping in a 25p timeline will keep the overall look, just not as smooth with the motion.

[Dave Baker]

Isn't image quality what you really care about?

Yes of course and now my question is answered closely enough to know I won't lose bitrate if I shoot at 50p, as long as I put it on the timeline without transcoding first.

Is it because of the look of the footage at 50p?

Yes, I suppose it is. I am used to 25p and it feels right, so I prefer it.

[Cary Knoop]

Originally Posted by Dave Baker

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.

If you convert 50p to 25p you lose every other frame, but in order to do that you must re-encode the file unless you use an all-intra encoding.

You can calculate the bits per frame which, if you decimate every other frame in the video, becomes a total of half the amount.