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Transcoding IPB to all-I, how to calculate appropriate bitrate?

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Nick Ryan
Transcoding IPB to all-I, how to calculate appropriate bitrate?
on Jul 27, 2018 at 3:12:42 pm

Hey All!

Wow, I haven't been on here in forever, but as usual, when I have a question that I can't solve with searching the entirety of the internet, I return here.

So, it's like this (warning: over-technical codec question coming up): I manage a small in-house production and post-production advertising facility for a liberal-arts college. We shoot on mirrorless cameras (GH5, GH4, A&SII), and then edit on Premiere Pro on our HP workstations. Since we're shooting in IPB, we transcode our footage to an all-I editing-friendly codec (an odd hack flavor of DNxHR that we came up with), which runs much much much more smoothly on our workstations than the original footage. Previews load almost instantaneously, effects process in 1/4 the time, etc. For the most part, everything's been fine and happy in editing land.

Then, it recently hit me. I've always assumed that I should transcode to an editing codec of similar average bitrate. So, since we're shooting at 100mbps, we transcode to an all-I codec that's roughly 100mbps. I think I've been doing it wrong all this time.

The average bitrate per frame for 100mbps (at 24 frames per second) is roughly 4 megabits per frame. So I thought transcoding to an all-I codec at 4 megabits per frame would yield equal results. However, since IPB is packing all of the quality into the I-frames, those I-frame data rates are really much higher, maybe 20-30 megabits per I-frame, and then just pocket change for the B-frames. So in terms of quality, you're really getting the equivalent of 20+ megabits per frame, so if you're transcoding to an all-I codec, you should be transcoding to something with an average of 20+ megabits per frame, like DNxHR SQ or HQ or whatever. Even though your transcode will end up being so much larger than your original file, you're not really throwing away information, you need that bandwidth. In fact, if you don't end up with a much larger file you're throwing away quality with your transcode.

Make sense? So, my question is, how to I calculate an appropriate bitrate conversion for IPB to all-I? How big are the I-frames on the GH4 at 100mbps or the GH5 at 150mbps? I've tried to find an IPB bitrate analyzer with no luck.

And yes, I know that many people are moving away from the editing codecs in favor of editing proxies, which I'm open to but am not sure how to deploy easily in our multi-machine environment. Besides, a lot of those proxies are still h.264 based which still results in processor intensive slowdown, right?

I'd love to hear your thoughts.

Nick


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Jeff Pulera
Re: Transcoding IPB to all-I, how to calculate appropriate bitrate?
on Jul 27, 2018 at 7:45:33 pm

Hi Nick,

HD or 4K? That is a very important factor in this discussion as 4K frames are 4x bigger than HD.

Thanks

Jeff Pulera
Safe Harbor Computers


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Nick Ryan
Re: Transcoding IPB to all-I, how to calculate appropriate bitrate?
on Jul 27, 2018 at 7:52:06 pm

Ah - great note. 4K, definitely.

Nick


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Jeff Pulera
Re: Transcoding IPB to all-I, how to calculate appropriate bitrate?
on Jul 27, 2018 at 8:18:06 pm

In Premiere Export, I chose the DNxHR HQ UHD preset for 24p video -


According to MediaInfo app, pushing about 700mbps for the video stream itself.


Thanks

Jeff Pulera
Safe Harbor Computers


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