Key Frame Distance (please help!)

Community Beginner ,
Apr 12, 2017 Apr 12, 2017

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On the export setting video tab...  Under advanced, what in the hell should 'Key Frame Distance' be set at? This is for a fast paced music video. Should it even be ticked?

Ive watched a bunch of YouTube videos on this subject but most contradict each other! So I'm truly bemused...

Can someone really break down in layman terms; what this setting does. And is it true that its good to set it at half the FPS (I'm at 25 FPS - and so set the key frame distance at 12, for example?)

Any and all advice is so greatly appreciated!

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Adobe Community Professional ,
Apr 12, 2017 Apr 12, 2017

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Which CODEC are you using?

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Community Beginner ,
Apr 13, 2017 Apr 13, 2017

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I'm using H.264

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Enthusiast ,
Apr 12, 2017 Apr 12, 2017

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The keyframe distance is the maximum number of frames before the encoder inserts an i-frame, which is the highest-quality frame of an MPEG stream (ie, it has all the information for a single frame of video inside of it rather than relying on the frames around it to reconstruct). In between the i-frames are b-frames and p-frames, which are "difference" frames that reconstruct that frame of video by looking at the nearest i-frame and applying all the "difference" frames from that point forward (or backwards). The collection of an i-frame and all its connected p-frame and b-frames is called a GOP, or "Group of Pictures". So in essence the "keyframe distance" value determines the length of each GOP.

Note that Premiere automatically inserts a new i-frame for scene changes, irrespective of the keyframe distance value. This is to assure video quality at the start of a new scene since there wont be enough in common from previous b-frame and p-frames.

Generally speaking, the more i-frames you have, the higher the image quality of video, but also the larger the size of the video. So the value you choose for keyframe distance is a balance between the file size you want the video to fit in vs quality. If you're not concerned with file size or the file size generated is within reason then you can just leave it to its default value. Otherwise you can tweak it as part of the process to get the file size down to where you want it to be (in combination with the other encoder parameters such as bitrate).

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Community Beginner ,
Apr 13, 2017 Apr 13, 2017

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Thank you for the breakdown. I find this paragraph interesting:

Note that Premiere automatically inserts a new i-frame for scene changes, irrespective of the keyframe distance value. This is to assure video quality at the start of a new scene since there wont be enough in common from previous b-frame and p-frames

* noting this; I can leave the setting blank then? And assume premiere is doing its best? I have read that I should set a value of around half the frames per second... is this true?

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Enthusiast ,
Apr 13, 2017 Apr 13, 2017

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Yep, you can leave it blank. ME will select a default value that should work well for most content. You really only need to tweak it for specific image quality situations or file size considerations.

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Community Beginner ,
Apr 13, 2017 Apr 13, 2017

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Thank you for your reply. I will try leaving it blank and see the result.

It's just that SO many people say to set a value there of around half whatever the frames per second is... so it seems odd so many people say this!

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