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Question about bit rate

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Bobby Hall
Question about bit rate
on Jun 2, 2015 at 6:59:44 am

I have a DSLR camera that shoots footage in h.264 format with a bit rate of about 45-50 mbps. After I convert it to prores, edit it in FCP, and export, I convert it back to h.264 in mpeg streamclip. I've read several sources online that say if you're uploading videos to the web, the bit rate should be about 10-20 mbps. I have a couple questions:

1. Isn't this lower quality than the original file which had a higher bit rate? Does it even really matter?

2.I used to use imovie to capture footage from another camera and it would automatically convert the footage from .mts to AIC, and the bit rate would be about 150 mbps. Does this mean the AIC footage would look better than the h.264 footage due to it having a higher bit rate?

Thanks for any info!


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John Rofrano
Re: Question about bit rate
on Jun 2, 2015 at 12:56:00 pm

[Bobby Hall] "1. Isn't this lower quality than the original file which had a higher bit rate? Does it even really matter?"
At the same resolution the answer is yes, this is lower quality. As for "Does it even really matter?" the answer is "Probably not!". Think about the devices that are going to be used to watch this internet video. Mostly 4" phones, or 7" tablets, and 13" laptops. Hardly the "big screen". You also can't deliver high bit rates when most of your viewers are on bandwidth constrained connections so Internet delivery a balancing act between bandwidth/bit-rate and quality.
[Bobby Hall] "2.I used to use imovie to capture footage from another camera and it would automatically convert the footage from .mts to AIC, and the bit rate would be about 150 mbps. Does this mean the AIC footage would look better than the h.264 footage due to it having a higher bit rate?"
No. You can use all the bits in the world and the original picture is not going to get any better. You are just using more bits to represent the same image. The reason iMovie converts footage to Apple Intermediary Codec (AIC) is because formats like .mts (AVCHD/H.264) take a lot of compute power to decode. By converting them into a codec that is easer to decode, playback remains smooth even on devices with limited compute power. But iMovie is not just about playing back the original video as much as changing it with video processing (i.e., color correcting, titles, effects, etc.) By using more bits, you ensure that processing the video degrades the image as less as possible. Think of it as protection against math rounding errors. When you start to add video effects, pixels get modified and you want to keep these modifications in the highest fidelity possible. When you finally deliver the video, you can use AVC/H.264 again to encode it to a lower bit rate and still maintain most of the quality. So processing the video at a higher bit rate may look better than processing the same video at a lower bit rate because you have more bits to represent the changes. Does that make sense?

~jr

http://www.johnrofrano.com
http://www.vasst.com



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Bobby Hall
Re: Question about bit rate
on Jun 2, 2015 at 2:07:23 pm

Yes, that makes sense! Thanks a lot John! I have one other question. If you weren't going to add titles, effects, etc. to a video you were editing, and you just wanted to do very basic editing and nothing else, would it matter if the files that were converted to AIC or prores were a high bit rate or could they be much lower and achieve the same quality when you convert back to the original format? Example: If my camera records h.264 files with 40 mbps, and I convert them to prores but limit the bit rate to keep it at 40 mbps (as opposed to about 150 mbps when I don't limit it), then export and convert the video back to h.264, will that exported video be pretty much the same quality as if the prores files had been a much higher bit rate? Thanks.


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John Rofrano
Re: Question about bit rate
on Jun 3, 2015 at 12:28:25 pm

[Bobby Hall] "If you weren't going to add titles, effects, etc. to a video you were editing, and you just wanted to do very basic editing and nothing else, would it matter if the files that were converted to AIC or prores were a high bit rate or could they be much lower and achieve the same quality when you convert back to the original format?"
No it wouldn't mater. In fact, I don't convert any of my AVCHD footage to ProRes in FCP X. I just edit the native files for the same reason, i.e., I don't modify them heavily. It's perfectly fine to edit native formats as long as your computer can keep up with decoding them during the edit session.

My only exception would be something like motion tracking. Because of the way inter-frame codecs like AVC/H.264 store full, delta, and predictive frames, I would use an intra-frame codec like ProRes that stores every frame as a full frame. This makes tracking more accurate. So there are always exceptions to the rule.

~jr

http://www.johnrofrano.com
http://www.vasst.com



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Eric Strand
Re: Question about bit rate
on Jun 2, 2015 at 7:24:19 pm

[Bobby Hall] "I've read several sources online that say if you're uploading videos to the web, the bit rate should be about 10-20 mbps"

That's because 10-20 mbps has sort of become the standard bitrate for maintaing a good quality to file size ratio. Of course it all depends on the content of your footage, but most 1080 files will still look good at 10 mbps. For 720 the "standard" is arguably around 6-10, and for SD 1-4.

@ericstrand11


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Michael Gissing
Re: Question about bit rate
on Jun 3, 2015 at 9:06:20 am

AIC is an old legacy codec and is very lossy. It is a very poor choice compared to using ProRes for editing. Avoid it as even though the data rate is higher than H264 the codec is poor. There is more to video codecs than just data rate.

Final delivery codecs can be much lower than camera capture and editing. During editing with processing like colour grading you need the bit depth and data rate to minimise degradation. So higher data rates until the video is finished then a variety of rates depending on how the final is being delivered. Web delivery can have quite low data rates. H264 and H265 are designed to perform well at low data rates.


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Bobby Hall
Re: Question about bit rate
on Jun 4, 2015 at 4:03:38 am

If you convert native h.264 files to AIC and convert back to h.264 when you export, is the quality of the finished video lower than if you had converted the h.264 files to prores? When you say AIC is a lossy codec, does that mean it's just lossy during the editing process or also when you export as well? Thanks.


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John Rofrano
Re: Question about bit rate
on Jun 4, 2015 at 12:59:23 pm

[Bobby Hall] "If you convert native h.264 files to AIC and convert back to h.264 when you export, is the quality of the finished video lower than if you had converted the h.264 files to prores?"
ProRes is a more advanced codec than AIC and is higher quality, That being said, the question is will you see a difference in your work. I doubt it. Both AIC and ProRes are digital intermediary codecs meant to withstand several encodings without loosing visual quality and FCP editors used to use AIC before ProRes was introduced in 2007 so if that's all you have to work with using iMovie then go for it. If you have FCP or FCP X and ProRes then use that instead. Of course, if you're just doing minimal cuts then just edit in the native H.264 camera format and forget about transcoding at all.
[Bobby Hall] "When you say AIC is a lossy codec, does that mean it's just lossy during the editing process or also when you export as well?"
The loss happens during the encoding process when you first convert it to AIC. When you export, any loss is determined by the codec that you are exporting to.

Let's understand what "lossy" means because it sounds worse than it is. All codecs that are not using 4:4:4 chroma subsampling are lossy. That's the majority of codecs (since most are 4:2:2 or 4:2:0). Also almost every camera uses a lossy codec because cameras can't afford to shoot 4:4:4 due to the extremely large file size it produces. The whole idea of using a codec vs uncompressed is to throw away information that will not be missed because the human eye can't perceive it. The question is how much loss will you encounter. Some codecs are better at it than others. When we can't see the difference we usually use the term "visually lossless". Personally I don't believe you will see a difference going from H.264->AIC->H.264 vs H.264->ProRes->H.264 (unless you start magnifying frames 200% and scrutinizing every pixel) but as it was pointed out, AIC is an older codec and you should use ProRes instead if you have it.

~jr

http://www.johnrofrano.com
http://www.vasst.com



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Bobby Hall
Re: Question about bit rate
on Jun 5, 2015 at 3:26:08 am

Thanks John! That was extremely helpful!


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Bobby Hall
Re: Question about bit rate
on Jun 5, 2015 at 3:31:01 am

Hey John (or anyone else who wants to answer),
I have a question about audio formats and bit rate. I edited a video in FCP7 and the format was prores 422 and the audio format was linear pcm. I want to upload this video to the web, so I'm using mpeg streamclip to convert it to h.264 but I'm not sure what the audio format should be. The default option is linear pcm, but should I change it to aac? I know pcm is lossless but if there's not really a noticeable difference between pcm and aac and it will make the file much smaller I'd rather use that. What do you recommend? And if I choose aac, what rate should I set for the kbps? Thanks!


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John Rofrano
Re: Question about bit rate
on Jun 5, 2015 at 5:02:55 am

I would use AAC instead of PCM and set the bit rate at 160 kbps or higher depending on the file size you are trying to achieve. You should not hear any appreciable difference in quality.

~jr

http://www.johnrofrano.com
http://www.vasst.com



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Bobby Hall
Re: Question about bit rate
on Jun 5, 2015 at 10:43:12 am

Thanks!


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John Rofrano
Re: Question about bit rate
on Jun 5, 2015 at 12:35:07 pm

You're welcome. ;-)

~jr

http://www.johnrofrano.com
http://www.vasst.com



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