Output resolution when transcoding 4k source: 720p vs 405p

[C.S. 62]

If I understand what's happening, there is a lower bitrate limit, depending on source resolution, where the transcoder switches the output from 1280x720 to 720x405.

For example: Starting with say a 10 mbit 1080p video, the output resolution will stay at 720p down to 2 mbit or so. This makes sense, as anything below 2 is not really enough for 720.

But when starting with 2160p, the lower limit seems to be set around 5.75 mbit. Anything below this and the output will change to 405p. This doesn't make much sense, because 5 mbit is plenty for a decent 720p stream. Is it not?

I've tested this with a number of files in hardware and software. Results are consistent.

But I've only tested playing through a Shield, so could this be a client-side issue?

[visproduction 122]

CS,

Look at how much bandwidth audio takes, framerate and HDR settings.  That might have something to do with the math and border limits of what gets what resolution.

Programming automation logic to handle all situations is tricky, in my opinion.  You are working with a lot of 'if' conditions.  I think it's a mess and very hard to make an automated system work for every situation.  Hence, the sliding scale of tweaking up the bitrate and throwing higher end hardware at the issue if you want better quality.  

To further complicate conversions, the source material is most always never the master and often is more than 5 generations removed from the master.  If any of those generations are created automatically, they probably have some small conversion errors, framerate, aspect ratio or added artifacts.  So, making a new automatic conversion of content, another generation down from the master, always has issues.

This doesn't really answer your question of what settings can be tweaked to get the results you want. Let's see what everyone else comes up with.

My approach is to 3rd party, manually custom test and convert everything.  But that means I have no DVR.  On the other hand, I can have any size, aspect ratio, color correction, audio conversion, fix some errors and have any compression quality I want.


 

Edited September 21, 2022 by visproduction

[C.S. 62]

For the record I'm talking both HDR and SDR 4k, 23.976 fps. Both software and hardware transcoding. (8th gen i5 UHD 630)

I'm sure you're right it's a resource issue like everything else, but to me it seems like 720x405 should be avoided at all costs. I think it's ironic that the next step below 1280x720 turns out to be the equivalent of a non-anamorphic dvd. What happened to 1024x576? And could the math justify a 5 mbit encode at 720x405? That bitrate seems like massive overkill for that resolution.

[visproduction 122]

CS,   23.976 fps is a speed that is encoded on blu-ray and DVD, but it comes from DVD that has 29.97 fps of a 24 fps movie and has added 3:2 pulldown.  The DVD is set to playback on SD TV which runs at 29.97 fps with 4 frames of moving motion, followed by 1 duplicate frame.  If you pull out the duplicate frame the new speed is 29.97 / 5 * 4 = 23.976.  Audio has to sync back if the original master is running 24 fps, but it's so small it just needs to go 1 second in 1000 slower.  Unfortunately, blu-ray releases are about 95% 23.976 fps, which is technically wrong and unecessary.  HD TV's can playback 24fps.   23.976 It might actually sync better with a SD TV if you output the content either in coaxial or RCA or component out, if available.  But use of SD TV's is probably under 1/2% at most.

23.976 is old hat and should be retired.  Maybe we will see more 30 and 60fps coming up.

[C.S. 62]

Just tested it, and yes it's the same story with 24 fps.

[Luke 37064]

@C.S. 

 

Hi there, let's look at an example. Please attach the information requested in how to report a media playback issue. Thanks!

 

[Happy2Play 8281]

Yes this topic it true as the quality settings are not linked, 1080p - 4Mbps will not give you 1080p as these options are only bitrate factors not resolution factors.

But there are a few topics that have has similar discusions.

 

[C.S. 62]

Many thanks for that link. I should have searched more.

[Luke 37064]

10 hours ago, C.S. said:

Many thanks for that link. I should have searched more.

@C.S. 

 

Hi there, let's look at an example. Please attach the information requested in how to report a media playback issue. Thanks!

[C.S. 62]

Thanks, Luke, I'm happy to provide a transcode log if you really want, but I think this is essentially the same issue as in the thread Happy posted.

What I'm really wondering though is why the output would ever be 720x405 at all. Is it technically impossible to use 1024x576 or 848x480? 405 makes sense at 1 mbit and below, but anything above that deserves 576 or at least a true 480. (405 is not 480)

The reason I bring this up is because up until now, I've only had 1080p content and below on my server, so all this time when family members with sketchy internet have had their quality set to 4 or 5 mbit, they have been getting a perfectly fine 720p transcode when needed. But now with the 4k titles I've added, they get a substantially worse image than they should have to live with; 720x405 should be the absolute last resort for the encoder, imo.

Quote

The only little issue that I'm aware of is that HEVC source videos are weighed a bit too strong in the calculations

I'm thinking this must be it. All the 4k stuff I have is HEVC. I will try encoding some H.264 at 2160p and see what happens.

But to me, the bottom line is that it should be possible to start with a 4k HEVC source and come out with a 5 mbit 720p transcode at the other end. If that's impossible for some technical reason, then that is unfortunate, and I hope that technical reason gets addressed.

[Luke 37064]

Quote

What I'm really wondering though is why the output would ever be 720x405 at all. 

We'd have to look at a specific example to answer that,  but it's probably just preserving aspect ratio. Your original video may have black bars embedded into the video, and these are not in the transcoded output.

So yes the more info the better. Can we see an example? Thanks.

[C.S. 62]

Ok here are two logs - one with the bitrate set to 5.5, the other 5.75:

ffmpeg-transcode-332728f0-98b3-411e-ba02-c11dfdcd007b_1.txt ffmpeg-transcode-ad251cd4-4ff4-4155-8fa8-54b2190e96d0_1.txt

[Luke 37064]

13 hours ago, C.S. said:

Ok here are two logs - one with the bitrate set to 5.5, the other 5.75:

ffmpeg-transcode-332728f0-98b3-411e-ba02-c11dfdcd007b_1.txt 1.51 MB · 1 download ffmpeg-transcode-ad251cd4-4ff4-4155-8fa8-54b2190e96d0_1.txt 1.27 MB · 1 download

OK we're looking into this. Thanks.

[C.S. 62]

Glad to see this appears to be fixed in 4.8

Thanks all!