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More Education Please - Transcoding


shackbill
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shackbill

I am looking for opinion and /or facts to help me understand more about the quality of my video.

 

Say i have 4 videos - same title - different transcoding or format as follows

 

1.  4K Movie at 8bits and 12.9 Mb/s bit rate (NVENC)

2.  4K Movie at 8bits and 4.5 Mb/s (NVENC)

3.  4K Movie at 10bits and 60.0 Mb/s  (i suppose this one should be the best quality)

4.  4K Movie at 10bits and 4.5 Mbs

 

I have a 4K/HDR Quantum Dot LED Tv...

 

What is the real perceivable difference in these files.  Can/Should we be able to see a difference with 10bit HDR vs 8 bit and does the bit rate make a significant difference in the quality.  Do i lose effective resolution by compressing way down? 

 

I realize disk space is cheap but it seems that even my Shield Pro which is SUPPOSED to be able to stream multiple 4K streams will struggle with the higher bit rate stuff and im just not so sure the difference in quality is worth 50GB files (even though i have 20TB) 

 

Im an older guy (who also happens to be color blind) so my vision isnt great but im not lost in the fact that others watch my content and i want everyone to get good quality.

 

thanks in advance.

Edited by shackbill
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Guest asrequested

That's a little more complicated than the 4 options you gave. You didn't specify codecs. But let's assume you are talking about HEVC. (2) and (4) are going to be the worst, in comparison to the others. What these codecs/compression algorithms do is analyze the data stream for the parts of the picture that can be removed without being noticed. So the more that gets removed, the more you will notice it. (1) can be good, if the person who compressed it knew what they were doing. 10 bit or 8 bit? These should always match your display. 10 bit is better than 8, but only on a 10 bit display. 10 bit video on an 8 bit display will look like crap, as it can't display all of the picture information. There's more technicality to it, but does that at least help? 

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shackbill

yea a bit. the codecs are HEVC. All of my 4K TVs 'supposedly' support HDR10 so i have to assume they are 10 bit displays.  So lets compare 3 and 4 - i realize these are extreme opposites but do you think you will lose enough information at the higher compression level to where HDR  is not really noticeable. in other words am i losing enough color information to where i might as well not be using 10 bit? its hard to determine the happy medium but the problem is, i dont seem to have enough computing power to reliably stream a high bitrate.

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There's a misconception: Even though the bit depth of a video (e.g. 8 vs. 10 bit) has some (rather small) impact on the overall bitrate, these two are very different things which are affecting the video quality in different ways. 

 

Bitrate is by far the more important one.

 

An extended color range will expose its benefits only in certain kinds of scenes (e.g. very dark, very bright, very mono-colored).

 

For more details - there's plenty of information available when you search for it....

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Guest asrequested

My personal rule of thumb is 4k HDR below 30Mb/s is a roll of the dice. If done well, there's no macro-blocking and details are mostly retained. Below 20Mb/s I try to avoid. Streaming services like Netflix stream 4k stuff somewhere between 20 and 25Mb/s, but they have professionals working on their content.

Edited by Doofus
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shackbill

Thanks for the feedback. I have read various points of view regarding bit depth and color range and some of them are a bit more technical than i can process and leaves me to wonder if i should really worry about HDR at this point. But i guess i will try a few different sizes and see what seems to work best but ultimately, it still has to stream efficiently for me with the hardware i have. 

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well yea but wont you lose the HDR effects if you compress too much or not use enough bit depth?

 

There doesn't exist something like "HDR effects". This is not some kind of 3D gaming feature.

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shackbill

HDR is a wider color gamut right? so maybe the term i am using is wrong but the concept of maintaining the color space. more color range is an 'effect' of HDR right?

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Simple recipe:

  • For now, forget about HDR and focus on video compression and playback experience
    .
  • A bit later, you choose some kind of movie with a lot of dark scenes.
    Get both versions, HDR and SDR and compare
    .
  • Then get HDR and SDR versions of another movie with rather constant lighting and try to see a difference

    => That's probably the best way to get and understanding of HDR
    (at the visualization side, HDR at the recording side is a different story)
     
  • Like 1
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shackbill

i guess i can forget about HDR for now but i still may want the capability in the future but not have to repopulate my library.

 

@.. good read. so, is mpv leveraged on emby apps such as Roku, APpleTV, or Samsung TV built-in apps just Emby Theater or does it occur at the server level such as Nvidia Shield TV?

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bman212121

Surprised no one touched on the most obvious thing yet. The transcoding can only ever be as good as the source. Unless you have a 4K 10 bit HDR movie at 60mbps bitrate, then option 3 is pointless. You can never make a video look better than the source, only worse. If the source is a TV recording at 720p H.264 @5mbps, then there is no reason to stream it more than 720p H.264 @ 5mbps.

 

 

Keep in mind the source could be 60mbps H.264 / MPEG4 Part 10 / AVC, but if you're transcoding it into H.265 / MPEG-H Part 2 / HEVC then the bitrates are not the same. HEVC at peak efficiency uses about half the bitrate for the same quality. So a 60mbps H.264 source could be transcoded at 30mbps H.265 and in theory should look exactly the same. (Assuming 4k resolution, it's not quite as efficient at lower resolutions) So you really need to know what source material you're using to know if what you're transcoding it into is overkill or not.

 

 

HDR is honestly not easy to explain in depth. I doubt I could do it correctly. But basically it's not about a range of colors, but rather the difference between how bright and how dark a pixel can be. The easiest way to explain this is to use greyscale. A black is never really 100% black, and white is never really 100% white. You could faithfully reproduce a white that is almost pure white, but then there isn't enough information to represent true black on the other end of the scale. So you make a compromise to where your white is very close, but then your black is now dark grey. Using HDR means you now have extra room to work with and the brightest value can be at a greater range from the darkest one. So in the same case, it means your darkest black can have a lower value while maintaining the same bright white as before.

 

Wide color gamut is basically what it sounds like. There are more possible color values to choose from, and it should also represent a greater amount of overall values. This is more about how correctly represented a particular red or green color might be.

 

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shackbill

@@bman212121 thanks for that response. Yea i pretty much have all original Discs that are ripped or high quality rips already. And ive come across some other 'remastered' content along the way that is very good quality. I'm looking for the happy medium where i can have very good quality, somewhat lower file size and still be able to stream without getting buffering. 

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Keep in mind the source could be 60mbps H.264 / MPEG4 Part 10 / AVC, but if you're transcoding it into H.265 / MPEG-H Part 2 / HEVC then the bitrates are not the same. HEVC at peak efficiency uses about half the bitrate for the same quality. So a 60mbps H.264 source could be transcoded at 30mbps H.265 and in theory should look exactly the same. (Assuming 4k resolution, it's not quite as efficient at lower resolutions) So you really need to know what source material you're using to know if what you're transcoding it into is overkill or not.

 

@@bman212121 - All of that is correct. I like that you wrote "peak efficiency" because that's an important point: To achieve this "same quality at half the bitrate" situation it takes a lot of prerequisites

  • It can't be achieved for lower-end bitrates (lower-end in relation to resolution and framerate)

    .

  • It takes a lot of processing power and video memory to achieve this

    (often more than what is available for live streaming)

    .

  • The latency of live-streaming will increase

    (because the encoder need to work over larger GOPs)

    .

  • The keyframe interval needs to be larger than 3s

    (for hls streaming, each segment needs to start with a keyframe and segment size is 3s)

 

 

That's why HEVC encoding is not as easy as it sounds sometimes.

 

Video encoding is nothing like creating and extracting a zip file.

 

So, my advice - can't say it often enough: Don't think about converting your library content to save space or something. It's a bad idea in most cases.

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  • 5 months later...
PaulZussma

I think the best option for u is 4K Movie at 8bits and 12.9 Mb/s bit rate (NVENC). By the way, guys, if someone takes cisco exams and does not know how to prepare for them as well as possible, I advise this site SPOTO.These guys helped me prepare for passing the cisco exams, and thus I got a cisco qualification and now work in a good company. By the way, I wrote the exam as quickly and easily as possible. :)

 
Edited by PaulZussma
mistake
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