Need help understanding the affect bitrate has on quality

[bolgnacake]

I'm experiencing a Jellyfin bug where a lot of my 4K titles will transcode, using Amazon Fire Stick TV when they should not. However, this question isn't to address that error (my first post had a member link me to an open bug about the issue). Due to the bug, though, with certain 4K titles I must drop the maximum bitrate within the Jellyfin client application. Its default is 100 Mbit/s, but I seemed to have found the sweet spot of 70 Mbit/s, where many problematic 4K titles will transcode to HEVC (even though that's what they originally were?).

Today, I found that my 4K Titanic wasn't happy with the 70 Mbit/s bitrate set by the client, so I dropped it to 60 Mbit/s and the movie started to transcode and play - no problem. Then I tried to watch the movie with director commentary, and once again, the open bug created problems - crashing the movie and/or client altogether. After dropping the bitrate to 20 Mbit/s (arbitrarily chosen...maybe it could have played at higher rates), the movie once again played, transcoded fine, and picture quality and commentary were very good.

In all of the aforementioned scenarios, Jellyfin is transcoding to HEVC for video and ACC for audio. I'm less concerned about audio at this time, since I don't have the best setup, nor do I fully understand all the nuances/differences between codecs. However, I have a nice 4K LG C2 Oled, so naturally I would love the best picture quality. In my opinion, whether the bitrate is 80, 70, or even 20, I'm not seeing a substantial drop in video quality. Should I? I imagine the higher the bitrate the better the quality, but is that not true? What even is the ideal bitrate for 4K streaming?

[TheDreadPirate]

The bit rate selector is, essentially, a cap. If the selected bit rate is HIGHER than the video's native bit rate, nothing will happen. Once you select a bit rate lower than the video's native bit rate, only then will it trigger a transcode.

[bolgnacake]

The bit rate selector is, essentially, a cap.  If the selected bit rate is HIGHER than the video's native bit rate, nothing will happen.  Once you select a bit rate lower than the video's native bit rate, only then will it trigger a transcode.

Thanks, that makes sense. Can you kindly clarify a couple of points for me?

1. In a transcoded scenario, the difference in bit rate will have no visual difference in video quality between bit rates. For example, let's say the max bit rate set in the client is 70 Mb/s, and the video's native is 80 Mb/s.

Scenario 1 - HEVC transcoded
Scenario 2 - The client bit rate is dropped down to 40 Mb/s. HEVC transcoded. Same video quality as scenario 1.

Is this correct?

2. In my case, original videos are MKV, 4k HEVC HDR. Due to the limitation in the client software at the moment, these are transcoded to HEVC, though my hardware supports the codecs, 4K, HDR, etc., so essentially the same codec. Is it fair to say that even though it's transcoded to its native codec of HEVC, there is some degradation in quality of the video? Not that I can tell from my eyes - picture looks great to me, but I just assume transcoding = downgrading quality to make the video/audio compatible with the client system.

[TheDreadPirate]

1) There will be quality loss in both scenarios. Is that quality loss noticeable? Depends on a lot of factors. Specifically what encoder you are using, like Quick Sync or NVENC or just your CPU, and what settings you're using. With the default settings in Jellyfin, modern iterations of both Intel Quick Sync and Nvidia NVENC will produce very close to original quality at both bit rates for 4K video. You'd have to pause and A/B compare to notice the differences.

2) Again, yes. At a minimum, you lose HDR since Jellyfin does not currently support HDR to HDR transcoding. Whenever you are re-encoding a lossy codec/lossy configuration, there will be quality degregation. Again, probably not noticeable if the bit rate is sufficiently high.

[bolgnacake]

1) There will be quality loss in both scenarios.  Is that quality loss noticeable?  Depends on a lot of factors.  Specifically what encoder you are using, like Quick Sync or NVENC or just your CPU, and what settings you're using.  With the default settings in Jellyfin, modern iterations of both Intel Quick Sync and Nvidia NVENC will produce very close to original quality at both bit rates for 4K video.  You'd have to pause and A/B compare to notice the differences.

Thank you so much for your quick replies. One last follow-up, because in my mind there is one distinction I'm unsure about. There's quality loss in both scenarios, I understand, but is there MORE quality loss depending on the bit rate number? For example, both transcode at 50 and 40 Mb/s but technically, does the 40 Mb/s transcoding degrade visual quality MORE?

Thanks again!

[TheDreadPirate]

As the difference between the original and transcode bit rates increase, the loss in quality increases. The difference starts off small, but there is a point where further decreasing the bit rate massive affects the quality.

[alleycat]

I'm curious about this also. What should an Android client (remote Tmobile 4G) be set to "receive" a 4k HEVC HDR stream?

[TheDreadPirate]

However fast your mobile connection can sustain. A raw 4K HDR video is like 50-80 Mbps. And that needs to be a fast and stable mobile connection.

And then you need to consider data caps, either absolute data caps where you starting paying more or "3G speeds after you hit this limit" type of caps.

Auto should adjust the max data rate according to your network performance, but it only performs the network speed test when you start the video.

IMO, 5-10Mbps is probably a good cap for a consistent mobile experience.

[alleycat]

However fast your mobile connection can sustain.  A raw 4K HDR video is like 50-80 Mbps.  And that needs to be a fast and stable mobile connection.

And then you need to consider data caps, either absolute data caps where you starting paying more or "3G speeds after you hit this limit" type of caps.

Auto should adjust the max data rate according to your network performance, but it only performs the network speed test when you start the video.

IMO, 5-10Mbps is probably a good cap for a consistent mobile experience.

Ok, Thanks. I understand. Didn't know a good starting point, I'll play around with it.