Output bitrate different than bitrate set up


#1

hi
I searched for that kind of problem but didn’t found.
When I set bitrate to for example 80M I’m getting as an output around 34M. When I set 1500M I’m getting around 80M as an output. Practically what I set as an bitrate has nothing to do with what I get as a result of encoding. I use constant bitrate, hardware encoding.

Does anybody have the same issue?


#2

What hardware encoder are you using? Also, please show some evidence like how you are measuring the bitrate.


#3

encoder h264_qsv. I check input videos 4k using mediainfo and it says
Overall bit rate : 104 Mb/s
When I set constant bitrate 100M output video is 14.5 Mb\s. And it’s probably correct value as quality is poor.
After spending one day on this issue I figured out that to get the highest possible bitrate (65.3Mb\s) need to set set “constant bitrate” to 1900M. Any other higher value resulting with lower, apparently random, bitrate.

Afterspending another day I figured out, that when I use variable bitrate. E.g. 98% quality results in 723 Mb\s, 96% - 520Mb\s and so on (where input is 103…). File is huge but at least I dont see the difference in quality to original.

When I encoded 1080 video input has around 33Mb\s, I set up 100Mb as an output and in practice got 31 as a result. If I’d set 30 in shotcut I’d get aroun 5-7 Mb\s. This is strange


#4

I kind of reproduced this, but the problem lies in Intel Quick Sync Video (qsv) and out of our control. It has a maximum for CBR that appears to be dependent on your chip. When you right click the job Quick Sync reports the bitrate mode and rates that it is using. When you supply 100M it exceeds some threshold. On my system, 100M gives “TargetKbps: 34464” and MediaInfo reports 34.5 Mb/s. Interestingly, when I tried with 50M it works, reports “TargetKbps: 50000” in the job log and 50.0 in MediaInfo. 60M also works, but not 70M (TargetKbps: 4464), 80M (TargetKbps: 14464), or 90M (TargetKbsp: 24464)! Thus, it appears to overflow some maximum as the TargetKbps increases linearly along with the overflow. Your chip and its driver give different results. Using ABR does not change the result. So, it appears that bitrate control only works up to a certain level.

If you want something high bitrate then this is your only option and to reduce the quality level to achieve it. Those quality levels are extremely high for video. The default preset uses ~60%, which still gives good quality for x264. Quick Sync and really any hardware encoder is not nearly as good quality as x264. The trade off is speed. Maybe try somewhere around 70%.


#5

thanks for spending time on this.
apparently 64M in Shotcut gives the highest possible result:

  • for software encoder it gives result of 58.9
  • for hardware 64.4
    Higher values set in shotcut results with some different bitrates:
  • software set to 100M - result of 52.9
  • hardware 100M - 380kB
    Exception: hardware 1900M - 65.3

As a side note, my laptop has i5 7200 and intel hd 620. I must check this problem on my old PC with AMD Phenom II X4 955 and gforce gt220

I checked on PC with AMD:

  • no hardware encoding is possible there
  • 100M setting gives 70.7
  • 160M gives 118
    on both machines as software encoder I used libx264

closed #6

This topic was automatically closed after 90 days. New replies are no longer allowed.