Intel QSV QuickSync H.264 CBR ignores target bitrate

Operating System Info

Windows 10

Other OS

No response

OBS Studio Version

28.1.2

OBS Studio Version (Other)

No response

OBS Studio Log URL

https://obsproject.com/logs/sLr1ezpFbDf0kOEa

OBS Studio Crash Log URL

No response

Expected Behavior

QSV encoder should respect the target bitrate when set below a threshold determined by the output resolution and framerate

Current Behavior

OBS outputs video at around 4300Kbps while streaming 1920x1080@120 after I lowered the CBR bitrate to 1800Kbps.

QSV appears to enforce a minimum bitrate depending on the output resolution and framerate. Higher bitrate values take effect as expected. I was able to replicate the issue on CBR, VBR, and ICQ rate controls.

OBS Studio Log is from a local recording demonstrating the issue. It was also observed in a 3 hour livestream to Twitch.

• Sample clip: https://clips.twitch.tv/RacyFurtiveGarageBrainSlug--hPP9JfqTGcZdP36
• Playback Info:
  
• RTMP Header received by live-sea.twitch.tv
  map[2.1:false 3.1:false 4.0:false 4.1:false 5.1:false 7.1:false audiochannels:2 audiocodecid:10 audiodatarate:320 audiosamplerate:44100 audiosamplesize:16 duration:0 encoder:obs-output module (libobs version 28.1.0) fileSize:0 framerate:120 height:1080 stereo:true videocodecid:7 videodatarate:1700 width:1920]
• Twitch Inspector Log: inspector.pdf

Steps to Reproduce

1. In the Advanced output menu, set QuickSync H.264 to CBR with an arbitrarily low bitrate like 100Kbps. If using ICQ, set ICQ quality to 51 (highest value, worse quality)

• Other settings used are target usage: slower, profile: high, keyframe interval: 4, latency: normal, bframes: 0, and no subjective video enhancements
  

2. In the Video menu, set the output resolution to a desired value. I tested with 1920x1080@120fps, but each resolution and framerate has its own minimum bitrate
3. Start streaming or recording

Anything else we should know?

Tested on Intel UHD 630 and UHD 770 on recent Intel Graphics drivers

Start by reproducing the issue with QuickSync H.264 using the Advanced output menu, a very low CBR or ICQ setting, and the reported resolution and frame rate. Compare the configured bitrate with the OBS log, stream output, and Twitch Inspector data; done means the encoder respects the requested target across the described rate controls.