Making Frag Vids with the OBS Replay Buffer

Last updated: 2026-09-02

The problem with recording frag videos the obvious way is that you have to record. You start OBS, you play for three hours, you end up with a 90 GB file, and then you spend an evening scrubbing through it looking for the twelve seconds where the shock combo actually landed. Nobody does this twice.

The replay buffer fixes it. OBS holds the last N seconds of gameplay in RAM and writes nothing to disk. When something good happens you hit one key and OBS dumps that window out as a clip. You play normally, you press a key when you do something worth keeping, and at the end of the night you have a folder of short clips instead of a feature film.

Set it up

1. Install OBS and get a picture first. Grab OBS Studio from obsproject.com/download, add your capture source to a scene, and confirm the preview shows the game and the audio meters move. Do this before touching anything below — there is no point tuning a buffer that is recording a black rectangle.
2. Switch to Advanced output. Settings → Output, set Output Mode to Advanced. You now have separate Recording and Replay Buffer tabs.
3. Point the recording somewhere with room. Settings → Output → Recording. Set the recording path to a fast drive — ideally not the one UT is installed on. Note that this path belongs to the active OBS profile, and the replay buffer writes there too; if you keep separate profiles for streaming and recording, they can point at different folders.
4. Set the format to MKV. Same tab, Recording Format. Do not pick plain MP4: if OBS or the game crashes mid-write, a plain MP4 has no index and the file is unrecoverable. MKV survives it, and you can convert to MP4 afterwards with File → Remux Recordings — a container swap, not a re-encode, so it is instant and lossless. Newer OBS builds (30.2+) also offer hybrid MP4, which is crash-recoverable; it works, but MKV plus remux is the path with the most miles on it.
5. Split the audio onto separate tracks. Still on the Recording tab, tick more than one Audio Track. Then in Edit → Advanced Audio Properties assign each source to the track you want it on:

   Track	Carries	Why
   1	Everything, mixed	The track that plays if you never open an editor.
   2	Game only	The one you'll actually use. Clean audio cues, no chatter.
   3	Mic only	Keep your reaction, or drop it. Your choice, later.
   4	Discord / Mumble	Separate so a teammate's hot mic doesn't kill an otherwise good clip.

   The replay buffer writes whatever tracks the Recording tab is set to record, so getting this right here is what makes the clips usable. You cannot un-mix audio after the fact.

6. Turn the buffer on. Settings → Output → Replay Buffer, tick Enable Replay Buffer, and set Maximum Replay Time. 30–60 seconds is plenty if you're quick on the hotkey — long enough to catch the approach and not just the kill, short enough that you're barely trimming. Push it to 90 or 120 seconds if you'd rather catch the whole setup, or if you tend to remember the hotkey a beat late. The only cost is RAM, and 90 seconds is a forgiving default. Leave Maximum Memory at 0 and OBS sizes it from your bitrate and replay time.
7. Bind the save key. Settings → Hotkeys, find Save Replay, and bind it. Pick something UT does not use — F9 or F10 are usually free, and a mouse side button is better still because you can hit it without leaving the movement keys.
8. Start it — and only it. In the OBS main window, the Controls panel has Start Replay Buffer. Click that. Do not click Start Recording: the buffer is independent of recording, and running both just puts you back to writing the 90 GB file you came here to avoid. The buffer has to be running for the hotkey to do anything — this is the step everyone forgets on the first night. Set Settings → General → Automatically start the replay buffer when streaming or recording, or just make starting it part of the routine when you launch the game.

Encoder settings

UT is a twitch shooter, so the goal is a clean 60 fps with no encoder stalls, not the smallest possible file. Use the hardware H.264 encoder for your GPU:

GPU	Encoder to pick
NVIDIA	NVENC H.264
Intel	QuickSync H.264 (QSV)
AMD	AMD HW H.264 (AVC)
None of the above	x264 — and it will cost you frames

Hardware encoding runs on the GPU's dedicated encoder block, so it isn't stealing CPU from the game. Stick to H.264 even though your card almost certainly offers HEVC and probably AV1: those two make smaller files, but every editor, every phone, and every upload path handles H.264 without argument. The free build of DaVinci Resolve on Windows in particular has awkward HEVC support, and AV1 needs a recent version of just about everything. Frag clips are short; the disk you save is not worth the evening you lose to a timeline that won't scrub.

Then, roughly:

Setting	Value	Notes
Rate Control	CQP (or ICQ)	Constant quality. Bitrate rises during chaotic fights, which is exactly when you want it to. NVENC and AMD call it CQP; Quick Sync may offer CQP or ICQ.
CQ Level	18–22	Lower is better quality and a bigger file. 20 is a good default.
Keyframe Interval	1s or 2s	Matters more than usual here. A saved replay begins at a keyframe, so a long interval means the clip starts somewhere other than where you asked.
Preset	Quality-oriented	P5 (Quality) on NVENC; Quick Sync and AMD expose their own quality/balanced presets. The slowest preset looks marginally better and costs real GPU time — not worth it mid-match.
Profile	high	The standard H.264 profile. Nothing to think about here.
Look-ahead	off	Adds latency for a benefit you will never see in a 20-second clip.
B-frames	2	Default. Fine.

In Settings → Video, set the Output (Scaled) Resolution to what you intend to publish, not what you play at, and set Downscale Filter to Lanczos. Set the FPS to 60 — or 120 if you want the headroom to do half-speed slow-motion on a combo in the edit, which is the one good reason to record above 60.

How much RAM the buffer costs

The buffer lives in memory, and the cost is just bitrate times duration:

megabytes ≈ bitrate in Mbps × seconds ÷ 8

  20 Mbps ×  90s  ≈  225 MB
  40 Mbps ×  60s  ≈  300 MB
  40 Mbps × 120s  ≈  600 MB

CQP has no fixed bitrate, so watch the actual figure in View → Stats during a busy match and size from that. If you set Maximum Memory by hand and set it too low, OBS silently shortens the buffer and your clips come out shorter than the replay time you asked for.

Capturing UT specifically

• Use Game Capture, not Display Capture. Game Capture hooks the game's swap chain directly and is measurably cheaper. Display Capture is the fallback when the hook won't take.
• Run the game borderless or windowed-fullscreen. Exclusive fullscreen can swallow the hotkey before OBS ever sees it, and it makes alt-tabbing to check a clip a whole production. This is the usual reason "my hotkey doesn't work".
• UT99 and other old D3D9 titles sometimes refuse the Game Capture hook. If the source stays black, run OBS as administrator, and failing that fall back to Display Capture with the game windowed.
• Cap your in-game framerate. An uncapped 400 fps gives OBS inconsistent frame pacing and the capture judders even though the game feels fine. Cap to a clean multiple of your recording FPS.
• Turn off the in-game HUD scaling weirdness before a serious recording session — or don't, since the scoreboard is half the evidence that the frag happened.

Before the match

Four things, once, before you rely on any of it. Discovering on Sunday that the buffer was never started on Saturday is a rite of passage you can skip.

• The preview shows the game and the audio meters are moving.
• Start Replay Buffer is running — the Controls panel says Stop, not Start.
• You pressed the save hotkey and OBS acknowledged it.
• You found the resulting file.

After the session

You'll have a folder of short clips already trimmed to roughly the right length, which is the entire point. Run them through File → Remux Recordings if your editor would rather have MP4 than MKV, then it's an editor's job: drop them on a timeline in DaVinci Resolve or Shotcut (both free), mute the tracks you don't want, tighten the in and out points, and cut. This is the step that pays back recording H.264 and splitting the audio — both decisions exist to make this evening painless.

And when you've got something worth showing, it belongs on the TV.