CPU Encoding

CPU encoding is the fallback path when no supported GPU is available, and it is also the right choice when you want the best software quality and do not care about throughput.

Encoders

Codec	Encoder
AV1	SVT-AV1
HEVC	x265
H.264	x264

Presets

cpu_preset controls the software speed/quality tradeoff.

Preset	Effect
slow	Best compression, lowest throughput
medium	Balanced default
fast	Lower CPU time, larger output

Thread configuration

threads = 0 means automatic. Alchemist lets FFmpeg choose the thread count per job. Set a manual value only if you are tuning around a busy shared server or a known NUMA/core layout.

Performance expectations

These are reasonable 1080p expectations on modern CPUs:

Codec	Preset	Expected speed
AV1	medium	~0.5–1.5x realtime
HEVC	medium	~1–3x realtime
H.264	medium	~3–8x realtime

When to use CPU

• No supported GPU is present
• Maximum software quality matters more than speed
• The batch is small enough that wall-clock time does not matter

Thread allocation

CPU cores	Suggested starting point
4	1 job, auto threads
8	1 job, auto threads or 2 jobs with care
16	2 jobs, auto threads
32+	2-4 jobs, benchmark before going wider

Use Settings → Hardware to allow or disable CPU encoding.