For generic default in fixed encoder: 1 or 2 B-frames?

[benwaggoner]

Very broad question:

In general, for H.264 encoders that don't have adaptive B-frame cadence, and for generic content, do folks have any opinion on 1 or 2 B-frames as being the optimum default?

It seems like encoders that don't expose the setting at all (QuickTime, Adobe Media Encoder) use a fixed 1 B-frame rate. But some where it's configurable like Carbon (Main Concept) and Episode (Dicas) use 2 B by default.

We've certainly found that 1 B is the optimal default in ~95% of VC-1 encoding, but have folks found any significant differences either way with H.264 implementations?

[Dark Shikari]

Any encoder worth using should have adaptive B-frame decision.

[benwaggoner]

Any encoder worth using should have adaptive B-frame decision.
(oops, forums crossed again. Fixed)

No argument there, but some don't seem to.

I guess I should drive into the bitstreams of those encoders and see if they're actually treading the B-frame value as a maximum.

What's the proper response to companies who provide poor encoder parameter documentation? WTFM?

[Dark Shikari]

(oops, forums crossed again. Fixed)

No argument there, but some don't seem to.Then they aren't worth using... ;)

[benwaggoner]

Then they aren't worth using... ;)
I won't argue that point either, but I'm still curious :).

[Manao]

DVD uses a strict gop structure with 2 bframes. Most broadcast h264 encoders use at least 3 bframes, with a pyramidal structure.

[akupenguin]

DVD uses a strict gop structure with 2 bframes.
Except when they don't. I don't know what the spec says, but I've seen DVDs with adaptive B-frames (max 2) and scenecuts detected.

[Manao]

but I've seen DVDs with adaptive B-frames (max 2) and scenecuts detected. Do you mean 2 bframes, except when a scene cut is detected ? Or a really adaptive number of bframes ? Because I think 2 bframes was strongly advised (perhaps even mandatory, I don't know since I've never seen DVD specs) in order to allow trick play (in the early days of DVD). In any case, I have only seen DVDs with 2 bframes except on scenecuts (which - sometimes - the encoder managed to detect...)

[akupenguin]

Adaptive numbers of B-frames, everywhere in the GOP.

[benwaggoner]

Adaptive numbers of B-frames, everywhere in the GOP.
Yeah, that's always been supported in the spec, even if not used that much in practice.

2B is pretty strongly optimal in MPEG-1/2, so even a fully adaptive encoder would probably use it most of the time.