LAME 3.92 stable let the users to feed in values < 80 for --alt-preset now.

Warning: this is not official and is not documented. (you should understand: not properly tuned)

I'd tried to encode my songs with 8, 16, 24, 32, 48, 56, 64 for --alt-preset e.g. --alt-preset 16. 16, 24, 32, 48, 56, 64 are all with acceptable quality (left->right, worse->better). --alt-preset 8 is with very terrible artifacts.

It is good to have ultra low bitrates settings for LAME.

You can encode using bitrate < 64 even before 3.92.

Though that's not using alt presets. But it is still available.

It's been my impression that --alt-preset cbr xxx is only tuned for bitrates of 128 and above. Not sure if that's correct, though.

Well, alt-preset xxx will give you better results than just lame -h -b xxx.

But you're right, >80kbps values are better tuned

So the --alt-preset cbr command invokes some form of special tuning (more than -h -b) for the whole range of bitrates, not just 128 and above? Not that I'd expect to do much below 128, perhaps 112 on some samples that have proven (with other codecs in vbr) to be very easy to encode.

Well I don't mean the --preset or any custom settings done by ourselves. I trust --alt-preset rather than those.

--alt-preset cbr xxx had already been tuned for > 80kbps since LAME 3.90.2.

By the way, I am talking about --alt-preset only (ABR).