As you know -d is to "allow channels to have different blocktypes". I don't see an option to disallow different block types.

Is -d on already...or...when is it not on? I have compared different encodings with & w/o -d but see no difference in the few I tried.

-d does nothing with Lame 3.92. Files are bit identical whether -d is used or not. I created a clip which has sharp attacks on the right channel and mouth-organ on the left channel. Then I did some testing with mp3x frame analyzer. The default settings seems to be "allow channels to have different blocktypes" since right and left channel did have different block types even though -d was not used in the commadline.
So -d does nothing, because it's enabled by default in Lame 3.92.

Thanks for your effort in answering my question. I saw the same results.

A bit clarification after talking with Naoki. -d was indeed defaulted some time ago. But Lame can only use different block types for left and right channels when using stereo coding. My test track did of course use stereo frames all the time, because there was not much channel correlation (2 different tracks for left and right channel).
But when a frame is coded using m/s-stereo, then Lame can't use different block types for the channels.

FhG encoders can't use different block types for left and right channels even with stereo coding (according to Naoki), so this is definitely one advantage for Lame.

Thanks again, man. You've been helpful...gee...I feel like I owe you something.

Currently, LAME and most MP3 encoder could not set "channels to have different blocktypes" when joint stereo.

It is allowed only when LR frame and not allowed in MS (Mid-Siide) frame by the spec. Because currently LAME determines frame type(MS or LR) after it determines block type, it cannot see whether it is allowed or not in the frame to "set channels to have different blocktype". so it always sets the same block type.

But on the latest CVS of my experimental branch, I changed the order of them to "detemining frame type first" and "detemining block type". Of course this makes LAME allow to set different block type.

By some test with "STEREO" samples like main_theme, taxman, and so on, it reduces about 5~10kbps by setting the different block type with --preset standard.

This is great! So, is this enabled by default in your latest CVS?

Yes, and could not disable it currently

This is too much inconvenient so I will make some new option to disable it (maybe next weekend. sorry I am only a weekend programmer).