I just found an issue with the .scp encoding for single-sided disks. I had a bunch of people recently trying to convert .dsk and .dmk TRS-80 disk images to .scp format. The HxC conversion for a 40 track single-sided image comes out as 20 tracks (2 heads), and a 80 track single-sided image comes out as 40 tracks (2 heads). The disk ID also seems to be Atari ST.
Per the .scp specification, with a single sided image you should be skipping every other TDH entry. Even TDH entries are the bottom head, and odd TDH entries are the top head. You should also be setting the HEAD byte (0x0A) to 1 (not 0 or 2) in these cases to indicate a single bottom head. There's not likely to be too many images with just the top head, but it's there for completeness. When imaging a double-sided disk (like Amiga, IBM 1.44MB, Atari ST, etc.) this is automatic. I am not sure if I made this very clear, so I have revised the specification. In my software I don't look at anything except the TDH entries. I know if an entry contains 0x00000000 then there is no data for that head (top or bottom). So, I don't really need to know how many heads were actually used in the image via the HEAD byte (0x0A). It's just there to make it easy for whatever is looking at the data to display the number of heads used.
So, as a recap - single sided disks use the bottom head only and so there should be 40 entries (for 40 tracks) or 80 entries (for 80 tracks) that use TDH entries 0,2,4,6, etc.