Optical media don’t even have a bitstream. When you dd from something other than a CD-ROM, it’s not clear what you’re getting. You might just get an error saying it’s not a CD-ROM. Different kinds of CDs use completely different encoding layers in between “the bitstream” and the actual pattern of dots engraved on the disk. Obviously a CD-ROM has “a bitstream” because it’s designed purely for computer use, but for other kinds it’s not so clear.