The raw OPL format was used by the DOS utility Rdos Adlib Catcher (RAC) by Roman Dolejsi of the group "Dimension" (RDOS stands for "Roman DOlejsi Software"). The files were then meant to be played with RDOSPLAY.
All values are little-endian. It features a simple header featuring an 8-byte signature and a two-byte initial clock speed. This format is unique in that the clock speed can be dynamically changed during playback.
|char||Signature (always RAWADATA)|
|uint16||Initial clock speed|
The clock speed is given in number of cycles of the PC's interrupt timer, which itself has a frequency of 1193181 Hz. So the clock speed can be converted into a clock rate in hertz by dividing 1193181 with it. The OPL chip's own update rate is 49716 Hz, so there is 1193181 / 49716 = 24 (approximatively) timer cycles for one OPL cycle. This means that the given clock speed divided by 24 gives the number of samples per OPL tick.