Setting this switch is done by enabling NTSC Clock Speed plus Force Song Speed.
Beware - this setting doesn't make much sense , but needs a bit more explanation. This setting forces SIDPLAY
to pitch up a little bit and replay any sidtune with NTSC timing, regardless of whether the file
is set up to play at any other intended speed except variable timer speed.
If you just want the sound of an NTSC C64 (that is the slightly
different pitch), enable NTSC Clock Speed and disable the Force Song Speed option.
In NTSC Song Speed mode SIDPLAY calls the music replaying routine at 60 Hz like a vertical blanking interrupt (VBI) of a NTSC based machine would do. But explicitly forcing this speed is not necessary and is most likely the wrong speed for a song, because all sidtunes already contain specific information about their original and intended replaying speed. This information is supposed to be correct for any existing sidtune that has been set up properly even if the corresponding C64 program has been PAL-NTSC fixed or vice versa. Thus, if a game and its music have been designed for a NTSC based machine, the extracted music generally has been set up by someone to play at 60 Hz by default. You can verify this by taking a look at the speed information display or by examining the sidtune file contents. Musics which are called via timer interrupts are intended to run at variable timer speed and have not been affected by PAL-NTSC conversion anyway. The difference between the PAL and NTSC CIA timer speed is equal to the clock speed difference, that is a factor of 1.038 only.
Due to the fact that PAL based screens (TV, monitor, ...) generally are able to display more lines of pixels than NTSC based ones, the NTSC screen refresh rate (60 Hz) is 1.2 times faster than the PAL (50 Hz) rate. Therefore, on NTSC based machines a VBI driven music routine is called more often, thus leading to falsification of the originally intended music timing. Such programs require a socalled PAL-NTSC conversion which makes them run at intended speed on another system with different clock speed.
Unfortunately, a lot of European games haven't been PAL-NTSC fixed. And as the musics have often
been played via VBI, they have been played too fast on NTSC based machines. Thus it seems, as if
former users of NTSC based machines now think that a lot of tunes are played too slow in comparison
to what they were used to on their old C64 computer .
If you don't care whether a tune is playing at its originally intended and correct replaying speed, and you are used to listen to tunes that are played too fast, enabling NTSC Clock Speed as well as Force Song Speed is a cure. But please believe the fact that any sidtune is supposed to run at proper speed without explicitly enabling the NTSC mode. If you are sure the default speed of a sidtune is wrong, please contact the maintainer of the respective sidtune collection.