I think I figured it out now. At least it now produces samples in 4-D Boxing seemingly at the correct speed while not negatively affecting Sim City and Space Quest III v1.0P with its Tandy DAC driver. It does not sound as clean as your recording, but then again, your recording seems to be low-pass filtered anyway. As this is a patch against SVN r4000, it also includes my correction to low-frequency notes in Zak McKracken, which still has not been included into the main source.
Note that direct hardware access to the Tandy DAC was, and still is, disabled whenever the Sound Blaster is active. I have added a warning message to notify users of this. When direct hardware access to the Tandy DAC is disabled, Int1A-based accesses were, and still are, redirected to the Sound Blaster, which is why most people, including me, were not even aware of that limitation.
Edit: Direct hardware access to the Tandy DAC is now only disabled if the DAC actually does occupy the same port, IRQ or DMA. The original source did not bother to check whether they did, only if a Sound Blaster was active at all.
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Last edited by NewRisingSun
on 2016-12-20 @ 11:34, edited 7 times in total.