whoops! The ESS DMA transfer mode value should be "2" (10b) not "3"
(11b). 11b is a reserved value that so far happened to work! No wonder
the ESS1887 chip in the Compaq Presario was playing audio at 3x the
sample rate! ESS 688 and ESS 1869 code now uses value (10b) instead.
Added TODO to self that the "play with ESS registers" mode should
provide textual documentation on the side of the screen on what that
register is supposed to hold, to aid with poking at the register. Added
code to dim ESS registers that are not present.
Noted: ESS sample rate & lowpass filter clock divider behavior. Setting
the lowpass filter divider to 0xFF silences output. Setting the sample
rate divider to 0x7F or 0xFF causes the DAC to stop playing back audio
(until you set the divider to another value).