PCM Refactoring

PCM interpolation factored out to a different module.
PCM filter is now softer, still noticeable sound improvement

PCM interpolation

YM2612 did not have internal interpolation for samples Thus, all PCM sounds not sampled at 55kHz have distortion due to spectrum duplication. This version adds a fixed x2 rate scaler for PCM samples using a CIC filter (second order, M=4). The results are very nice with much clearer samples. Bass might be too strong. I may consider compensating for bass and adding automatic sample rate detection to vary the interpolation filter rate on later releases.

Genesis Mixer

-Added sound mixer for FPGAgen
-Uses JT89 with signed output
-Proper interpolation/decimation filter to interface with DAC at 54MHz

This version provides a nicer sound (cleaner and with more potent bass) because the use of filter prevents aliasing and spectrum replication inside the DAC.

Titan 2

-Operator register data is updated every 12FM ticks, like YM3428. This fixes Titan 2 demo.
-Removed multiplexed output. Instead the linear output is now amplified by 1.25 to better use the dynamic range.
-Part bit (address MSB) is now latched when address write occurs (suggested by Alexey)

Better BRAM usage

Changed jt12_op module. Now it doesn't resemble original logic as much, but it saves some logic cells and two M9K memory blocks in MiST implementation.

Full YM2203

Full support for YM2203. Use together with JT49 tag "YM2203".

-Full operator accuracy
-Correct clock dividers
-Pipeline data stream is composed only of three channels
-YM2612 operation unchanged

YM2203

Preliminary support for YM2203. Note that frequency scaling is wrong, so songs will be played at a different pitch than the original. This will be fixed.

Scooby Doo

-Phase increment width fixed
-Scooby Doo now sounds correct

CSMx1

-CSM fixed
-Gain reverted to x1, as it was creating problems with at least one game ("Crying Asia Senmei Sensou")

Todos los santos

-New phase and envelope generators with many bugs fixed
-No known issues at the time of release
-Minor improvement to x2 volume sound accuracy