I finally had time to probe a few pins inside the Miracle Piano.
AS0012 - Pin 21 (PWM)
It turns out this pin controls the master volume and NOT the envelope as I had originally assumed.
Turning on the keyboard, this pin defaults to about 50%, but as you can see, it is not exactly 50%. How odd.
When the volume is reduced, the pulse width is widened and when the volume is increased, the pulse width is reduced. That is because the integrator, U4A is inverting the output.
Each photo has the scope set to 10 microseconds per division:
U4 - Pin 1
This pin goes to the LM13700 to control the master volume, or gain of the sound. It appears as a DC voltage and is remarkably solid. There is no visible ripple as would be expected from an integrator.
No photo is needed of this signal.
J004 - Pins 21-24 (OUT1-OUT4)
The signals from these pins are too small to view on my scope, so I chose the amplified versions from U10. Pins 1, 8, 7 and 14 respectively.
OUT1 is not active for all sound samples, apparently. When Harpsichord and Synthesizer are selected, nothing comes out at all.
When the other four sounds are selected however, the waveform is perfectly clear.
OUT2 is the opposite. Only when Harpsichord and Synth are selected, do we see any activity on this pin.
OUT3 mimics OUT1 exactly, or at least as far as I can tell.
OUT4 on the other hand does not seem to react to any button or key. It simply outputs a DC voltage which measures 5.03v at pin 14 of U10.
I can speculate as to why there are three OUT pins. While probing both Pins 8 and 14 of U4 which has the mixed and filtered outputs split to both left and right output terminals, the waveforms are identical for Harpsichord and Synthesizer but are not identical for the other four sounds. It would seem that Harpsichord and Synthesizer were recorded and digitized in mono while the other four sounds were recorded and digitized in stereo.
I am a little saddened that the envelope is not broken out to any specific pin or sub-circuit.
Some other Notes
J004 - Pin 40 (Bus Strobe)
This pin reacts differently for different sounds, however, when the key is pressed with a sound that has an attack and release; the pin will strobe for the full length of the sound and go to 0 when the note is finished.
For a sound that ends only when the key is released, the pin strobes at key press for a moment, then goes to 0 and strobes once again when the note is released.
I could use this pin for creating a gate and trigger, but it would require that I charge a capacitor then feed it through a comparator and set up some logic to keep the signal high when the key is pressed and low when the key is released. This wouldn't require too much effort, but the function would change from sample to sample since the envelopes are all different. I don't thin the modification is worth it in this case.
"Piano" sample is very hard to photograph since the amplitude changes so rapidly in software. It looks very similar to "Organ, but is more rounded whereas the Organ is more triangular.
This photo shows a sample that is identical on both sound channels. Trust me, they are identical but the channels are not being displayed the same way.
These two photos show sounds that are not identical from OUT1 and OUT2.
PS: Yes, the reflection in my oscope is naked. Deal with it.