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Oct 16, 2013, 10:19 PM
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Another round of CP33 hacking begins

Finally discovered the CP33 can be plugged into USB & recorded with arecordmidi. The result can by played back through the CP33's synthesizer using aplaymidi. Should have gotten around to that, 4 years ago.

After many days of banging on Cubase, Ivory, Virtualbox, alsa-utils, & waiting for Adsense prerolls, finally got the CP33 output to feed through a German Steinway synthesizer. Audio doesn't work on VirtualBox, so everything has to be rendered. Cubase crashes a lot.

Unfortunately, Ivory truncates the notes to 1.4 seconds, pedaling or no pedaling. This might be a problem with the pirated version, the virtual machine, or an obscure Cubase setting. The Mystic synthesizer in Cubase doesn't have the truncation problem.

There might be another piano synthesizer, but the only problem with the CP33 sound is the extreme hissing. It's otherwise a better sound than Ivory. The Ivory sound is more out of tune & uneven. It might just bring out finer detail in the velocity. The problems are masked by ambiance & EQ. People like Ivory because of the realism, not because of how ideal the sound is.

The CP33 sound now has a lot of history to it. It's the sound of the Jesus Heroine's era. There's a lot of motivation to keep it instead of changing to a Steinway.

Ivory is everywhere & nowhere. There are no recordings of complete performances, no documented problems with it, no documented presets for it. The default settings are worthless. It needs a lot of tweeks to sound decent.

Trying to find any information on Ivory is like being in 1995 again. Goog has a lot less information on the Ivory/Cubase world than it does on the Android/science/politics world. There's no for Ivory, no google code for music. Like the early internet, it's still a ghost town outside the core computer science material because the people who create most of the internet are still manely in the gadget/computer science world.

Getting digital audio out of the CP33 would probably eliminate all the hissing & provide the highest quality recording. It was long believed to be impossible, since there was no external DAC in the original photo.

It was time to open it again & see if any digital signals were exposed on the system on chip or if there was a higher quality analog signal. A quick feel of the board's underside revealed more flat plastic not detected 4 years ago, because it required flexing a lot of cables to get to.

There was indeed a double sided load, with an identical Yamaha SOC as the one on top, a few flash chips, & right next to the audio output header

an AKM4385 192khz 24 bit DAC. It was the only DAC on the entire board, so it was probably where the magic happened. The Yamaha SOC was still old enough that all the analog conversion was done offboard. Newer pianos probably aren't as lucky.

The Yamaha SOC's must be chained somehow, with each one mixing 32 voices into the single digital audio stream. That would allow different products to have multiples of 32 voices.

The DAC runs at 44.1khz. It's fed by an SPI at 2.8Mhz. The framing is done by a left/right select pin. Each channel gets a 32 bit word representing the value. Only the most significant 24 bits are converted. The configuration pins are all floating. It just reads interleaved 32 bit words.

The SPI signals & left/right framing pin are connected to a line driver on the top of the board, allowing a bundle of wire to easily reach the signals.

Since an 8 bit PIC requires major effort to get above 2megabits of throughput, an STM32 will be required to capture the digital audio & stream it over USB. The mane unknown is where to have the USB connector.
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