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 Forum index » Clavia Nord Modular » G2 Patches - Completed » Classic
Yamaha CP-30
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jdd



Joined: Dec 01, 2015
Posts: 11
Location: Portland Oregon USA
G2 patch files: 3

PostPosted: Sat Jan 02, 2021 9:29 pm    Post subject: Yamaha CP-30
Subject description: Late 1970's electronic piano
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I originally know the Yamaha CP-30 from The Cars' first album. It's a big instrument, and I don't think I have space for a real one. Lately I have researched it from YouTube videos, old service manuals on the internet, and the great new Lenhoff & Robertson book "Classic Keys".

The CP-20 and CP-30 are electronic pianos of the late 1970's. The CP-20 had 5 octaves of weighted, pressure-sensitive keys. It could produce two "piano" sounds and two thin "harpsichord" sounds, which can all be combined in ways that resemble various electric pianos. There is a tuning knob, a knob to vary the decay rate, and an on-board tremolo effect. When you combine two patches and turn up the volume, the percussive note attacks can overdrive the preamp in a nice gritty way.

The 76-key CP-30 was a big, heavy keyboard with an integrated flight case, meant to more reliably take the place of various electric pianos on tour. You might call it the "Nord Electro" of its day. It was Yamaha's flagship electronic piano from its introduction in 1976 until the DX-7 arrived in 1982. The CP-30 substitutes a third piano for one of the harpsichords, and is often described as two full CP-20s of slightly differing timbres. The two voices are hard-panned left and right and and usually detuned slightly from each other to produce a chorus effect. None of the piano sounds by itself is very impressive, but the touch response, the overdriven transients, and the chorus add up to make the CP-30 a nice instrument.

How would you build a fully polyphonic and touch-sensitive electronic piano in the late 1970s? Yamaha based the design around a custom 15V hybrid analog/digital IC, alternately called the YM253 or YM25300. The chip has 12 analog input pins, which each accept a control voltage from one key that represents key velocity and sustain. It also has one digital input, for a high frequency clock signal, presumably around 2 MHz in the top octave. Taking a key input, it divides the clock signal to derive a frequency, amplifies it according to the voltage on the input, and generates two separate audio signals called "Attack" and "Sustain" (or "A" and "S"). A single "A" pin and a single "S" pin carry the audio for all 12 keys in the octave. The chip also divides the clock frequency by 2 and provides this on a digital output pin, to supply the chip for the next octave down. On each per-octave circuit board, the "A" and "S" signals are mixed together and filtered in various ways to produce the instrument's four tones. Yamaha put all of the filters on the octave cards, but following the G2's architecture I have put the P1 and P2 filters in the top half of the patch, and P3 and H1 in the bottom half.

Yamaha's CP-30 service manual reveals the harmonic content of the A and S outputs, before filtering, when 5V is applied on a keyboard input pin. On the YM253, "S" is a 75% pulse wave, and "A" is a complex pulse wave you could call "00111101" (I drew this on a sequencer that runs at audio rate). The second voice of the CP-30 is produced by a variant of the chip called the YM252, where "S" is an 87% pulse wave and "A" is 01111101. The bottom octave of the CP-20 uses the third member of the family, the YM25301, but I don't know how it differs from the other two. The service manual also gives the amplitude for the A and S signals at 5V input; I assumed that the relative amplitudes vary with the input voltage and decay time, but that the waveforms are constant.

Officialy, tremolo speed varies from 0.5 to 12 Hz; maximum tremolo intensity is 70%; and pitch can vary from 437 to 453 Hz. I recreated the front panel pretty faithfully, but did not enforce these limits.

I have some possible future improvements in mind. The top octave is currently useless. I got an idea, listening to the originals, that P2 and P3 have slightly different decay characters and somehow combine to form a complex decay -- though I don't actually see in the schematics how that would happen. And I'd like to do a better job of reproducing the "bark" in the attacks when multiple patches are combined.

And I might post some more patches that take some of the good ideas in the CP-30 design and apply them in a way that's more idiomatic to the G2.


CP-30.pch2
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tristanshout



Joined: May 20, 2005
Posts: 4
Location: half moon bay, ca

PostPosted: Sat Oct 02, 2021 3:39 pm    Post subject: Re: Yamaha CP-30
Subject description: Late 1970's electronic piano
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An important thing to know about the YM chips is that the attack and sustain wave level response to the input control voltage is different. The attack wave dies away fairly early while the sustain wave continues to sound. That's what give it its bark. My guess is that there's some non-linear process applied to at least the attack wave levels, possibly square or cube.
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jdd



Joined: Dec 01, 2015
Posts: 11
Location: Portland Oregon USA
G2 patch files: 3

PostPosted: Sat Oct 02, 2021 5:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

That's interesting!

I have not come back to this patch in a while. I suspect first that I could find a better way to produce the basic waveforms, and then apply this little bit of information about their different responses.
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