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Pressure Sensor & Interface: Q&A

By David Clarke [21030085++] <>

Q1: There are 64 separate pressure pads. Does that mean that there are 64 different pressure values controlling the Chroma at any one time?

A1: No. At most the Chroma is 16-note polyphonic. From that point of view, there can only be 16 notes sounding at any one time, and hence only 16 notes can be affected at any one time.

The Chroma manages pressure through a "key pressure list." In effect, it knows what notes it is currently playing and only requests pressure information for those notes.

Q2: If the Chroma internally manages pressure by note, what would happen if the Chroma hooked up via MIDI, and had 8 different patches selected (8 different channels), all playing the same note, at the same time as pressure was applied to the keys on the Chroma.

A2: This would not cause a problem. While pressure is managed by note, the firmware does also have the concept of an 'instrument.' Pressure data is also instrument specific. By default, the keyboard will generate data for Instrument 0, or Instrument 0 and Instrument 1 (if a link/split is in effect).

Q3: In sequencer mode, I can have many more than 16 notes queued to be played. How does pressure information interact with those notes? What about if notes are arpeggiated?

A3: As noted in the writeup for the Pressure command in the Chroma Computer Interface Manual, pressure commands are only really relevant between the corresponding Attack and Release command for that note.

In the case of a sequence, only the note numbers/velocity are stored, not their pressure. So - when the sequence is played back, there is an Attack event. If pressure is applied before the Release event for that note, then pressure can affect the sound of the note. If a different pressure is played the next time that note comes up in the sequence, then it is used to affect the sound. The same situation exists for notes played due to an arpeggio.

Q4: If I don't have the Pressure Sensor installed, can I still use Pressure Commands via MIDI?

A4: Yes. The Chroma's incoming support for pressure commands over the Chroma Interface (and therefore, via MIDI) is unrelated to whether or not the Pressure Sensor is installed.

The absence of a Pressure Sensor simply means that true key pressure can not be generated by the Chroma's keyboard.

Q5: The Chroma is supposed to have Polyphonic aftertouch/pressure response, yet when I look at the pressure output from my Syntech/Chroma Cult MIDI interface I see channel pressure not key pressure. What gives?

A5: This MIDI interface can be configured to send either Channel Pressure or Key Pressure. To configure it for the other mode of operation, go into the configuration menu (SET SPLIT 46) and select parameter 16.

Q6: Can I build my own pressure interface?

A6: Technically - yes.

From the Chroma's point of view, it really only needs to see a voltage from 0 to 5v present on the PRESS signal on J22, the 14-pin connector from the I/O board.

In terms attempting to duplicate/replicate the original interface, that might be a bit tougher, given that the specific parts that make up the pressure sensor (felt, rubber, conductive bit, foam, etc.) were a custom build. It is unlikely that an identical configuration could be made today. Of course, you might not have to be exact to get something that would 'work' - but in terms of making a 100% identical replacement, it is unlikely that the exact piece-parts would be available now.

Q7: I issued a SET SPLIT 35 command to connect the Pressure Sensor. I then saw that I did seem to get pressure data out of the Chroma - but I've opened it up and I don't seem to have the Pressure Sensor installed. How can this be explained?

A7: Two things could come into play here. First, with no pressure sensor installed, the PRESS input to the ADC is not connected to anything - and so it could pick up spurious noise inside the chassis. The ADC doesn't know if the data is correct or not - it simply interprets any voltage it might see as being valid. So, in effect, any noise will be interpreted as a valid pressure signal.

There is a second reason, and this has more to do with how the Chroma designers decided to handle "initial" pressure. The keyboard will attempt to take its best guess at an initial key pressure, based on the velocity of the key strike. For very fast key strikes, a high initial pressure is assumed. For slow (and medium) key strikes, no initial pressure is assigned. Given this mode of operation, it is possible for the Chroma to respond with this 'initial' pressure value, even without a pressure sensor installed.

Q8: There looks to be green felt residue on the top of my Pressure Sensor Damper Bar - why?

A8: It appears that instead of using new damper bars, existing ones were converted for pressure sensor use. In those cases, the original green felt was removed, the bar was flipped over and the pressure damper interface was connected to the other side.

Q9: I've heard that the pressure sensor really affects the way the keys feel - is this true?

A9: Well - it depends what you mean. The regular key-press is about the same (since the depth of the felt on the original damper bar and the height of the pressure interface is about the same).

That being said, if you wish to make use of the pressure, a different amount of force would be needed to elicit pressure responses from the keys.