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The Chroma Pressure Sensor & Interface

By David Clarke [21030085++] <>

While the Chroma was reportedly offered with optional pressure support, it appears that keyboards with this option installed are few and far between. [But see Chris Borman's [21030194+] Pressure Sensor Retrofit Kit.] That being the case, not much has even been said about the implementation and details on the option have been sketchy at best.

The following is an attempt to provide some detail on the offering, based on an analysis of the Pressure Sensor recently found in a Rhodes Chroma. The level of detail was chosen so that it was in line with what might have been included a Pressure Sensor entry in the Service Manual (had one been provided.)

See also Chris Borman's [21030194+] Pressure Sensor Retrofit Kit.

What is the Pressure Interface?

The Chroma offers polyphonic pressure support, with the ability for each note being played to be individually affected by a different pressure value.

References to pressure support are usually asterisked and indicated as optional (case in point, the Chroma Product Brochure).

In order to provide support for pressure control, two main elements are required.

First, the Chroma's firmware must possess an ability to manipulate pressure data.

The second (and harder to implement) element is the physical interface to translate mechanical key pressure into something that the electronics inside the Chroma can understand and use.

This physical interface will hereafter be referred to as the "Pressure Sensor."

The Pressure Sensor and the software elements to support it will collectively be referred to as the "Pressure Interface."

Does my Chroma have a Pressure Sensor Installed?

Based on the hardware, it appears that the Pressure Interface was always intended to be part of the Chroma's design. That being said, the Pressure Sensor portion of the interface was not available to consumers during the initial Chroma offering. As a matter of fact, as late as December 1983, the Pressure Sensor was still not complete (ref the note from CBS/Fender in "Chroma Notes #1").

As the interface was only available later in the Chroma's life, and was only offered as an optional addition - most Chromas will not have the Pressure Sensor installed.

That being said, there are no external indications to allow a user to check one way or another.

While some experimentation can be done to attempt to assert pressure on the keys and see if it will affect the sound of a patch, the only certain way to determine if the Pressure Sensor is installed is to open the case of the Chroma and look to see if it is physically present.

From a hardware point of view, the installation of the Pressure Sensor means that the stock damper bar is replaced with an alternate damper. This alternate damper contains a pressure sensing pad and a small amount of electronics to condition the resulting electrical pressure signal.

The pressure damper assembly bolts in exactly as the original damper did, and then interfaces to the I/O Board electronics in the Chroma via a 14-pin ribbon cable.

Figure 1 shows what the a stock (non-pressure sensor equipped Chroma) would like when opened up. Note that the view of the key contacts is unobstructed.

Figure 1: Damper Bar - No Pressure Sensor

Figure 2 presents the same view as Figure 1; however, in this case, the Pressure Sensor (pressure equipped Damper assembly) is installed. Note that the pressure sensor's circuit board partially obscures the view of the key contacts thus giving a clear indication that the Sensor is installed.

Figure 2: Damper Bar - Pressure Sensor Installed

Figure 3 shows the connection of the Pressure Sensor's ribbon cable to the Chroma's I/O board (via J22). On a non-Pressure Sensor equipped Chroma, the socket for J22 is present; however, there is no cable in it.

Figure 3: J22 Connection to I/O Board (Pressure Sensor Installed)

Figure 4 and Figure 5 show the Pressure Sensor as it appears outside of the Chroma. Two views are presented - one from the Top, where the electronics are visible, and one from the Bottom where the pressure sensitive interface is located.

The pressure sensor is constructed with two identical circuit boards affixed to a wooden bar. The right-most circuit board has the electronics populated, and it terminates the ribbon cable. The left-most board connects to the right board via a pair of Mylar connectors, and it does not have any active electronics populated.

Figure 4: Pressure Sensor (Standalone) - Top

Figure 5: Pressure Sensor (Standalone) - Bottom

Figure 6 presents a close-up view of the portion of the pressure sensor assembly containing the interface electronics.

Figure 6: Sensor - Electronics Close-Up

Figure 7 provides a close-up view of the pressure sensitive bar.

Figure 7: Sensor - Key Interface Close-Up

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