The long promised RPCube is finally available for purchase on RadioPopper’s online store for $29.95, according to an announcement in a blog post on RadioPopper website.
With this little “cube”, users of RadioPopper JrX Studio receivers can remotely control the power levels of their Canon/Nikon-compatible flashes by turning the dials on the JrX Transmitter or pushing buttons on PX transmitter. Upto 3 groups of flashes can be controlled.
How does it work?
It is no secret that the flash output levels of most Xeon-tube based portable flashes are controlled by controlling flash duration. The camera first triggers the flash to fire then shut it off when a pre-determined flash duration is reached or amount of reflected light is received. On the camera and flash hot shoes, there is one terminal for triggering the flash (X-Sync) and another one for stopping the flash (Quench).
On Nikon TTL hot shoe, the largest contact terminal in the middle is the X-Sync terminal. The bottom right on is the Quench terminal. Both contacts are hot (with a voltage above the ground) during standby. The camera pulls the X-Sync terminal to groud to fire the flash and pulls the Quench terminal to ground to stop the flash. The Canon system has different hoe shoe contacts arrangement and the opposite stop voltage. The Quench pin is at ground level during idle and are only raised to higher voltages when a quench signal is sent.
(The following is a guess!)
The JrX system controls the flash output by controlling the flash duration. The transmitter reads the dial position (potentiometer) and converts the reading to corresponding flash duration then sends the data over RF to the receivers. The receivers receives the flash duration data then waits for the final trigger signal. When the final trigger signal is received the receiver first sends the flash fire signal through the X-Sync terminal of the flash the sends the Quench signal through the Quench terminal after the specified flash duration.
Since Nikon and Canon have different contact terminal arrangements and signaling specifications, a Nikon RPCube is needed for Nikon flashes and a Canon RPCube is needed for Canon flashes. The Canon version has circuit to invert the Quench signal level from the JrX Studio receiver so the output is compatible with Canon TTL flashes.
Not all flash are created equal in terms of flash tube discharge characteristics. The flash duration for certain amount (Guide Number) of flash output is different for different flashes. Therefore a warning is provided in the RPCube User Manual.
Each 1/8th turn of the dial on the JrX Transmitter will represent 1 stop of flash power. Note that due to the wide variety of flashes available, each 1/8th turn may be slightly greater or slightly less than 1 stop of flash power.
In addition, the JrX with Nikon RPCube does not work with Nikon SB-900. The SB-900 appears to only support iTTL, not the old TTL. In iTTL, flash stop is done within the flash using the amount data transmitted from the camera digitally. If you are interested, please read this post.
Third party product
Do-it-Yourself (DIY) RPCube
If you know some basic electronics and are handy enough, you can try the DIY route. The Nikon version is fairly easy to make at very low cost. You only need a $4 Nikon AS-E900 TTL adapter and a stereo audio plug.
The author and the website (DPTnT) is not affiliated or associated with RadioPopper.
Keywords: Flash photography, JrX, JrX Studio, PX, RadioPopper, RPCube, TTL