A testable assembly that includes a plurality of recording elements such as LED's for burn-in testing or the like is described. The assembly includes LED chip arrays, integrated circuit driver chips and spreader boards for distributing signals to the driver chips. The spreader boards each include a minimum number of test pads for providing power and control signals during burn-in testing. The driver chip is designed for grey level recording using a plurality of data bits to control the pixel recording period for each LED. Burn-in testing is accomplished without even provision for real data signals. Rather, pseudo data signals are generated internally by a weak current pull-up circuit associated with each data input lead of the driver chip. This pull-up circuit is easily overridden by normal data signals during normal printhead operation. In addition to power signals, an exposure clock and latch signal are provided at the input pad for varying power and duty signal to the LED's during testing.

A method of improving the grayscale resolution of a bi-level, light emitting diode type page printer includes increasing the frequency at which printing data is entered into an LED head and correspondingly increasing the frequency at which the LEDs are actuated. The rate at which an image drum is moved past the LEDs is decreased and the feed rate of paper flowing through the printer is correspondingly decreased. The duration of each actuation of the LEDs is decreased to produce dots on the image drum and on the paper which are of a small size.

A reflex printing device having multiple print heads mounted at different locations around the circumference of the drum at different "angles" and an encoder disk mounted on the drum to allow for detection of the drum position as a function of time. An image defect due to a misalignment in the print process direction of the output from the multiple print heads is corrected by detection of an encoder position error function subtracted from itself shifted by the angle between the print heads.

Methods and apparatus are provided for forming images on a moving, charge retentive surface using a stream of radiant energy, modulated in imagewise fashion, wherein the intensity of the stream of radiant energy is controlled based on measured variations of the actual speed of the imaging surface from a set speed. In particular, a motion encoder, which is preferably the same motion encoder previously used to control the proper location of each line of information on the imaging surface is used to monitor the actual, instantaneous speed of the imaging surface to produce an actual speed signal. This actual speed signal is compared to a set speed signal to produce a speed variance signal which represents the difference between the actual imaging surface speed and the set speed. The variance speed signal is then used to control the intensity of the stream of radiant energy.

In an ionographic device having a printhead, including a source of ions, a modulation channel defining a path for a stream of ions from the the ion source towards a moving imaging surface, an array of modulation electrodes arranged along the path to modulate the stream of ions in imagewise fashion, a gating electrode is provided at a location adjacent to the path of ion stream, biased to create a field to block or allow passage of the ion stream in accordance with movement of the imaging surface, the biasing arrangement allowing passage of ions for a maximum period over which ions for a line of the image are deposited on the moving imaging surface. The bias on the electrode is controlled in accordance with movement of the imaging surface, as detected by a motion detecting encoder, and in accordance with a predetermined period to produce a constant charge density on the imaging surface at the writing position.