Laser printing facsimile receiver

Abstract

A facsimile receiver of the type in which a laser beam modulated by a received video signal scans a light sensitive recording paper to print an image corresponding to the video signal. A feedback loop insures that the output of the laser beam acousto-optical modulator (AOM) accurately tracks the demodulated received video signal. A variable transmissivity filter in the path of the laser beam is automatically positioned to provide increased transmittance as the laser beam intensity decreases with aging of the laser, so as to maintain a constant light output intensity. A lens in the optical system which focuses the laser beam on the paper is moved mechanically as the paper is transversely scanned, so that the focus of the beam is varied to compensate for the planar (non-cylindrical) configuration of the paper, thus keeping the beam focused on the paper throughout the length of each scan line. To minimize bleeding or spreading of the image on the paper, the laser beam output is pulse width modulated inversely with variations in the video signal amplitude, so as to maintain a substantially constant energy per pulse. The amplitude of vertical dither is also varied inversely with the video signal amplitude, to minimize spreading of the scan lines at higher light beam intensity levels. Ripple and patterning in the printed image due to D.C. offset are reduced for video signals transmitted by frequency modulation (FM) or by amplitude modulation (AM), by measuring the duration (for FM) or amplitude (for AM) of each carrier half-cycle and adding a D.C. level to the modulated carrier to minimize the differences in duration or amplitude between successive half-cycles. The demodulated FM video signal is automatically offset and scaled to utilize the entire gray scale dynamic range of the paper on which the image is printed.