ction of the hammer flight times. Any variation in the hammer times results in misregistration and slur. Matrix line printing can also be performed by a drum configura- tion. A "bar helix" line printer uses rotating helical wires wrapped in a drum and a horizontal hammer bank with sharp hammer faces to form print dots. By properly timing the impacts of the line ham- Carriage motor Paper Ribbon Print hammers Platen Type band Type band motor Timing marks Emitter pick-up Fig. 4-6. Band printer. 60 TELECOMMUNICATIONS mers, the intersection of the helical wires and hammer surfaces cre- ates a line of dots as the paper moves vertically. Because of the continuous paper motion, bar helix printers have the advantage of high speed. However, bar helix printers usually do not have supe- rior print quality. The electrophotographic printers (laser printers) (Fig. 4-7) are based on the same basic principle as a copier machine. This non- impact machine uses an optical system to image a document onto a rotating photoconductor drum. The charge image on the drum is toned by charged toner and transferred to plain paper electrosta- tistically. The transferred toner on the paper is fused by a hot roll or radiation for the final output. A laser printer uses a modulated laser beam which scans across the photoconductor drum to paint the charge image on the drum point by point. An ink jet printer can be operated either in a continuous synchron- ous form or in a drop-on-demand mode. As shown in Fig. 4-8, a continuous ink jet forms a stream of ink droplets about 2 to 4 mils Polygon mirror Rotation axis Cylindrical lens Toroidal lens Beam expander Doublet Acousto-optical modulator spherical lens Beam compressor Laser Scanline Photoconductor surface Fig. 4-7. Laser printer. TERMINAL TECHNOLOGY 61 High-voltage electrode Gutter Charge electrode High-voltage Nozzle electrode Fig. 4-8. Ink jet printer. in diameter by pumping ink through a nozzle. These ink droplets are charged to different voltages by a voltage charge electrode and then deflected