Electrostatographic single-pass multiple station printer with improved colour registration

An electrostatographic single-pass multiple station printer for forming an image onto a printing medium, which comprises:

• (i) a plurality of toner image-printing electrostatographic stations comprising
• (ia) rotatable endless surface means onto which a toner image can be formed;
• (ib) means for forming an electrostatic toner image on each said surface means;
• and
• (ic) means for transferring the formed toner image onto a printing medium;
• (ii) means for conveying a printing medium in succession past said stations in synchronism with the peripheral speed of said rotatable endless surface means;
• and
• (iii) register control means for controlling the operation of each of said stations in timed relationship thereby to obtain correct registering of the distinct toner images on the printing medium, wherein said register control means comprises
• (iiia) a main encoder means for producing a first signal indicative of printing medium displacement at a first position, and
• (iiib) delay means for initiating the operation of subsequent image-printing stations after a predetermined printing medium displacement, as measured by the main encoder means, has occurred,
• wherein said register control means comprises a further printing medium advancement measurement means comprising a second encoder means for producing a second signal representative of printing medium displacement at a second position, said printing medium advancement measurement means being adapted to co-operate with said main encoder means for automatic adjustment of said first signal using said second signal.