A page leaves as reflected light, crosses a telephone line as a changing voltage, and comes back as heat on new paper, one row at a time.
A fax never sends paper. The sending machine measures reflected light along one fixed row, turns 1,728 samples into black and white bits, and the receiving machine builds a brand-new image from what it decodes. This exhibit puts two representative Group 3 thermal fax machines on one telephone circuit so you can follow a single row, and a single pixel, from the original to a heated mark.
The data path is computed, not animated. Rows are rasterized from the page, compressed with the real ITU-T T.4 Modified Huffman tables, scrambled and mapped to V.29 signal points, decided at the receiver and decoded independently, so line noise can genuinely corrupt a row. A line-by-line lesson slows one row through exposure, readout, encoding, modulation, decoding, the head's shift register, latch, grouped strobes and paper feed, and a missing ribbon, missing paper or pulled line stops exactly the stage it should.
Scanned, sent, received and printed rows are counted separately, because they are different events: with the paper out, decoded rows wait in the receiver's memory; when the line is pulled, the partial page is kept. Some parts are deliberately simplified. The call setup is a teaching state machine rather than full T.30 signaling, the modem decides symbols at ideal instants with no equalizer, light rays, currents and heat colors are illustrations, and the machines are engineered representations rather than a certified replica of one model.
