Weather charts to be transmitted are fastened to a revolving drum and illuminated by a light source. The drum is then scanned by a light sensor moving along the axis of the drum. The voltage output from this sensor is converted into tone frequencies modulating the transmitter.

The number of revolutions per minute (RPM) is a measure of the speed of the drum on the transmitting side. The index of cooperation (IOC) is a measure of the speed with which the sensor moves along the axis of the drum.

A fax transmission begins with a tone of 300 or 675 Hz. The start tone has duration of 5-10 seconds and is very well suited for precise tuning. The frequency of the tone determines the IOC value. Then 30 seconds of alternations between the frequencies representing black and white levels are transmitted, the switching frequency being 1-4 Hz. This sequence carries the RPM information, and the receiver is now synchronized so that the picture will start in the right position. Subsequently the transmission of the picture proper begins.

At the end of transmission the stop signal is sent; this consists of a switch-off signal of 450 Hz having duration of 5 seconds followed by 10 seconds of the frequency representing black level.