Data transmission system

996,409. Telegraphy; multiplex pulse code signalling. INTERNATIONAL BUSINESS MACHINES CORPORATION. Nov. 21, 1963 [Dec. 18, 1962], No. 45925/63. Drawings to Specification. Headings H4L and H4P. In a data receiver to counteract drift in the clamping circuit that occurs when the data has one value for some time a D.C. feed-back path is provided from the output of the receiver to the input. In the embodiment described two binary messages are transmitted as a single 4-level message by adding one to the other doubled in amplitude. This message is preceded by a signal which causes a clamp in the receiver to place the received data at the correct level. The data is fed to threshold circuits which provide three signals indicative of the level of the data. These signals are fed to logic circuitry to derive output signals the equivalent of the original two binary messages. The outputs from the logic are combined to give a four-level signal which acts in the D.C. feed-back loop to stabilize the D.C. level at the receiver input.