The final architecture this training module will examine is the sigma-delta. The basic sigma-delta architecture had its origins in systems which used delta modulation or differential PCM for signal transmission. The major part of the early work was done by Bell Labs in the 1950s and 1960s. The concepts of oversampling and noise shaping were studied as part of various transmission system techniques. In a 1954 paper, Cutler of Bell Labs describes single and multi-bit oversampled noise shaping transmission systems. The objective was to transmit oversampled, noise shaped data at a high rate, not to make an ADC. However, the only thing required to turn these systems into a true sigma-delta ADC was the addition of the digital filter, which was a real problem until solid-state digital technology became available.