The data-channel circuit receives an analog signal from the preamplifier and sends detected data in digital form to the controller. IBM shipped the industry's first PRML channel in 1990 and has continued to focus channel design efforts primarily on PRML. In a PRML data channel the signal is filtered and equalized to a partial-response target. Using synchronous clocking, the signal is analyzed at specific points on the readout waveform where the partial-response target has removed any inter-symbol interference. Data is detected using a Viterbi detector in which all combinations of allowed data patterns are compared with the actual data signal and the best matching pattern is selected. Periodically, the channel is also multiplexed to perform servo-signal processing.