A circuit that creates a queue or "bucket brigade" of input values. Both analog and digital shift registers exist and are used in synths. An analog shift register consists of a sample and hold input circuit, a clock input, a number of intermediate storage stages, and one or more outputs. Each time the clock input goes from low to high (or vice versa, depending on the circuit), the sample and hold captures the input value and stores it. On the next clock pulse, before the sample and hold captures another input, the previous value is passed down to the next stage; the previous value that was in that stage is passed to the further next stage, and so forth. The value passed to the final stage on a given clock pulse appears at the output. Intermediate stages might also have outputs. Analog shift registers are often used to create arabesque patterns, which repeat in a semi-random manner, by using various functions to feed the outputs back to the input and cause them to alter the input in some fashion.
A digital shift register works in a similar fashion, but the input captures digital bits instead of analog values, and each intermediate stage stores one bit. Digital shift registers usually have at least one intermediate-stage output, which will often be combined with the final output using logic functions to generate the input for the next shift. Various mathematical formulas exist which can be applied to a digital shift register to create pseudo-random bit patterns, ranging from short repeating patterns to patterns which can shift many thousands of times before repeating.