This analogy also works for a delay-line based computational model, as depicted in Fig.5.5. The magnetic tape is now the delay line, the tape read-head is the read-pointer of the delay line, and the write-head is the delay-line write-pointer. In this analogy, it is readily verified that modulating delay by changing the read-pointer increment from 1 to (thereby requiring interpolated reads) corresponds to listener motion away from the source at speed . It also follows that changing the write-pointer increment from to corresponds source motion toward the listener at speed . When this is done, we must use interpolating writes into the delay memory. Interpolating writes may be called de-interpolation , and they are formally the graph-theoretic transpose of interpolating reads (ordinary ``interpolation'') . A review of time-varying, interpolating, delay-line reads and writes, together with a method using a single shared pointer, are given in .