Abstract: A modulator apparatus for modulating arrays of input data V.sub.in to be stored in a holographic recording medium is disclosed wherein the final output data array V.sub.out has frequent transitions from light to dark and from dark to light in either dimension across the data page and has the total amount of illuminated regions throughout the entire data page held constant. These two constraints are achieved by a first set of control arrays obtained from two fixed sets of m.times.n binary arrays {A.sub.0, A.sub.1, . . . , A.sub.n } and {B.sub.0, B.sub.1, . . . , B.sub.m } which in turn are obtained from fixed sets of binary control vectors {a.sub.0, a.sub.1, . . . , a.sub.n }, {b.sub.0, b.sub.1, . . . , b.sub.m }, respectively. The control vectors a.sub.0, a.sub.1, . . . , a.sub.n are any n+1 fixed elements of the inverse mapping, .phi..sup.-1 (C.sub.1), of the (t-2) error-correcting code C.sub.1 of length m. The control vectors b.sub.0, b.sub.1, . . . , b.sub.

Abstract: Two-way telephone communication is established between a subscriber-dwelling and a remote station using the automatic dialer of existing security apparatus. A speaker and a microphone are placed in the dwelling. A diplexer electrically coupled to the automatic dialer and responsive to a tone of a given frequency, operates a switch which selectively applies either the microphone or the speaker to the audio circuitry of the dialer. When the dialer is triggered, the switch couples the microphone to the audio circuitry of the dialer whereby sounds in the dwelling are communicated to the remote station. When an employee at the remote station applies the tone to the link, the diplexer causes the switch electrically to isolate the microphone and couple the speaker to the dialer whereby the employee of the station can communicate with the dwelling. Alternate operation of either the speaker or microphone prevents accoustical feedback between the two.