Abstract: An overload detector for an analog-to-digital converter. A series of logic gates are connected to the output of an analog-to-digital converter for determining the presence of an upper limit and a lower limit of an overload condition. A signal indicative of such a condition is input to circuitry which extends the length of the signal so that it is visible or audible to a user. An algorithm for a computer causes a latch to be engaged when an overload condition occurs. The latch is coupled to a pulse stretching circuit which permits a visible or audible signal to be generated.

Abstract: An electronic cryptographic encoder-decoder is disclosed which performs the equivalent operations of a known simple hand-held cryptographic encoder-decoder. The electronic cryptographic encoder-decoder includes a parallel addressable, programmable read only memory receiving parallel data input from a serial to parallel converter and providing parallel data output which is converted to a serial data output by a parallel to serial converter. The memory element has stored therein a plurality of different predetermined encryption-decryption character code groups addressed by the parallel input data and controlled by a character counting arrangement including a multiplicity of multi-contact switches to establish a unique code and to select appropriate ones of the plurality of code groups as determined by the unique code to provide the parallel output data from the memory means.

Abstract: The channel selector employs a technique in which a tone calling system is combined with a scanner. The combination of these devices provides the user a convenient method of effectively monitoring all channels and automatically selecting the channel upon which communications can take place.

Abstract: Upon operating a key on a phonetic keyboard, the clock circuit gate is opened to permit the counters, normally held in reset condition, to begin counting. Each of the memories (organized in 2048 .times. 8 bits) that are connected in parallel to the counters begin to emit the digital representation in CVSD (Controlled Variable Slope Delta Modulation) form of the repertoire of phonetic sounds with each memory containing the entire phonetic alphabet. The keyboard switch selects the desired data stream output from an addressed memory which represents the chosen phonetic sound, and guides this data stream to a CVSD demodulator, thus producing an analog representation of the desired sound. This sound may be amplified and reproduced by means of a loudspeaker or alternatively, it may be amplified and placed on a telephone line. A feature of the invention is a stop decode gate which automatically stops the counting for a short phonetic sound.

Abstract: A two-tone decoder decodes simultaneously at least two calling tones each having different frequencies coupled from a calling station to a called station and produces a first control signal upon cessation of the two calling tones. A timer responding to the first control signals produces a second control signal in the form of a pulse having a predetermined duration. A relay is activated by the second control signal and couples a confirmation tone to the calling station from a confirmation tone generator in the called station for the predetermined duration of the second control signal.

Abstract: A stable generator produces a reference square wave signal having a stable given frequency. Logic circuitry is coupled to the generator to provide a plurality of square wave signals each having a different predetermined frequency, each of the predetermined frequencies having a different predetermined relationship to the given frequency. A switching arrangement is coupled to the logic circuitry to select at least one of the plurality of square wave signals of the logic circuitry. A ring counter, resistor tone mixing circuit and filter are coupled to the logic circuitry to provide as an output signal of the digital tone generator a sine wave tone having a frequency equal to the predetermined frequency divided by a given factor of the selected one of the plurality of square wave signals.

Abstract: A microfilm instruction system provides detailed service instructions for diagnosing and repairing complex electronic instrumentation. Troubleshooting procedures are recorded on microfilm along with a series of programmed instructions for directing the service repairman step-by-step to pinpoint the source of the problem and to make the necessary repair. Each frame of microfilm containing a sequence of question sets which appear upon the display plate of the microfilm viewing system further provides a corresponding series of response switches also located on the microfilm viewing system. Each of the questions relate to a possible condition for the malfunctioning electronic apparatus to be repaired and each series of questions requires an affirmative or negative response by means of the response switches.

Abstract: Two-wire telephone lines connecting between subscriber telephones and the central office telephone system are coupled to four-wire transmission lines within the central office by means of phototransistors and light emitting diodes. The use of the phototransistors and light emitting diodes provides two-wire to four-wire conversion between balanced two-wire lines.