Abstract: A direct digital frequency synthesizer generates an analog waveform of a predetermined frequency from accumulated digital frequency words which, as accumulated, represent the phase of a sine wave of the predetermined frequency. The synthesizer includes a phase accumulator, a 4-bit non-linear digital-to-analog converter (DAC) and a sample and hold circuit. The phase accumulator includes a 4-bit coarse-component accumulator for accumulating coarse phase components of the digital frequency words and a fine-component accumulator for accumulating fine phase components of the digital frequency words. The phase accumulator increments the coarse-component accumulator in response to the accumulated fine phase components exceeding a predetermined value. The 4-bit non-linear DAC converts the four bits accumulated in the coarse-component accumulator into an analog waveform of the predetermined frequency.

Abstract: A system utilizing bandpass sampling for converting an analog input signal A.multidot.sin ((.omega..sub.o +.omega..sub.s)t+.phi.)) into (I) in-phase and a (Q) quadrature-phase digital signals for digital signal processing, in which image rejection is enhanced. .omega..sub.s is the baseband frequency, .omega..sub.o is the IF center frequency, and .phi. is the phase of the analog input signal. A sequence of digital signal samples obtained by bandpass sampling is processed to derive either the I digital signal or the Q digital signal as a first digital output signal from a first series of alternate samples and to derive the other of the I and Q digital signals as a second digital output signal by computing the average of the absolute values of alternate samples that immediately precede and follow the samples of the first series. In accordance with this processing scheme, there is no phase error, but instead an amplitude error in each I/Q pair equal to the value of cos .omega..sub.s .tau.

Abstract: A system for controlling and accounting for retrieval of data from a CD-ROM memory containing encrypted data files from which retrieval must be authorized. The system includes an authorization and key distribution terminal and a plurality of customer data retrieval terminals having such memories loaded therein. The authorization terminal authorizes retrieval of data by authorized customer terminals by providing to the authorized customer terminals an encryption key for enabling retrieval of the data and a authenticated credit signal for use in limiting the amount of data to be retrieved from the files. The encryption key is encrypted for communication to the customer terminals in unique unit keys that are stored in the respective customer terminals.

Abstract: A phase accumulator for accumulating digital frequency words, which, as accumulated, represent the phase of a cyclic waveform of a predetermined frequency. The phase accumulator includes a coarse-component accumulator for accumulating coarse phase components of the digital frequency words; a fine-component accumulator for accumulating fine phase components of the digital frequency words; and means for incrementing the coarse-component accumulator in response to accumulation of the fine phase components.