Abstract: An improved bias control circuit (FIG. 2) is disclosed for a radio receiver (122) used in a communication system wherein a signalling preamble is sent prior to sending other information. The bias circuit (140) rapidly determines the DC voltage level representing the average frequency of the received signalling preamble (215), develops a DC bias voltage (275) proportional to the error in frequency between the receiver and the transmitted signal, and then selectively applies this bias voltage to the series coupling capacitor (240) located between the receiver detector (121) and the receiver processing circuitry (123). The circuit is adapted for a 0.2 GMSK receiver in a TDMA communication system, where a known 1100 bit synchronization preamble is sent in each user's TDM time slot.

Abstract: A zener diode random number generator circuit is described which produces a random binary number output having a statistical distribution exhibiting a controlled degree of randomness determined in response to an input control signal. A microprocessor feedback circuit monitors the random number output and produces the input control signal in response to the difference between the degree of randomness of the output signal and that of a pre-determined statistical distribution. The digital feedback automatically adjusts the zener diode biasing point and the limiter threshold such that part-to-part tolerance, component aging, temperature variations, or voltage fluctuations will not adversely affect the randomness of the bit stream output. In the preferred embodiment, the microprocessor tests the ratio of ONES bits to ZERO bits of the random number such that a desired 1:1 ONES/ZERO ratio is approximated.

Abstract: A method and means for reducing multipath interference distortion in a time-division multiple access (TDMA) communications system is disclosed. A transmitting station (100) formats one or more equalizer synchronization words with a first user's data word, time-multiplexes this formatted user message with at least one other user message, and transmits the combined signal. The receiving station (200) receives the transmitted TDMA signal, de-multiplexes the first user message from the other user messages, and equalizes the time delay spread characteristics of the data word portion of the first user message (or a stored version of such) in response to this de-multiplexing. Performing the adaptive equalization process more than once per user time slot provides an even higher data throughout for the system. Storing the first user message in a data buffer permits the adaptive equalization process to be performed in non-real time, thereby lowering the required signal processing data rate in any particular receiver.

Abstract: A method and apparatus is disclosed for improving the quality of speech samples communicated via sub-band coding utilizing adaptive bit allocation, by providing error detection only on the adaptive bit allocation information. A first error detection code, such as a cyclic redundancy check (CRC), is calculated on the bit allocation parameters in the transmitter and sent to the receiver, where a second error detection code is calculated based upon the reconstructed bit allocation parameters. The transmitted error detection code is then used to determine if the received bit allocation information is correct, and if not, the frame of speech data is discarded. By protecting only the bit allocation information, additional speech frames may be salvaged from the error-prone channel, thus further increasing speech intelligibility.

Abstract: An automatic frequency control (AFC) circuit is disclosed for use in an SSB radio transceiver having the capability of transmitting and receiving SSB and FM data signals over any one of a number of communication channels having equal bandwidths. The dual-mode AFC circuit includes AFC circuitry which is capable of tracking the frequency of either: (a) a first radio frequency (RF) carrier having voice signals modulated via a signal sideband amplitude modulation within a first channel bandwidth by locking to the reduced pilot carrier transmitter with the SSB signals; or of (b) a second RF carrier having high speed data signals modulated by narrowband frequency modulation within the designated channel bandwidth. The AFC circuit also includes control circuitry to memorize the AFC control voltage during the time the mobile is transmitting, and to assist the acquisition of the FM data carrier by varying the AFC sweep rate on the signalling channel.

Abstract: An improved excitation vector generation and search technique (FIG. 1) is described for a code-excited linear prediction (CELP) speech coder (100) using a codebook of excitation code vectors. A set of M basis vectors V.sub.m (n) are used along with the excitation signal codewords (i) to generate the codebook of excitation vectors u.sub.i (n) according to a "vector sum" technique (120) of converting the selector codewords into a plurality of interim data signals, multiplying the set of M basis vectors by the interim data signals, and summing the resultant vectors to produce the set of 2.sup.M codebook vectors. The entire codebook of 2.sup.M possible excitation vectors is efficiently searched by using the vector sum generation technique with the M basis vectors--without ever having to generate and evaluate each of the 2.sup.M code vectors themselves. Furthermore, only M basis vectors need to be stored in memory (114), as opposed to all 2.sup.M code vectors.

Abstract: An improved noise suppression system (800) is disclosed which performs speech quality enhancement upon the speech-plus-noise signal available at the input (205) to generate a clean speech signal at the output (265) by spectral gain modification. The improvements of the present invention include the addition of a signal-to-noise ratio (SNR) threshold mechanism (830) to reduce background noise flutter by offsetting the gain rise of the gain tables until a certain SNR threshold is reached, the use of a voice metric calculator (810) to produce more accurate background noise estimates via performing the update decision based on the overall voice-like characteristics in the channels and the time interval since the last update, and the use of a channel SNR modifier (820) to provide immunity to narrowband noise bursts through modification of the SNR estimates based on the voice metric calculation and the channel energies.

Abstract: A digital communication system for transmission and reception of speech signals and utilizing sampling, filtering and sub-band coding techniques for reduction of the bit rate. The energy levels of each sub-band are determined with respect to the band having the maximum energy level. The energy level information is encoded and divided into protected and unprotected bits. Only the protected bits are used for the determination of the bit allocation of the sample information, but both protected and unprotected bits are utilized in reconstituting the original levels of the bands.

Abstract: A method and means for maintaining continuous bit synchronization of data transmitted from a remote unit through a base unit to a landline unit is disclosed. An input data stream at a first bit rate, such as digitized or encrypted speech, is interleaved with a plurality of signalling words, and transmitted over an RF channel at a second bit rate. The base site recovers the clock of the received data, strips off the signalling word, modifies the bit rate of the received data, and adjusts the recovered clock rate to provide an output data stream which is in bit synchronization with the input data stream and within a predetermined modem specification. The encrypted data is then sent over landlines to a decryption unit which requires bit synchronization.

Abstract: A system and method for error correction in communication systems for transmission of digitized speech signals. Sub-band coding is preferably applied to the input signal, and the sample information and the side information provided. An error detection code (EDC) is applied to at least a portion of the side information and an error correction code (ECC) is applied over the side information and the EDC. A second EDC is applied to the ECC. These information signals and error detection and correction codes are multiplexed with the protected portion of the side information and the error correction code separated by at least the sample information. Speech signals are thus transmitted and reconstituted with improved quality with a minimum data rate required.

Abstract: A method and means for filtering the quantization noise from the output of a 1-bit analog-to-digital converter (noise-shaping coder) is disclosed. The A/D utilizes oversampling of the analog input signal, and decimation of the digital output signal. The multiplierless low-pass filter is comprised of a coefficient ROM for storing the filter coefficients, a counter for addressing the memory, and a true/complement gate for selectively complementing the output of the memory in response to the 1-bit data stream from the noise-shaping coder. An accumulator sums the selectively complemented output words for all samples, and the accumulated output is then applied to the decimator. A second embodiment is also disclosed which utilizes an overlapping digital filter approach, wherein a plurality of digital multiplierless filters are overlapped to provide an arbitrary length filter capable of producing an arbitrary filter response.

Abstract: A memory programming system which provides a method and apparatus for programming and reading an electronic device memory through its power source connections. Circuitry is provided for programming and reading internal EEPROM memory of a portable radio with information provided by an external programming apparatus through its existing battery terminals, wihout removing the EEPROM from the unit, and without the use of supplementary connectors. The radio battery is disabled and a substitute power source is connected to the existing battery terminals, the substitute power signal is encoded with program data by the programming apparatus circuitry and decoded by the radio circuitry, and a programming voltage is applied to the EEPROM such that the program data is programmed into the radio memory through the battery terminals. The program data can be verified by encoding the substitute power signal with verification data in the radio and decoding the signal externally to provide a visual indication for verification.

Abstract: An improved hands-free user-interactive control and dialing system is disclosed for use with a speech communications device. The control system (400) includes a dynamic noise suppressor (410), a speech recognizer (420) for implementing voice-control, a device controller (430) responsive to the speech recognizer for controlling operating parameters of the speech communications device (450) and for producing status information representing the operating status of the device, and a speech synthesizer (440) for providing reply information to the user as to the speech communications device operating status. In a mobile radiotelephone application, the spectral subtraction noise suppressor (414) is configured to improve the performance of the speech recognizer (424), the voice quality of the transmitted audio (417), and the audio switching operation of the vehicular speakerphone (460).

Abstract: A retractable antenna assembly is disclosed for tuning an antenna having a helical section to frequency by selectively positioning an appropriate tuning core within the helix. The antenna frequency can either be raised or lowered through the use of conductive or permeable tuning core compositions. The core positioning mechanism is implemented by affixing the tuning core to a portion of the antenna supporting rod slideably located within the helix. This helical antenna/tuning core configuration is readily adaptable to miniature portable radios by providing a helical antenna assembly which is retractable within the radio housing in the receive-only or standby mode, and which is outwardly extendible from the radio housing for use in the active transmit/receive mode. A unique barrel-cam latching mechanism is also described.

Abstract: An improved antenna configuration is disclosed for a fully duplex portable radiotelephone that is normally operated in the nearly horizontal position next to the user's ear and mouth. A notch antenna is provided in the bottom portion of the portable radio transceiver parallel to the major longitudinal axis of the housing. The notch aperture is cut in the conductive radio housing at a transverse angle to the major face plane of the radiotelephone to form a notch antenna which radiates predominately vertically polarized E-field waves when the transceiver is positioned such that the major longitudinal axis of the radio is approximately 30 degrees to the horizontal. The notch is an effective quarter-wavelength long and is dual-fed from a pair of coaxial cable transmission lines near the shorted end of the notch.

Abstract: An electronic apparatus comprises a housing having walls of an electrically conductive material and together defining an inner space. The walls of the housing comprise outer walls, and partition walls extending from one of the walls of the housing into the inner space of the housing dividing the inner space into different compartments. The apparatus further comprises an electronic circuit board having electronic components mounted thereon and is provided with apertures through which the partition walls extend. In order to shield the electronic circuits of the individual compartments in relation to each other a first shielding component is provided.

Abstract: An improved switched capacitor circuit configuration is disclosed which exhibits improved low voltage supply operation and improved charge injection cancellation performance. In the preferred embodiment of a switched capacitor integrator, the improvement is realized by the particular configuration of a switched tub switch on the input, single N-channel devices at the summing junction, skewing the analog ground V.sub.AG downward, and delaying the clock signals to turn off the switches at the summing junction before the charge injection of the switched tub switch is realized.

Abstract: A differential input circuit for a switched capacitor CMOS voltage comparator is provided which minimizes offset voltages by configuring the load devices to utilize a single switched capacitor biasing network initialized from internally-generated bias voltages, while configuring the initialization switches for the differential input devices to also utilize internally-generated bias voltages such that the offset voltages are stored on the input capacitors. The power supply rejection performance of the voltage comparator is also optimized by connecting parallel load devices of opposite switching topology such that the same input impedance is seen at both load terminals.

Abstract: A CMOS data clock oscillator circuit is disclosed which provides a simple, inexpensive, high-specification oscillator with an accurate duty cycle. The data clock oscillator 100 includes an oscillator stage 121 providing an AC output signal having an average DC value determined by an applied bias voltage, a limiting stage 124 having MOSFETs 102 and 103 configured to form a back-to-back limiter network to limit the amplitude of the AC output signal, a CMOS biasing stage 122 including complementary MOSFETs 104 and 105 configured to form an active resistor voltage divider network providing the bias voltage for the oscillator stage, and a CMOS buffer stage 123 including complementary MOSFETs 106 and 107 configured to form an inverting amplifier network having a predefined input switching threshold. Buffer stage MOSFETs 106 and 107 have conduction types, geometries, and device parameters matched to those of bias stage MOSFETs 104 and 105, respectively.

Abstract: An improved method of adjusting piezoelectric devices to frequency by particularized laser trimming is disclosed. The laser removes a single contiguous area in the center of the electrodes to appropriately increase the individual resonator frequency without significantly degrading the filter "Q". In an AT-cut monolithic crystal filter, the particularized laser trimmed area can be positioned such on the face of the electrodes to simultaneously control frequency and bandwidth. This method can be substituted for the more cumbersome process of frequency adjustment by vapor deposition to provide a rapid inexpensive technique for monolithic crystal filter manufacture.