Abstract: A cordless telephone has receive (12) and transmit (14) signal paths for passing voice signals. Sidetones normally appear in the receive signal path from the near party's voice. A signal strength comparator (34) monitors the transmit signal path and the receive signal path and asserts a gain control signal when the transmit path signal strength exceeds a threshold set to a predetermined value below the receive path signal strength. The gain control signal decreases the gain (42) in the receive signal path to reduce undesirably loud sidetones in the speaker earpiece. When the transmit path signal strength is less than the predetermined threshold, the gain control signal is not asserted allowing maximum amplification in the receive signal path as the sidetone is sufficiently small as to not interfere with the main received voice signal, or otherwise become noticeably loud in the speaker earpiece.

Abstract: A frequency inversion scrambler in a cordless telephone utilizes an integrated high-pass filter (14) between a first stage low-pass filter (12) and modulator (16) to reduce the filter order while maintaining low group delay in the audio signal. The first stage low-pass filter and high-pass filter remove high frequency components and any DC offset from the filtered audio signal. The modulator translates the spectrum of the filtered signal to sum and difference frequencies. A second stage low-pass filter (18) removes the upper portion of the spectrum such that the resulting frequency spectrum is inverted with respect to the original audio signal to prevent eavesdropping of transmissions between the handset and base unit of the cordless telephone. Another frequency inversion circuit (30, 32, 34, 36) in the base unit inverts the frequency spectrum again back to its original state for transmission along telephone lines.

Abstract: A gain stage circuit having automatic level control is provided. In particular, the gain stage circuit provides an output voltage signal that is a predetermined multiple of the input voltage signal when the input voltage is less than a predetermined threshold. Moreover, the gain stage circuit provides an output voltage that is maintained at a constant predetermined level and independent of the input voltage signal for input voltages that are above a predetermined threshold. The gain stage circuit includes a rectification circuit, a variable gain stage circuit, a current switch and an operational amplifier. The current switch circuit of the present invention provides either an output current of the rectification circuit or a constant current to a reference input of the variable gain stage circuit. Moreover, the variable gain circuit includes a gain input that is coupled to receive a constant reference current.

Abstract: A speakerphone with a varying gain current mirror circuit is provided. The varying gain current mirror circuit is within the DC control loop of the speakerphone such that by varying the gain of the current mirror circuit, the attenuation range of the speakerphone is correspondingly varied.

Abstract: A compandor circuit uses a DC-coupled compressor to reduce the required IC package size to 8-pins. A delta gain stage and rectifier in the feedback path of the compressor circuit receive the AC and DC components of the compressed analog output signal Features of the delta gain stage and rectifier eliminate the need for any AC-coupling capacitors in the feedback path thereby reducing the number of external components The remaining external components may be made smaller and manufactured as surface mount devices An expandor circuit receives a second compressed analog signal and expands it to full amplitude dynamic range.

Abstract: A compressor with a DC bias control circuit is provided. The DC bias control circuit provides a predetermined DC current to a variable gain stage of the compressor circuit such that when the input signal to the compressor is substantially equal to zero, the variable gain stage is biased by the DC current and provides a DC feedback path for the compressor.

Abstract: A full wave rectifier/averaging circuit for processing an input signal having a large dynamic range. A rectification circuit is responsive to the input signal for providing a first signal to a current mirror whereby the current mirror provides a second signal in response to the first signal. An averaging circuit is coupled to the current mirror to perform averaging of the second signal. An output circuit is coupled to the averaging circuit to provide an output signal at an output terminal.

Abstract: A current switch that is responsive to logic switching control signals is used in a telephone circuit comprising a handset receive signal path and a speakerphone receive signal path for separation a received voice signal into either the handset or speakerphone receive path depending on the selected mode of operation of the telephone circuit. The current switch includes a pair of switched current mirrors and clamp circuits which are rendered active or inactive depending on the particular mode of operation. The clamp circuits maintain the dc voltage appearing at the input of each current mirror constant as the particular current mirror is switched between active and inactive states thereby inhibiting audible dc transients.