Abstract: A battery heater uses the internal resistance of the battery as the battery heating element. In one embodiment, a DC battery charger and a programmable battery load, is used in a closed loop temperature control system. In another embodiment, a DC battery charger and a resistor or small auxiliary battery heater, is used in a closed loop temperature control system. The programmable load or the small auxiliary battery heater, is enabled during full charge, low temperature conditions. The resistor or small auxiliary heater serves to both heat the battery and draw down the battery charge. In yet another embodiment, an AC heater power supply and a DC battery charger, is coupled to the battery in a closed loop temperature control system.

Abstract: A method of operating a digital signal processor to detect DTMF tones in a digital voice telephone system in which the digitally encoded signals appearing on the telephone channel are decimated to compress the spectrum to be monitored for the appearance of call signalling tones. The signals received in a decimated block are "correlated" or convolved with one another on a forward and backward time-shifted basis and each forward and backward correlation product is summed to form the elements of a 5.times.5 modified covariance matrix. The modified covariance matrix exhibits the desirable property that its eigenvectors will be symmetric. Since all eigenvectors of the modified covariance matrix are orthogonal and the eigenvectors associated with the signal span the signal subspace, the signal subspace is orthogonal to the eigenvector associated with the noise. The dot product of the noise eigenvector with the signal subspace is set to zero.