Abstract: Equipment for labelling audio signals with identification information has an encoder which inserts the binary information into two very narrow notches of center frequencies 2883 and 3417 Hz, between semi-tones in the tonic scale to minimize music breakthrough into the decoding circuits, and to ensure that no fundamental frequencies in the tonic scale will be excluded in the reproduction. The notches are derived from a 3-stage biquad filter, and are approximately 50 dB deep and 150 Hz wide at the top. The encoder includes a wide bandpass circuit consisting of a 1 KHz highpass filter and a 6 KHz lowpass filter introduced to ensure that the code insertion level is not determined by frequencies, either high or low, which do not adequately mask the code frequencies. The code amplitude is kept a fixed level below the programme, initially adjustable by a suitable control.

Abstract: Nuclear magnetic resonance is excited in surrounding fluids by pulsing a first winding which with a capacitor forms a resonant circuit. In order to observe signals induced by NMR in the first winding between pulses, the pulses are given an envelope which rises linearly to a peak and then immediately decays linearly to zero. A waveform generator generates bursts of constant amplitude oscillations having a 180.degree. phase change at the centre of each burst. When such bursts are applied to the resonant circuit by way of a drive amplifier and a second winding wound over the first winding, the required envelope is generated in the first winding.

Abstract: Nuclear magnetic resonance is excited in surrounding fluids by pulsing a first winding which with a capacitor forms a resonant circuit. In order to observe signals induced by NMR in the first winding between pulses, the pulses are attenuated by connecting resistors across a second winding inductively coupled to the first winding. Connection of the resistors is made when required by forward biasing two MOSFETs connected "back to back" to reduce transients when the MOSFETs cease conduction. A switching pulse for the MOSFETs which has a steep rising edge and an exponential trailing edge has been found advantageous.