1. In a disc drive which includes a plurality of read/write magnetic heads and a servo magnetic head mounted for simultaneous movement by an actuator in response to a servo signal from the servo head, and in which the positions of the read/write heads are individually adjustable with respect to the servo head, a system for indicating any misalignment of the individual read/write heads as the particular head produces a cyclic position signal in response to position signals pre-recorded on alignment tracks of an alignment disc, each cycle of said cyclic position signal having a first segment with positive and negative peak amplitudes and a second segment with positive and negative peak amplitudes, the positive and negative peak amplitudes of said first and second segments being equal when the particular head is in registry with the corresponding alignment track, and the positive and negative peak amplitudes of one or the other segments decreasing when the particular head moves out of registry with the alignment rack to one side or the other thereof; said system including balanced gating circuitry having four output circuits for respectively producing four gating signals respectively timed to occur in time coincidence with respective ones of the positive and negative peak amplitudes of said first and second segments; a balanced input circuit connected to the particular head and including first and second outputs respectively applying the cyclic position signal and its complement to said gating circuitry to cause said gating circuitry to cause said gating circuitry to produce said four gating signals at the respective output circuits thereof; peak detector circuitry including four peak detector circuits respectively connected to said four output circuits of said gating circuitry to be individually gated by respective ones of said four gating signals, means connecting two of said peak detector circuits to the first output of said balanced input circuit and further means connecting the other two of said peak detector circuits to the second output of said balanced input circuit, said four peak detector circuits collectively detecting the positive and negative peak amplitudes of each of the two segments of each cycle of said position signal and providing four analog outputs corresponding thereto; and output circuitry connected to the outputs of said four peak detector circuits in said peak detector circuitry for producing an analog output signal having an amplitude corresponding to the difference between the algebraic sum of the positive and negative peak amplitudes of the first segment and the algebraic sum of the positive and negative peak amplitudes of the second segment.
2. The system defined in claim 1, in which said balanced input circuit includes a linear amplifier, and in which said system includes a summing circuit connected to said four peak detectors in said peak detector circuitry for producing an output signal representing the algebraic sum of the outputs of the four peak detector circuits, and an automatic gain control circuit connected to said summing circuit and responsive to the output thereof for producing an automatic gain control signal for said linear amplifier.
3. The system defined in claim 1, in which said gating circuitry contains a frequency-independent phase-shifting circuit so that the system may be used with a wide variety of recorded signals without adjustment.
4. The system defined in claim 1, and which includes circuitry connected to said peak detector circuitry for limiting the analog outputs thereof to a predetermined maximum.
5. The system defined in claim 1, and which includes an analog/digital converter coupled to the output of said output circuitry to convert the analog output signal therefrom into a corresponding digital signal; and a digital display device coupled to the output of the analog/digital converter.