the tracking threshold voltage in the positive direction an n comp signal is also provided representing the output of the negative data comparator and indicating that the analog data amplitude has exceeded the tracking threshold voltage in the negative direction a data signal is also provided this represents that if either p comp or n comp signal output is valid concurrent with the derivative of the analog data signal changing polarity a fixed width pulse is created on an alternating polarity basis i e once a positive peak is detected the next valid transition must be negative the rising edge of this data signal is taken as coincident with the peak of the analog data finally a clock signal is provided which represents a variable frequency clock locked in phase to the data pulse train thus this signal represents data speed referring to fig1 an exclusive nor circuit 12 is illustrated circuit 12 has dual inputs receiving respectively the p comp and n comp signals described hereinbefore the exclusive nor circuit provides an output signal that is connected to the base of a normally conducting transistor 14 the emitter of transistor 14 is connected to a constant current source provided at terminal 16 and the collector of the transistor is connected to ground the emitter of transistor 14 is also connected to one terminal of a capacitor 18 having its other terminal connected to ground the common emitter capacitor connection is connected to one input terminal of a comparator 21 and the other input of the.