a circuit arrangement for transmitting binary data elements ` 0 ` and ` 1 ` by frequency shift using two tone frequencies f0 and f1 . the circuit arrangement comprises a modulator , an exclusive or - gate and an integrator . a data signal ds produces from the modulator a digital signal dss which by decoding of the integrator results in a tone signal ts comprising f0 or f1 . the action of the or - gate to which both signals ds and dss are applied causes the amplitude of the tone signal ts to be substantially the same for each of the tone frequencies f0 and f1 . the or - gate may change either the effective resistive input into the integrator for each tone frequency or it may change the amplitude of the signal dss for each tone frequency .