or a test instrument at a remote location tayloe discusses an analog product detector in u s pat no 6 230 000 wherein an input analog signal is passed through a commutating switch with 4 outputs each output being connected for successive 90 degree portions of the frequency of the input signal supplied to detecting capacitors each of the 4 capacitors provides baseband product detection values at 0 90 180 and 270 degrees of the input the 0 and 90 degree outputs multiplied provide the original information signal the above fm discriminator can be implemented algorithmically with a digital system referring to fig4 a block diagram of the implementation in a preferred embodiment incoming signals from phone line 20 are sampled at 4 times the carrier rate by a to d converter 52 the signals are suitably fsk signals in a particular embodiment comprising 1200 and 2200 hz and sampling of the incoming signals is done at 6800 hz assuming a carrier frequency of 1700 hz being the average of the 2 frequencies 1200 2200 2 sampling at four times the carrier frequency provides sufficient data to create the fm discriminator mathematically fig5 illustrates an example sampled signal output from a to d converter 52 synthetic discriminator 54 receives the 4 times carrier rate sample data and a current sample datum from the a to d is multiplied by a previous sample datum from the a to d which represents the sample 90 degrees prior to the current sample providing the output function of an fm discriminator by multiplying the 0 and 90 degree signal samples for example the output of discriminator 54 is passed through.