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Abstract

An FM stereo generator for use as a modulator or in a test instrument or the like to provide the conventional 19 KHz and 38 KHz signals. The phase of the generated 19 KHz pilot subcarrier signal is fixed relative to the 38 KHz carrier signal. The 38 KHz double sideband suppressed carrier signal is generated in a digital synthesis circuit to provide a stable 38 KHz carrier signal. The left and right stereophonic audio signals are applied to operational amplifiers to provide sum and difference audio components. The difference component is applied as an input to a double sideband suppressed carrier digital-to-analog converter which is responsive to the synthesis circuit to provide the 38 KHZ carrier signal. The 19 KHz pilot subcarrier signal is generated by a digital-to-analog converter that is clocked by a crystal timer which is common to both signals thereby to lock the phase of the pilot signal to the 38 KHz signal. The 19 KHz and 38 KHz signals are added without filtering which would otherwise adversely affect phasing precision. A master timing address chain is used to address memory sine tables stored in an EPROM. The 38 KHz circuit utilizes differentially cross-connected D/A converters each of which provide two-quadrant multiplication in response to a digital input word address derived from the EPROM. The combined outputs of the connected converters provide four-quadrant multiplication and carrier suppression. The EPROM includes sine wave tables for both the 19 KHz and the 38 KHz signals. An address counter, which is driven at the clock rate of the crystal timer, scans the data in the EPROM tables and applies the data as digital input words to the respective digital-to-analog converters.

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First Claim