From a 1983 US ES catalog:
"[...] Quartz synthesis tuners were inherently more noisy than the best analog models. So Sony produced one of the most highly acclaimed tuner circuits ever invented. The Direct Comparator.
The Direct Comparator improves audible reception because it functions above the audible frequency range. By making the required PLL comparison directly at higher frequency, the D.C. in tuners like the ST-S555ES cuts through the blanket of noise so prominent in FM reception. Result? A spectacular S/N ratio of 92dB in mono and 86dB in stereo."

From a 1988 US ES catalog:
"The D.C. circuitry provides the accuracy of quartz frequency-synthesis without the commonly associated pre-scaling noise."