[en] A data acquisition method has been developed at the SLAC Linear Collider (SLC) that provides accurate beam parameter information using sub-tolerance excitation and synchronized detection. This is being applied to several SLC sub-systems to provide high speed feedback on beam parameters such as linac output energy spread. The method has significantly improved control of the linac energy spread. The linac average phase offset (φ), used to compensate the effects of longitudinal wakefields, is adjusted ±1 control bit (about 0.18 degree S-band or 20% of tolerance), in a continuous fashion. Properly coordinated beam energy measurements provide a measure of the derivative of the accelerating voltage (dE/dφ). The position of the beam on the RF wave can thus be determined to ± 0.3 degree in about 5 seconds. The dithering does not contribute significantly to the energy jitter of the SLC and therefore does not adversely affect routine operation. Future applications include control of the interaction region beam size and orientation