Early analog synthesizers used technology from electronic analog computers and laboratory test equipment. They were generally "modular" synthesizers, consisting of a number of independent electronic modules connected by patch cables into a patchbay that resembled the jackfields used by 1940s-era telephone operators. Synthesizer modules in early analog synthesizers included voltage-controlled oscillators (VCOs), voltage-controlled filters (VCFs), and voltage-controlled amplifiers (VCAs). The control voltage varied frequency in VCOs and VCFs, and attenuation (gain) in VCAs. Additionally, they used envelope generators, low-frequency oscillators, and ring modulators.