String synthesizers are specialized polyphonic synthesizers designed specifically to make sounds similar to that of a string orchestra. Dedicated string synthesizers occupied a specific musical instrument niche between electronic organs and general-purpose synthesizers in the 1970s and early 1980s, until advances in digital signal processing technology allowed the production of inexpensive general-purpose polyphonic synthesizers and samplers which made the existence of a separate type of instrument unnecessary.
The development of the string synthesizer was originally motivated by the need for a cheaper alternative to the Mellotron, which was itself a cheaper alternative to human string ensembles. The availability of string synthesizers was influential in adding string orchestration to popular music that would not otherwise be able to afford the use of a human string ensemble, and their characteristic sound, which was almost, but not quite, like that of a real string ensemble, was one of the distinguishing sounds of the era.
To keep costs down, string synthesizers typically used a frequency divider architecture similar to that of electronic organs, with the addition of specialized in-built vibrato and chorus effects to mimic the ensemble effect of multiple string instruments playing at once. Classic string synthesizers included the Freeman String Symphonizer, Eminent 310, Elka Rhapsody, Roland RS-202, ARP String Ensemble, and Moog Opus 3.