(Contains extracts from wikipedia's page)
The Sound Interface Device (SID) is a sound chip designed by MOS Technology SIDin 1981. The It is the "legendary" sound chip used by the [[Commodore 64|C64]].
It was indeed better than the [[AY]] used in many other computers of the era (Amstrad's too) because it could produce many different (3) wave signal while AY could only produce rectangular wave signals.