The present invention relates to a knock control system for controlling a knock controlling factor such as an ignition timing, a boost pressure, an air-fuel ratio, an amount of exhaust gas sent back in an EGR system, or the like in accordance with the condition in which a knock is produced in an internal combustion engine. 
Knock control systems generally include a knock sensor for detecting vibration of an internal combustion engine.  If an electric signal produced by the knock sensor is in excess of a certain level (hereinafter referred to as a "knock discrimination level"), then the knock control system determines that the engine is subjected to knock and retards the ignition timing.  Conversely, if the electric signal from the knock sensor is below the knock discrimination level, i.e., if no knock or detonation is detected over a prescribed period of time, then the ignition timing is advanced.  In this manner, the knock control system controls the ignition timing to be kept close to a critical point beyond which knock would be produced, thereby improving the fuel economy and power output capability of the engine.  See Japanese Laid-Open Patent Publication Nos.  51-46606 published on Apr. 21, 1976, 56-115861 published on Sept. 11, 1981, and 58-180766 published on Oct. 22, 1983, for example, for conventional knock control systems. 
The knock discrimination level is of great importance in the proper operation of the knock control system.  If the knock discrimination level were too high, knock that is actually produced would not be detected and the ignition timing or spark would be advanced.  As a result, the engine would knock many times and lead to damage.  If the knock discrimination level were too low, the spark would be retarded regardless of the fact that no knocking occurs, preventing the engine from generating enough power output. 
It has been conventional practice to establish an appropriate knock discrimination level as by applying the knock sensor signal to an integrating circuit, multiplying the output signal from the integrating circuit by a constant K (hereinafter referred to as a "K value") closely commensurate with the engine speed, and adding an offset voltage to the product. 
However, since engine blocks or other engine components are susceptible to different manufacturing errors from engine to engine even if the engines are of the same design, the knock discrimination levels of some engines may not be appropriate and no accurate knocking may be detected regardless of a closely matched K value. 
Therefore, establishing the knock discrimination level for the prior knock control systems has required the close matching of a K value, and has been disadvantageous in that accurate knocking detection is rendered impossible by engine manufacturing errors.