When neither "A" nor "B" changes, it is assumed that the encoder has not moved and so the quadrature decoder negates its "count enable" output, thereby causing the counts to remain unchanged. If both the "A" and "B" logic states change in consecutive AB samples, the quadrature decoder has no way of determining how many increments, or in what direction the encoder has moved. This can happen if the encoder speed is too fast for the decoder to process (i.e., the rate of AB state changes exceeds the quadrature decoder's sampling rate; see Nyquist rate) or if the "A" or "B" signal is noisy. In many encoder applications this is a catastrophic event because the counter no longer provides an accurate indication of encoder position. Consequently, quadrature decoders often will output an additional "error" signal which is asserted when the "A" and "B" states change simultaneously. Due to the severity and time-sensitive nature of this condition, the "error" signal is often connected to an interrupt request. A quadrature decoder does not necessarily allow the counts to change for every incremental position change. When a decoder detects an incremental position change (due to a transition of "A" or "B", but not both), it may allow the counts to change or it may inhibit counting, depending on the AB state transition and the decoder's "clock multiplier". The clock multiplier of a quadrature decoder is so named because it results in a count rate which is a multiple of the "A" or "B" pulse frequency. Depending on the decoder's design, the clock multiplier may be hardwired into the design or it may be run-time configurable via input signals. The clock multiplier value may be one, two or four (typically designated "x1", "x2" and "x4", or "1x", "2x" and "4x"). In the case of a x4 multiplier, the counts will change for every AB state change, thereby resulting in a count rate equal to four times the "A" or "B" frequency. The x2 and x1 multipliers allow the counts to change on some, but not all AB state changes, as shown in the quadrature decoder state table above (note: this table shows one of several possible implementations for x2 and x1 multipliers; other implementations may enable counting at different AB transitions).