A PLL may be used to synchronously demodulate amplitude modulated (AM) signals. The quadrature signals, once properly aligned/recovered, also successfully demodulate the signal. Another benefit of crystals was that they could demodulate amplitude modulated signals. Phase-locked loops can also be used to demodulate frequency-modulated signals. AM detectors cannot demodulate FM and PM signals because both have a constant amplitude. A product detector (or equivalent) is needed to demodulate SSB signals. A MLSE must be used to optimally demodulate any signal in the presence of ISI. A coherent detector is able to independently demodulate these carriers. On the receiver side, an analysis bank is used to demodulate the signal again. To demodulate BPSK, one needs to make a local oscillator "synchronous" with the remote one.