Pulse-coe modulation (PCM) is a digital representation of an analog signal where the magnitude of the signal is sampled regularly at uniform intervals, then quantized to a series of symbols in a numeric (usually binary) code. PCM has been used in digital telephone systems and 1980s-era electronic musical keyboards. It is also the standard form for digital audio in computers and the compact disc "red book" format. It is also standard in digital video, for example, using ITU-R BT.601. However, straight PCM is not typically used for video in standard definition consumer applications such as DVD or DVR because the bit rate required is far too high. Very frequently, PCM encoding facilitates digital transmission from one point to another (within a given system, or geographically) in serial form.