A PCM adaptor is a device that encodes digital audio as video for recording on a videocassette recorder. The adapter also has the ability to decode a video signal back to digital audio for playback. This digital audio system was used for mastering early compact discs. Operation High-quality pulse-code modulation (PCM) audio requires a significantly larger bandwidth than a regular analog audio signal. For example, a 16-bit PCM signal requires an analog bandwidth of about 1-1.5 MHz compared to about 15-20 kHz of analog bandwidth required for an analog audio signal. A standard analog audio recorder cannot meet this requirement. One solution arrived at in the early 1980s was to use a videotape recorder, which is capable of recording signals with higher bandwidths. A means of converting digital audio into a video format was necessary. Such an audio recording system includes two devices: the PCM adaptor, which converts audio into pseudo-video, and the videocassette recorder. A PCM adaptor performs an analog-to-digital conversion producing series of binary digits, which, in turn, is coded and modulated into a black and white video signal, appearing as a vibrating checkerboard pattern, which can then be recorded as a video signal. Most video-based PCM adaptors record audio at 14 or 16 bits per sample, with a sampling frequency of 44.1 kHz for PAL or monochrome NTSC, or 44.056 kHz for color NTSC. Some of the earlier models, such as the Sony PCM-100, recorded 16 bits per sample, but used only 14 of the bits for the audio, with the remaining 2 bits used for error correction for the case of dropouts or other anomalies being present on the videotape. Sampling frequency The use of video for the PCM adapter helps to explain the choice of sampling frequency for the CD, because the number of video lines, frame rate and bits per line end up dictating the sampling frequency one can achieve. A sampling frequency of 44.1 kHz was thus adopted for the compact disc, as at the time, there was no other practical way of storing digital audio than by using a PCM adaptor and videocassette recorder combination. It is simplest if the same number of lines are used in each field, and, crucially, it was decided to adopt a sample rate that could be used on both PAL and monochrome NTSC equipment. Since monochrome NTSC has a field rate of 60 Hz, and PAL