nonPCM

NonPCM is a descriptive term used to refer to digital audio data that is not stored as uncompressed linear PCM (pulse-code modulation). It encompasses a wide range of formats and encoding methods, including both lossy and lossless codecs that compress or differently represent audio data before storage or transmission. In practice, files labeled as nonPCM are typically encoded with formats such as MP3, AAC, Vorbis, Opus, WMA, and Dolby Digital, or with lossless codecs like FLAC and ALAC that compress PCM samples while preserving the original signal upon decoding. The label also covers alternative modulation schemes used in specialized contexts, such as Direct Stream Digital (DSD) used on SACD.

Non-PCM formats differ from PCM in several ways. PCM encodes audio as a sequence of uniform samples with a defined bit depth and sampling rate, producing a straightforward, scalable representation. Non-PCM formats apply psychoacoustic models, perceptual coding, or delta/sigma modulation to reduce data size or alter the signal representation. As a result, decoding non-PCM data typically yields PCM output within playback devices, even though the stored data is not PCM on disk or in transit.

Common considerations include quality, bitrate, and compatibility. Lossy non-PCM formats trade some original information for smaller file sizes, while lossless formats aim to reproduce the exact PCM signal when decoded. Some non-PCM systems, such as DSD, use fundamentally different signal representations and may require specialized DACs or processing to convert to PCM for playback. The term nonPCM is mainly cosmetic and descriptive, rather than a standardized technical category.