LPCM

LPCM (Linear Pulse Code Modulation, 线性脉冲编码调制) is a form of pulse code modulation that represents audio signals using linear quantization.

The analog waveform is sampled at fixed time intervals, and each sample value is quantized into a signed digital value. The sampling rate determines the number of samples taken per second, and the bit depth determines the number of quantization levels available for each sample.

LPCM typically does not compress the sample data, nor does it use auditory models to discard information. Excluding packaging overhead, its data rate can be calculated by multiplying the sampling rate, the number of bits per sample, and the number of channels. For example, a stereo 44.1 kHz, 16-bit LPCM has a data rate of 1,411.2 kbit/s; while an 8-channel, 96 kHz, 24-bit LPCM requires 18.432 Mbit/s. The data volume increases linearly with an increase in any of these parameters.

“Linear” refers to a linear relationship between the quantized values and the signal amplitude, distinguishing it from non-linear compression-expansion PCM formats such as μ-law and A-law. LPCM and PCM are often used interchangeably in consumer audio documentation, but PCM is a broader concept. Differential PCM, floating-point PCM, and compressed telephone voice PCM are all related forms of pulse-code modulation, but they do not necessarily fall under the commonly understood definition of integer linear PCM.

LPCM describes only the encoded representation of samples; it does not specify file extensions, byte order, channel arrangement, or metadata. WAVE, Broadcast Wave, AIFF, CAF, Matroska, and MPEG transport streams can all carry linear PCM, but each marks bit depth, sample rate, and channel mapping in its own way. The binary structure of files containing the same LPCM samples differs depending on the byte order, padding, or encapsulation used.

Audio CDs use 44.1 kHz, 16-bit, stereo linear PCM; however, CD-DA organizes data into continuous audio sectors and subcodes and is not a collection of WAVE files. DVD-Video can contain various combinations of 48 or 96 kHz LPCM; DVD-Audio further expands to sampling rates such as 44.1, 88.2, 176.4, and 192 kHz, and allows for 16-, 20-, or 24-bit quantization. Due to the disc’s total bitrate limitations, not every maximum sampling rate can be used simultaneously with the maximum number of channels.

Blu-ray can multiplex multichannel LPCM directly into BDMV programs; commercial releases commonly feature 48, 96, or 192 kHz audio tracks with 16- or 24-bit quantization. The specification supports high-bit-depth PCM with up to eight channels, but the maximum allowed sampling rate varies depending on the number of channels and the limitations of the disc application. The terms PCM, Linear PCM, LPCM, or Uncompressed PCM in Blu-ray menus typically refer to the same type of uncompressed audio representation.

Digital devices also often decode other audio formats and output them as LPCM. When a player decodes Dolby TrueHD, DTS-HD Master Audio, FLAC, or AAC and sends PCM via HDMI or the operating system’s audio interface, this does not mean the source file was originally LPCM. If a display panel reports only “PCM,” this usually reflects the transmission format on the current interface rather than providing a complete description of the original encoding on the media.

Sampling rate and bit depth define the technical range of digital representation. According to sampling theory, the sampling rate must be at least twice the highest frequency to be represented; practical systems also require a transition band for filtering; bit depth, meanwhile, affects the quantization step size and the theoretical signal-to-noise range. Higher values provide greater production headroom but cannot correct source signals that are bandwidth-limited, clipped, noisy, or have undergone lossy processing, nor do they prove that the program originates from a native recording in the corresponding format.

Lossless codecs such as FLAC, ALAC, MLP, Dolby TrueHD, and DTS-HD Master Audio can reduce storage or transmission data volume without altering the decoded PCM. Their differences from LPCM lie primarily in bitstream organization and compression methods, rather than in necessarily different decoding results. When comparing CD audio tracks, it is necessary to verify whether each version originates from the same mix and master; one cannot determine whether the content is identical based solely on labels such as “uncompressed” or “lossless.”