LPCM Linear Pulse Code Modulation

What LPCM is

Linear PCM is the variant of pulse-code modulation in which the quantization steps are evenly spaced on the amplitude axis, in contrast to PCM encodings whose step sizes vary with amplitude such as the A-law and mu-law schemes used in telephony. The underlying PCM mechanism is simple: the analog waveform is sampled at uniform intervals, and each sample is rounded to the nearest value within a fixed range of digital steps. Two properties alone determine how faithful the resulting stream is to the original signal — the sampling rate (how often samples are taken) and the bit depth (how many possible values each sample can take).

A single LPCM stream is monophonic by definition; surround sound is built by synchronizing several parallel LPCM streams inside a transport or container. That is why LPCM is format-agnostic — every consumer codec, from Dolby Digital and DTS to TrueHD, MP3, and AAC, eventually decodes back to LPCM before the digital-to-analog stage. LPCM is the common substrate across decades of consumer audio carriers, defined as part of the Compact Disc Red Book standard, the DVD specification since 1995, the Blu-ray specification since 2006, and HDMI since 2002.

Sample rate, bit depth, channel count

Raw LPCM bit-rate is the product of three numbers: sample rate, bit depth, and channel count. The formula is bit-rate = sample rate × bit depth × channels. A 5.1 stream at 48 kHz / 24-bit consumes roughly 6.9 Mbps (48,000 × 24 × 6 = 6.91 Mbps), and a full 7.1 stream at 192 kHz / 24-bit consumes roughly 36.9 Mbps (192,000 × 24 × 8 = 36.86 Mbps). That second figure is the design ceiling cited in HDMI Forum eARC documentation.

HDMI carries up to eight channels of uncompressed LPCM with 16-, 20-, or 24-bit samples at sample rates of 32, 44.1, 48, 88.2, 96, 176.4, or 192 kHz. HDMI 2.0 widened the audio ceiling to up to 32 audio channels and up to 1536 kHz audio sample frequency, but the 8-channel / 192 kHz / 24-bit figure remains the practical home-theater LPCM ceiling.

eARC raises the audio return channel's capacity from ARC's roughly 1 Mbps to 37 Mbps, sized specifically to carry eight channels of uncompressed PCM at 192 kHz / 24-bit. eARC also adds support for uncompressed surround, Dolby TrueHD and DTS-HD Master Audio passthrough, and up to 32 channels. The HDMI Licensing eARC page lists support for uncompressed 5.1 and 7.1, 32-channel uncompressed audio, and high-bitrate audio formats up to 192 kHz / 24-bit.

When LPCM gets used in a HT chain

On a UHD Blu-ray player, the choice between LPCM and Bitstream HDMI output decides who does the decoding. When the player is set to LPCM, it decodes any compressed audio bitstreams — Dolby Digital, Dolby TrueHD, DTS, DTS-HD Master Audio — internally and sends multichannel LPCM downstream. Oppo's UDP-203 manual recommends LPCM mode when the HDMI output goes directly to a TV or to a receiver without advanced audio decoding capabilities.

Bitstream mode does the opposite: it passes the original encoded data through HDMI for the AVR or processor to decode, which is the path used for Dolby Atmos, Dolby TrueHD, DTS:X, and DTS-HD Master Audio. Selecting LPCM instead of Bitstream on an Atmos disc decodes the underlying TrueHD 7.1 bed to channel-based LPCM and discards the Atmos object layer — the player has no way to insert objects into a plain LPCM grid.

That said, plenty of Atmos still arrives as PCM in practice. Streaming sources and game consoles transport Atmos by wrapping the object metadata inside Dolby MAT 2.0 (Metadata-enhanced Audio Transmission) frames carried over the HDMI LPCM payload. Listed MAT-encoding sources include Apple TV 4K (tvOS 12 onward), Xbox One X/S, and the PS4 for Blu-ray Disc playback. MAT is what makes "Atmos over PCM" possible — plain LPCM on its own has no slot for object metadata.

The ARC versus eARC split sits on top of this. ARC's roughly 1–3 Mbps capacity can only carry compressed Dolby Digital Plus, so a TV passing Atmos to a soundbar over plain ARC must transcode lossless TrueHD-Atmos down to lossy DD+ Atmos. eARC's 37 Mbps is what lets a TV forward full TrueHD-Atmos and Dolby MAT (Atmos-in-PCM) to a downstream receiver or soundbar without recompression.

Common LPCM misconceptions

"LPCM doesn't support Atmos." This is true in the disc-player case — selecting LPCM on a UHD Blu-ray player decodes only the TrueHD 7.1 bed and discards object metadata. It is misleading in general. Atmos absolutely is transported as PCM on Apple TV 4K, Xbox, and PS4 Blu-ray playback, via Dolby MAT 2.0, which embeds the object metadata inside the LPCM payload. The honest distinction is between plain channel-based LPCM, which cannot carry objects, and Dolby MAT 2.0 LPCM transport, which can.

"LPCM is a competitor to DSD or MQA." It is not. LPCM is the underlying representation that other formats are converted to (or contrasted against) before reaching the DAC, and it has been the substrate for Red Book CD, DVD (since 1995), Blu-ray (since 2006), and HDMI (since 2002).

"LPCM is lossless, so my audio is lossless." LPCM being uncompressed only describes the state of the bits in transport between the decoder and the DAC. It says nothing about the lossiness of the original encode: a 128 kbps MP3 decoded into a 24-bit / 48 kHz LPCM stream is still MP3-quality audio sitting in an LPCM container. The "lossless encoding" description applies specifically to the Red Book CD case, where the source itself was captured at LPCM bit-depth and sample-rate; for streaming and broadcast sources, LPCM transport sits downstream of whatever lossy decode happened earlier in the chain.