LPCM — Linear Pulse Code Modulation and Audio Formats

Linear Pulse Code Modulation

For EDID audio format negotiation, see glossary/edid. For DSP processing of these formats, see audio/dsp-fundamentals.

LPCM (Linear Pulse Code Modulation) is the digital audio encoding used in CDs, AES3 professional audio, HDMI audio, and most DSP interconnects. Unlike compressed formats (Dolby Digital, DTS), LPCM is uncompressed — every sample is stored at full bit depth with no perceptual compression. LPCM is the format DSPs process natively; compressed formats must be decoded before processing. Understanding which formats a source outputs and which formats the signal chain can handle determines whether a system produces audio at all.

LPCM Specifications

LPCM audio is defined by three parameters:

• Bit depth: 16-bit (CD quality), 24-bit (professional/hi-res), 32-bit (some professional interfaces). Bit depth determines dynamic range: 1 bit ≈ 6 dB, so 24-bit provides ~144 dB of theoretical dynamic range.
• Sample rate: 44.1 kHz (CD, consumer music), 48 kHz (professional AV standard), 96 kHz (hi-res), 192 kHz (hi-res archival). All Dante systems use 48 kHz. AES67 supports 48 kHz and 96 kHz.
• Channels: stereo (2.0), 5.1 surround, 7.1 surround, up to 32 channels via HDMI 2.1

HDMI carries LPCM from 2 to 8 channels at 48 kHz/24-bit as a baseline. Higher channel counts and sample rates require HDMI 2.0/2.1 and compatible hardware at both ends.

Compressed Audio Formats on HDMI

Sources (streaming boxes, Blu-ray players, game consoles, PCs) may output compressed audio over HDMI instead of LPCM. This is important because:

• A DSP or de-embedder that receives a compressed bitstream and treats it as LPCM produces a noise burst, not audio
• Control systems and DSPs cannot process or route compressed bitstreams — only decode

In most installed AV applications, the source should be configured to output LPCM stereo (2.0) or LPCM 5.1 — not bitstream compressed formats — so the DSP can process the audio. Only AV receivers and Dolby/DTS-capable amplifiers can decode compressed bitstreams.

Audio in AV Installed Systems

Conference Rooms and Meeting Spaces

Sources (laptops, video codecs) output LPCM 2.0 (stereo) at 48 kHz. The switcher or de-embedder extracts the audio from HDMI and sends it to the DSP (Q-SYS, Biamp Tesira, Extron DMP). The DSP routes it to amplifiers and speakers.

Key requirement: the source's HDMI audio output must be set to the correct output device. Windows laptops often default to "Speakers" (internal) rather than the connected HDMI display. Verify in Sound Settings → Output device.

EDID and Audio Format Negotiation

The EDID advertised by the switcher input determines what audio format the source offers. If the EDID only lists LPCM 2.0 at 48 kHz, the source will only send stereo LPCM — it cannot send 5.1 unless the EDID advertises it. For installed AV, configure the switcher EDID to advertise LPCM 2.0 at 48 kHz (or 5.1 if the system supports it) and no compressed formats, to ensure the source always sends a DSP-compatible signal.

ARC and eARC

ARC (Audio Return Channel) and eARC (Enhanced ARC) carry audio from the display back to an audio device (receiver, soundbar, DSP) over the HDMI cable without a separate audio cable. ARC supports LPCM 2.0 and basic compressed formats. eARC (HDMI 2.1 feature) supports lossless high-bandwidth formats including LPCM 7.1, TrueHD, and DTS-HD MA. See glossary/arc-earc.

Common Pitfalls

• DSP receiving a compressed bitstream and producing digital noise. A Blu-ray player set to "Bitstream" output sends Dolby TrueHD over HDMI; the de-embedder passes the raw bitstream to the DSP, which does not have a decoder. Fix: set sources to PCM/LPCM output mode in their audio settings; never send compressed bitstreams to a de-embedder feeding a DSP.

• Sample rate mismatch between source and DSP. A source outputting LPCM at 44.1 kHz connected to a DSP locked to 48 kHz produces audio artifacts or silence if the DSP does not have a sample rate converter on that input. Fix: configure the source to output 48 kHz (the AV standard); most professional AV sources support 48 kHz output.

• Laptop audio output set to internal speakers instead of HDMI. The most common "no audio from laptop" call — the laptop's audio output device is set to its built-in speakers, not the HDMI/DisplayPort output. Fix: right-click the speaker icon in Windows → Sound Settings → Output device → select the connected display/switcher.

• EDID advertising Dolby Atmos causing source to send Atmos bitstream. If the switcher EDID advertises Atmos support and the source sends an Atmos-encoded stream, a simple de-embedder or DSP cannot process it. Fix: configure the switcher EDID to advertise only LPCM formats; remove compressed and Atmos entries from the EDID.