PCM Audio over S/PDIF: Differences and Limitations

Many TVs, consoles, computers and audio interfaces let you choose PCM or bitstream for their optical or coaxial output. That setting changes what travels over the cable. It does not change the cable. This guide explains what PCM over S/PDIF can and cannot carry, and how to choose an output mode for a soundbar, receiver, DAC or audio interface.

Define the alternatives

PCM (pulse-code modulation) is a way of storing audio as a sequence of samples. A decoder produces PCM, and a DAC turns it into an analog signal. S/PDIF is a consumer digital connection, defined in IEC 60958. It carries digital audio from an output device to a receiver over a coaxial (RCA) or optical (TOSLINK) cable. Comparing PCM with S/PDIF is therefore not a choice between two connections. The real choice is between two ways of using the same S/PDIF link:

• PCM: the source decodes the audio itself and sends uncompressed two-channel PCM.
• Bitstream (often labelled Bitstream, Auto, Dolby Digital or DTS): the source sends compressed surround audio unchanged, and the receiver decodes it.

This article covers that choice on optical and coaxial digital outputs. HDMI appears only where it is the practical way around an S/PDIF limit.

Compare meaningful differences

Channels and formats

S/PDIF carries uncompressed PCM as two channels only. It cannot carry uncompressed 5.1 or 7.1 PCM. Multichannel sound can reach a receiver over S/PDIF only as a compressed bitstream, usually Dolby Digital or DTS, packed according to IEC 61937. Dolby TrueHD and DTS-HD Master Audio, and the Atmos and DTS:X tracks built on them, are not carried over S/PDIF. Dolby Digital Plus is generally not passed through either. Some sources convert it to Dolby Digital instead, depending on the device.

In practice, suppose a TV or console is set to PCM and connected to a surround receiver by optical cable. The source downmixes any 5.1 programme to stereo before sending it. The receiver may apply its own upmixing mode, but the result is not the original discrete channels.

Sample rate and bit depth

S/PDIF supports common sample rates such as 32, 44.1 and 48 kHz, and many devices also accept 88.2 and 96 kHz. Support for 176.4 and 192 kHz varies by device and connection type, and some optical transmitters and receivers do not accept those rates. Word length goes up to 24 bits. Check the specifications of both the output device and the receiver, because whichever one supports less sets the limit.

The receiver locks to the clock embedded in the incoming signal. If the source changes sample rate between tracks or apps, some receivers briefly mute or click while they re-lock. On a computer, you can set a fixed output sample rate that the receiver supports to avoid this switching. Whether the system then resamples audio depends on the operating system and driver.

Volume and input level

S/PDIF carries digital samples, not an analog signal that can be turned up or down along the way. In PCM mode the source can apply its own volume before sending, and some TVs let you choose a fixed or variable digital output. In bitstream mode the source usually cannot change the level, so you control volume on the receiver. An audio interface's S/PDIF input has no microphone-style preamp gain, so the sending device sets the input level. If the signal clips at the source, it stays clipped.

Optical versus coaxial

Optical and coaxial carry the same data format. An optical cable is electrically isolated, so it cannot create a ground loop between the devices it connects. That can remove one source of hum, but it does nothing about background noise in analog cabling, microphones or the room. Coaxial is an electrical connection. Supported rates and maximum reliable cable length depend on the hardware and cable, so rely on the manufacturer's specifications rather than general rules.

Privacy and file handling

An S/PDIF cable is a one-way, point-to-point link with no network function. What happens to the audio after it arrives depends on the receiving device and its software. That includes recording it, streaming it or passing it to an app, so check that device's settings and documentation. The format also includes copy-control status bits (SCMS), and whether a recorder acts on them varies by device.

Practical trade-offs

• TV to stereo soundbar or DAC: PCM is the safer setting. Many stereo devices cannot decode Dolby Digital or DTS and may stay silent or output noise if they receive a bitstream.
• TV or media player to surround receiver: bitstream keeps discrete surround for Dolby Digital or DTS content, while PCM gives stereo only.
• Game console to receiver: some consoles offer a Dolby Digital output option for game audio over S/PDIF. Without it, PCM over optical is stereo. Availability depends on the console and its firmware.
• Computer to external DAC or audio interface: use stereo PCM at a sample rate both devices support. This suits music, video calls and monitoring.
• Audio interface with an S/PDIF input: this lets you add a digital input device such as an external preamp. If the interface requires it, set its clock source to the incoming S/PDIF signal.

Published specifications tell you which formats, channel counts and sample rates each end accepts. They do not tell you about latency, lip-sync offset, dropouts when the format switches, or how a particular receiver handles a downmix. Those depend on the specific devices and firmware, so check them with your own content. That is why many TVs and receivers offer lip-sync adjustment. For video calls, also confirm that the call app is using the intended output device, because an app's own device choice can differ from the system default.

Choose for the use case

• Choose PCM if the receiver cannot decode Dolby Digital or DTS, if stereo is enough, if you want the source's volume control to work, or if you use the link for calls, music or monitoring from a computer.
• Choose bitstream if the receiver decodes Dolby Digital or DTS and you want discrete surround from films, broadcasts or games that provide it.
• Choose a different connection if you need multichannel PCM or lossless surround formats, because S/PDIF cannot carry them. Standard HDMI ARC cannot carry them either. eARC can, but both ends must support it.

If sound disappears after you switch modes, check the format setting on the source first. Then check the receiver's selected input and the sample rates it supports. A mismatch between what the output device sends and what the receiver accepts is a likely cause to rule out early.