Optical Audio Output: PCM or Bitstream in Your Menu
Quick answer / TL;DR: The optical audio output setting decides the format that leaves the TOSLINK port. PCM makes the television decode the audio itself and send two-channel LPCM, which any bar can play. Bitstream, sometimes labeled Auto or Dolby Digital, passes the original Dolby Digital or DTS stream through untouched, so the soundbar or receiver decodes it and can produce 5.1. Choose bitstream for surround hardware, PCM for a stereo bar or a DAC, and expect silence when the two ends disagree on the format.
The optical audio output is a setting before it is a socket: the row in a television's sound menu that decides what leaves the square port and in what format. Two identical panels can behave completely differently on the same soundbar because one is set to PCM and the other to bitstream, and neither set is broken. The difference lives in the menu, not in the hardware.
This is the settings view: where the control sits, what each option sends, and the three symptoms – silence, stereo only, and a delay against the picture – that the wrong choice produces.
Where the Control Lives
The path differs by brand but the shape is constant: a Sound or Audio menu, then an entry such as Digital Audio Out, Audio Out Format or SPDIF Output, then a choice between PCM, Bitstream, Auto and often Dolby Digital or DTS as fixed options. On a set running a streaming OS the same control can appear twice, once in the television settings and once inside the app, and the app usually wins while it is playing.
Fixed and variable, the second switch
A separate control decides level, not format. Fixed output sends a constant signal and leaves volume to the soundbar or receiver, which is what you want when the bar has its own remote and display. Variable output lets the television's volume buttons change the optical level, convenient for a bar with no remote and awkward when two remotes fight over the same number. Some sets hide this as TV Speakers versus Audio System, or as a Speaker Selection entry. If the optical output goes quiet whenever the television volume reaches zero, variable is the setting you are hearing.
PCM and Bitstream, Side by Side
| Menu choice | What leaves the port | Fits |
|---|---|---|
| PCM | Two-channel LPCM, decoded by the TV | A stereo bar, a DAC, a headphone amp |
| Bitstream | The original Dolby Digital or DTS stream | A receiver or bar that decodes 5.1 |
| Auto | Bitstream when the far end reports support | The set-and-forget default |
| Dolby Digital | Forces AC-3, drops DTS sources to PCM | Older bars that read AC-3 only |
| DTS | Forces DTS where the source carries it | Receivers with a DTS bias, rarely needed |
| TV speakers | Nothing leaves the port at all | The silent port, the usual mistake |
Read the table as a handoff, not a quality ranking. The port has a fixed budget, roughly 1.5 Mbps, enough for two channels of uncompressed PCM or one compressed 5.1 stream. PCM spends that budget on two clean channels. Bitstream spends it on six compressed ones. Neither is better in the abstract; the question is which end is going to do the decoding, and only one end should.
What PCM does downstream
PCM means the television decodes. A Dolby Digital broadcast arrives, the set unpacks it, mixes it down to stereo and sends two channels of LPCM down the fiber. The soundbar receives plain audio and plays it, no decoder involved and no handshake to fail. That is why PCM is the safe answer to almost every unexplained silence: it removes the negotiation entirely. The cost is the surround information, which the set has already collapsed before the port sees it. Some sources send multichannel LPCM over optical, at lower quality than HDMI carries.
What bitstream hands over
Bitstream passes the packet through. The television does not decode; it forwards the Dolby Digital or DTS frames to the bar or receiver, which recognizes them, lights its decoder badge and spreads six channels across six speakers. This is the only route to true 5.1 through a TOSLINK cable. It also creates a dependency: if the receiving end cannot decode what is being sent, the result is not softer sound but no sound, or a burst of digital noise.
Lip Sync and the Delay Against the Picture
Optical carries audio only, so the television has no way to tell the bar what the picture is doing. The video path, through HDMI to the panel with its own processing, runs a few milliseconds longer than the light path on some sets and shorter on others, and the mismatch shows as sound arriving before or after the lips. Most televisions expose an Audio Delay or Digital Audio Delay slider in the same menu, graduated in milliseconds, with a useful range around 0 to 250. Adjust in steps of 20 while a news anchor talks rather than while music plays, because speech is where the error becomes visible.
When the Wrong Choice Sounds Like a Fault
- Silence with the cable seated – the bar cannot decode the stream being sent
- Stereo only, no center channel – PCM selected, or a stereo source
- A pop or hiss at each ad break – format switching between PCM and AC-3
- Volume buttons do nothing – fixed output selected, the bar remote is the control
- Sound trails the picture – delay left at zero on a panel with slow video processing
Each of those has a setting behind it, and none is a hardware failure. The silence case is the one that sends people back to the shop: a bar that decodes Dolby Digital only, fed a DTS disc with bitstream selected, has nothing to say. Switch the television to PCM and sound returns at once, which proves the cable and the port are fine and points the fault at the format. The stereo case is the mirror image, with no surround mode able to invent missing channels.
Optical audio output is set per input on most televisions, so the format chosen for the HDMI 1 port does not automatically apply to the tuner or to an app; the catalog lists the cable lengths and plug forms that suit each panel layout.
A Short Sequence That Settles It
Set the television to bitstream, play a known 5.1 source and read the bar's display. If it shows Dolby Digital or DTS and the rear speakers are alive, the chain is correct and the menu can be left alone. If it stays silent, switch to PCM: sound returning proves the hardware and tells you the bar lacks that decoder. That three-step test separates a format problem from a cable problem from a source problem, and it takes less time than reading the manual.
Related reading: The socket this setting feeds: the optical port on a TV and soundbar — optical-port
Related reading: The physical connection, step by step: how to connect an optical cable to a TV — how-to-connect-optical-audio-cable-to-tv
Frequently Asked Questions
Should optical audio output be set to PCM or bitstream?
Bitstream if the far end is a receiver or soundbar that decodes Dolby Digital or DTS, because that is the only way 5.1 reaches it. PCM if the far end is a stereo bar, a DAC or a headphone amp, or if a bitstream setting has produced silence. The format must match what the receiving device can decode; each end assumes the other is handling it.
Why is there no sound with the cable plugged in?
Usually the format, then the source. A bar that decodes AC-3 only will fall silent when fed DTS with bitstream selected, and a set still routed to its internal speakers sends nothing to the port at all. Check the speaker selection first, then switch the format to PCM as a test, since sound returning under PCM proves the cable and the socket.
Does PCM over optical reduce sound quality?
It removes the surround channels rather than degrading the two that remain. PCM is uncompressed, so a stereo feed is clean; what it cannot do is carry the six discrete channels a Dolby Digital or DTS stream holds. If the room has rear speakers and the source is 5.1, PCM costs you the effect. If the bar is stereo, PCM costs nothing at all.
Why is the sound out of step with the picture?
Video processing in the panel adds delay that the optical audio path does not share. Most sets offer an Audio Delay or Digital Audio Delay slider in the same sound menu; raise it in 20 millisecond steps while speech is playing until the lips and the words line up. Cables and ports are not the cause, which is why swapping them never fixes the timing.
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