Audio Tracks and Subtitles: Formats, Sync Problems and Passthrough

A video file or stream is not a single block: it is a container (MKV, MP4, MPEG-TS...) that carries several tracks in parallel. It holds a video track, one or more audio tracks (original version, dubbed version, audio description) and sometimes several subtitle tracks. The player chooses which ones to show, decodes them or passes them to another device. When one of these links does not understand the format it receives, the result ranges from a mild nuisance to complete silence. This guide reviews the most common formats and the fixes for typical problems.

The main audio formats

Like video, sound is compressed by a codec. Some are universal, others are reserved for home theater and are not decoded by every device.

What to remember

• AAC plays everywhere: phone, tablet, TV, computer. It is the default format of most HLS streams.
• AC3 and E-AC3 are very common in television, but decoding them depends on licenses: some cheap devices or some apps do not decode them themselves.
• DTS is the format most often missing: several generations of TVs do not decode it internally and can only pass it to a compatible receiver.
• A full software player, such as those built on libmpv, decodes almost all of these formats itself, which works around the TV's limits.

The typical "picture but no sound" case comes from an AC3, E-AC3 or DTS track that no link in the chain can decode. The fix is to choose another audio track if the content offers one (often a stereo AAC track), to enable software decoding in the player, or to send the sound as passthrough to a compatible receiver.

Switching audio tracks

When content offers several languages, they appear as separate tracks, identified by a language code (eng for English, fre or fra for French) and sometimes by a title ("Audio description", "Original"). Every player offers a selection menu during playback, usually under a speech-bubble or speaker icon.

If a track is simply called "und" (undetermined), the publisher did not fill in the language: you have to listen to recognize it. Many players also let you set a preferred language that is selected automatically at the start of each video.

With live television, a channel may broadcast a main track and an audio description track for visually impaired viewers. If you hear a voice describing the action, that second track is selected: go back to the first one in the audio menu.

Subtitle formats

There are two main families of subtitles: text, which the player draws itself, and images, already drawn by the publisher.

Burned-in, embedded or external

• Burned-in (or "hardcoded"): they are part of the picture. You cannot turn them off or change their size or language.
• Embedded: they are a track of the container (common in MKV). You turn them on or off in the player's menu.
• External: they are in a separate file, usually with the same name as the video (film.mkv and film.fr.srt). Most players load them automatically if they are in the same folder.

Anatomy of an SRT file

An SRT file is simple numbered text, with timestamps in hours:minutes:seconds,milliseconds format:

1
00:00:12,400 --> 00:00:15,000
Good evening, and welcome everyone.

2
00:00:15,600 --> 00:00:18,250
Tonight, we will talk about the network.

WebVTT is very similar: it starts with the line WEBVTT and uses a period instead of a comma before the milliseconds. Both formats can be edited with a plain text editor.

Accents that display badly

If subtitles show "é" instead of "é", the file is encoded in Windows-1252 (or ISO-8859-1) while the player reads it as UTF-8, or the other way around. Fix: open the file in a text editor that lets you choose the encoding when saving, and save it as UTF-8. Some players also offer an option to force the subtitle encoding.

Fixing a sync problem

A mismatch between sound, picture and subtitles can have three distinct origins, which are not fixed the same way.

Constant offset

The subtitles or sound are early or late by the same number of seconds from start to end. This is the simplest case: set a delay in the player. In mpv on a computer, the z and x keys shift subtitles in steps of 0.1 second, and Ctrl + + or Ctrl + - shift the sound. On a TV, look for an "Audio sync" or "Subtitle delay" option in the playback menu. An effective method: find a short, clear line of dialogue, adjust the offset on it, then check on another line a few minutes later.

Growing offset

The subtitles are right at the start and increasingly off afterwards. The file was made for a version of the video at a different frame rate, typically 25 frames per second (European version) versus 23.976 (cinema version). The gap reaches about 4%, which is more than four minutes over a two-hour film. A simple delay is not enough: you need a subtitle file matching your exact version, or a subtitling tool able to convert the frame rate.

Sound offset caused by hardware

The sound lags behind the lips only when a soundbar or a Bluetooth headset is used. Bluetooth devices often add 100 to 300 ms of latency. The TV may also delay the picture through its processing (motion smoothing, noise reduction). Try Game mode, which removes most processing, or adjust the audio delay on the TV or soundbar.

Passthrough to a receiver or soundbar

There are two ways to send sound to an external audio system:

• Decoding by the player (PCM output): the playback device decodes the AC3 or DTS and sends already uncompressed sound. Simple and compatible, but multichannel is often reduced to stereo if the link cannot carry it.
• Passthrough (or "bitstream"): the player sends the compressed stream as is, and the receiver decodes it. You keep the 5.1 or the Atmos, and the receiver's display shows the format received.

ARC, eARC and optical

The classic symptom of badly set passthrough is total silence: the player sends a format the chain cannot decode. If that is the case, turn off passthrough for that format in the player, or set the TV's audio output to "PCM" or "Auto". On a computer, the choice is made in the player's audio settings; the Windows software page presents the playback environment on PC. The playback settings offered by AICKO IPTV are described on the video player page.

Common mistakes

• Enabling DTS passthrough on a soundbar that cannot decode it: complete silence on the affected films.
• Leaving the TV output on "PCM" when you want 5.1: the receiver only gets stereo.
• Mixing up the track and the link: a stereo AAC track stays stereo, whatever the receiver. Check the track format in the playback information.

To go further on the video formats that accompany these tracks, see the article H.264, HEVC and AV1 codecs.

In summary

• A container carries several audio and subtitle tracks; the player chooses which to use.
• Picture but no sound: an undecoded AC3, E-AC3 or DTS track; switch track or enable software decoding.
• Unreadable accents: re-save the subtitle file as UTF-8.
• Constant offset: a delay is enough; growing offset: the subtitles do not match the video version.
• Passthrough: only for formats the receiver can decode; eARC for lossless formats.

Frequently asked questions

Why do I get no sound with some films only?

Those films probably use a DTS or E-AC3 track that your device cannot decode or that your audio system does not recognize in passthrough. Choose another audio track, or let the player decode the sound itself.

Can burned-in subtitles be removed?

No. They are part of the picture, pixel by pixel. Only subtitles embedded as a track or supplied in a separate file can be turned off.

Are WebVTT and SRT interchangeable?

Almost: the content is the same, only the header and the millisecond separator differ. Most players read both, and a manual conversion is possible with a text editor.

Why is the sound late with my wireless headphones?

Bluetooth adds its own transmission latency, often 100 to 300 ms. Adjust the audio delay in the player or on the TV, or use headphones with a low-latency codec.