MTS to AU Converter

Extract AU Audio from MTS: What This Tutorial Covers

This converter pulls the audio track out of an MTS (AVCHD) camcorder clip and saves just the sound as an AU (.au / .snd) file — the H.264 video is discarded and you keep only the soundtrack. AU is Sun Microsystems' minimal late-1980s Unix sound format, so this is a deliberately narrow target: it makes sense when a legacy Unix tool, an older Java application, or a scientific or instrumentation system specifically wants an .au feed. This walk-through shows where the .MTS clip hides on the memory card, what happens to the audio depending on how your camera recorded it, why the AU comes out large, the one real gotcha — a slight length drift from AC-3's decode padding — and, importantly, when you should pick MTS to WAV or MTS to MP3 instead.

AU Format at a Glance

How to Convert MTS to AU

1. Upload Your MTS File: Drag and drop your .MTS (or .M2TS) clip onto the page, or click "+ Add Files" to browse. Queue several clips to extract them in one batch with the same settings.
2. Set Audio Channel (Optional): Audio Channel sits on Original, which copies the source layout into the .au. Set it to mono if a legacy reader expects single-channel audio, or to stereo to fold a surround track down to two channels.
3. Set Audio Sample Rate or Trim (Optional): Audio Sample Rate also defaults to Original; classic AU pipelines often expect 8000 Hz, so set it there only if the consuming tool requires it. Use Trim to keep only part of a long recording.
4. Convert and Download: Click "Convert" and save your .au file individually or as a ZIP. No sign-up, no watermark.

Walk-through: Finding the Right File, and What Happens to the Audio

MTS is the AVCHD camcorder format — the codec system Sony and Panasonic introduced in 2006 — and on the card it is a folder structure, not a single file. Footage lives under PRIVATE/AVCHD/BDMV/STREAM/, where each recording is a .MTS clip (the extension becomes .M2TS once the clip is copied to a computer; the bytes are the same MPEG-2 Transport Stream). This tool takes the individual stream file, so browse into that STREAM/ directory and upload the actual .MTS clip rather than the top-level AVCHD folder.

This converter writes 8-bit µ-law into the .au by default — AU's historical telephone-grade companding, stored in the fixed 24-byte .snd header Sun defined. That means a modest additional quality cut layered on top of whatever the source already was; select 16-bit big-endian linear PCM (PCM_S16BE) in the advanced options for an uncompressed copy with no extra loss. What the sound starts from depends entirely on how your camcorder recorded it, and the two AVCHD audio types behave very differently here:

• Dolby AC-3 (Dolby Digital) source — the common case. Most consumer AVCHD camcorders record AC-3, which is already a lossy format (up to 5.1 channels). Decoding it to 16-bit PCM (if selected) adds no new generation of loss — but it cannot rebuild detail the AC-3 encode already discarded, and the µ-law default layers a further telephone-grade companding on top. At best you get a clean copy of exactly the quality the source had, never an upgrade.
• Linear PCM source — some professional models. A few pro camcorders record uncompressed Linear PCM. Because that source is already lossless, writing it into a 16-bit PCM AU (select PCM_S16BE in the advanced options) is a genuinely lossless handoff — camcorder PCM straight to PCM-in-AU, with no transcode in between; the µ-law default would instead compand it to 8-bit. Keep Audio Sample Rate on Original so the AU inherits the source rate.

Either way, the AU stores a fixed byte count per sample, so expect it to be larger than the clip's audio bitrate suggests — roughly 5.3 MB per minute for the µ-law default at 44.1 kHz stereo, doubling to about 10 MB per minute if you select 16-bit PCM. That size is the cost of a legacy-friendly file:

• Feeding a Unix tool, Java application, or scientific pipeline that needs .au? Keep the defaults — µ-law in a .snd header is the classic profile those readers expect; select 16-bit big-endian PCM in the advanced options if the tool wants linear samples.
• A classic AU pipeline that wants telephone-style audio? Drop Audio Sample Rate to 8000 Hz and Audio Channel to mono to match the format's 8 kHz heritage, accepting the smaller, lower-fidelity result.
• Do not expect a setting that recovers fidelity. There is no "higher quality" option that rebuilds what AC-3 compressed away; a higher sample rate only makes the file larger.

Set against the two obvious alternatives, the trade looks like this:

Common Errors and How to Fix Them

• "I can't find a file to upload — just folders" — You are looking at the card's PRIVATE/AVCHD/BDMV/ tree. Browse down into STREAM/ and pick the individual .MTS (or .M2TS) clip; that is the file this tool needs.
• "The AU won't play on my phone or in my browser" — Expected. AU never became a consumer format; smartphones, browsers, and most media apps do not read it. AU is a feed for the specific legacy or scientific tool that asked for it — for everyday playback, extract to MTS to MP3 instead.
• "The AU is huge compared to the clip's audio" — Also expected. The default µ-law stores one byte per sample per channel, so a modestly-bitrated AC-3 track becomes about 5.3 MB per minute at 44.1 kHz stereo (about 10 MB per minute if you select 16-bit PCM). If size matters more than legacy compatibility, MTS to MP3 is far smaller.
• "The audio sounds no better than the original" — Expected; the µ-law default is in fact slightly duller than the source. Selecting 16-bit PCM gives a faithful copy of the AC-3 decode, not added fidelity; only an LPCM source written as 16-bit PCM is a truly lossless handoff.
• "The extracted audio is slightly longer than the video / drifts out of sync" — AC-3 (like all lossy audio) carries a few milliseconds of encoder padding, typically around 10–20 ms, and some editors read the camcorder's muxing delay differently. On a single clip this is usually inaudible, but in a tight edit you may need to nudge the track into alignment.
• "My clip has 5.1 surround but I only got stereo" — AVCHD AC-3 can carry up to 5.1 channels; this tool's Audio Channel control offers Original, mono, and stereo, so a surround source is most reliably handled as a stereo fold-down.

When This Doesn't Work — and the Simpler Routes

AU makes sense for one narrow case: a legacy Unix tool, an older Java application (the classic AudioClip era expected audio/basic), or a scientific or instrumentation system built around the format's trivially simple 24-byte header. If that is not your situation, AU is the wrong target. For editing, a plain WAV carries uncompressed 16-bit PCM and is accepted by virtually every audio editor on every platform, so MTS to WAV is the standard editing route; MTS to MP3 is the right pick if you only need the soundtrack to play on a phone, car stereo, or generic player. The conversion can also fail outright if the .MTS is partially corrupted — often from pulling the card before the camcorder finished writing — in which case the audio stream may be unreadable even when a player can still scrub the video; re-copy the clip from the card rather than fight a broken file. And if you would rather keep the picture as well as the sound, convert MTS to MP4 re-encodes both streams into one playable file.

Frequently Asked Questions

Does this keep the video, or just the audio?

Just the audio. This is an extraction: the H.264 video inside your MTS clip is discarded and only the soundtrack is written out as an AU file. If you want to keep the picture too, convert MTS to MP4 instead, which re-encodes both the video and audio into a single playable file.

Why would I convert MTS to AU instead of WAV or MP3?

Only when a downstream tool specifically wants an .au file. The realistic cases are narrow: legacy Unix sound tooling and shell pipelines, older Java applications that expected audio/basic, and scientific, embedded, or instrumentation systems built around AU's minimal 24-byte header. For everything else, MTS to WAV gives you the same uncompressed PCM in a format every editor reads, and MTS to MP3 gives you a small, broadly playable file. AU and WAV can hold the identical 16-bit PCM payload — WAV is just the one virtually everything accepts, so pick AU only when the .au extension itself is the requirement.

What encoding does this put inside the .au file?

8-bit µ-law by default — the telephone-grade companding Sun's format is remembered for, written into the fixed 24-byte .snd header (magic 0x2e736e64) defined in the late 1980s. The whole format is big-endian, including any 16-bit samples. Because µ-law packs each sample into 8 bits, it does add a slight, telephone-grade quality reduction on top of whatever the camcorder recorded; if you want a faithful uncompressed copy for a Unix tool, Java application, or scientific system, select 16-bit big-endian linear PCM (PCM_S16BE) in the advanced options — A-law is also available.

Will the AU sound better than the audio already in my MTS clip?

It depends on how the camera recorded it, and the honest answer is no. Most consumer AVCHD camcorders record Dolby AC-3, which is already lossy — converting cannot exceed the source, and the default µ-law output actually sits slightly below it. Selecting 16-bit PCM in the advanced options gives a faithful copy of the AC-3 decode. Some professional models record uncompressed Linear PCM, and that case is different: PCM straight into a 16-bit PCM AU is a genuinely lossless handoff with no transcode at all. Either way nothing here upgrades the original; only the LPCM source starts from true lossless audio.

Why is my AU file so much larger than the original soundtrack?

Because the AU stores a fixed byte count per sample regardless of the source bitrate. AVCHD audio is compressed (AC-3 packs a stereo track into a few hundred kbps), whereas the default 8-bit µ-law at 44.1 kHz stereo runs about 5.3 MB per minute — and 16-bit PCM, if selected, about 10 MB per minute. The size jump is the cost of a legacy-friendly fixed-rate format, not a setting you can tune away; for a small file, extract to MTS to MP3 instead, and to squeeze an AU back down later, AU to Opus is the reverse route.

Is a .snd file the same thing as a .au file?

Yes — same format, two extensions, and the file itself does not distinguish them. The giveaway is the signature at the very start of the file: the four ASCII characters .snd, which is where the alternate extension came from in the first place. Sun's Unix workstations and the Java tooling that followed used both names interchangeably, so if a program asks for a .snd you can simply rename the .au this converter produces. Nothing inside needs to change.

What sample rate and channel setting should I use for a classic AU workflow?

Only override the defaults if the receiving tool actually demands it. Audio Sample Rate and Audio Channel both sit on Original, which copies the camcorder's rate and layout straight through and gives the most faithful result. The historical audio/basic convention was 8000 Hz mono, so a genuinely old Unix or Java reader may expect exactly that — set the sample rate to 8000 Hz and the channel to mono in that case, accepting a much smaller and noticeably duller file. Raising the sample rate above the source rate does the opposite of what people expect: it inflates the file without recovering any detail, because the extra samples are interpolated rather than recorded.

How are my files handled, and how long are they kept?

Your MTS file is uploaded over an encrypted connection, processed on our servers, and deleted automatically a few hours after conversion — no sign-up, no watermark, never shared or made public. The main practical limit is upload size and time rather than the extraction itself: an MTS clip carries full HD video alongside the audio, so a long recording can take a while to upload even though pulling out the soundtrack is quick.