RM to M4B Converter

RM to M4B: Turning a RealMedia Recording Into an Audiobook File

.m4b is what Apple's ecosystem treats as an audiobook: an MPEG-4 audio file that Apple Books and most audiobook players will remember your position in, play at adjustable speed, and keep out of the music library. RealMedia is where a great deal of lecture, sermon, radio and conference audio ended up between roughly 1997 and 2007, because RealPlayer was how that material was streamed.

Moving one into the other is a sensible piece of archive rescue, and it mostly works well. But there is one property of your source file that decides whether it works at all, so this page checks that before anything else.

Check One Thing First: Which Audio Codec Is Inside Your .rm

RealMedia carried several different audio codecs over its life, and they do not all behave identically through this conversion.

We measured a .rm whose audio track was RealAudio 1.0 — the narrowband speech codec also known as ra_144 — and the resulting .m4b came back 0.15 seconds long instead of the full five seconds of the source. The audio decodes correctly; the problem is timestamps. The RealMedia demuxer hands every ra_144 packet a presentation time of zero, and the MP4-family container that .m4b uses requires timestamps that increase, so the muxer collapses the whole track into a fraction of a second. The same source file converted correctly to MP3 (5.09 s), WAV (5.02 s) and AU (5.02 s), which are containers that do not impose that requirement.

So: after converting, check the duration of the M4B before you delete anything. If it is wrong, your source is one of these older RealAudio tracks, and the right move is RM to MP3 — a small, universally playable file that most audiobook apps still accept — or RM to WAV if you want an uncompressed intermediate to re-encode yourself.

A .rm with a more conventional audio track converts without any of this. Our test with an AC-3 track produced a full-length M4B with the correct duration.

How to Convert RM to M4B

1. Upload Your RM File: Drag the .rm onto the page or click "Add Files". Several files can be queued and they all run with the same settings, which is useful for a lecture series split across many files. Any video track in the file is discarded — this conversion keeps only the sound.
2. Set File Compression: File Compression opens on Quality Preset. The alternatives are Specific file size, Custom Bitrate (a free-text field that opens at 128 Kbps) and Constant Bitrate (a dropdown running from 8 kbps to 384 kbps, sitting at 128 kbps). For spoken word, see the sizing table below before you accept 128.
3. Set Audio Channel and Audio Sample Rate: Audio Channel offers ORIGINAL, MONO and STEREO — Mono is the right answer for almost any speech recording and halves the data the encoder has to spend. Audio Sample Rate lists ORIGINAL first, then 8000, 12000, 16000, 24000, 44100 and 48000 Hz; leave it on ORIGINAL unless you know the source rate and have a reason to change it.
4. Convert and Download: Click Convert and save the .m4b. Files are uploaded over an encrypted connection, decoded and re-encoded on our servers, and deleted automatically after a few hours — no sign-up, no watermark. Check the duration of the result before you rely on it, for the reason in the previous section.

What the M4B Actually Is

We converted a test .rm and read the container structure of the output:

That fourth row surprises people, so it is worth spelling out: there is no such thing as an M4B codec or an M4B brand. An .m4b is an ordinary MPEG-4 audio file, and the extension is what tells Apple Books and other players to treat it as an audiobook — remembering your position, offering speed control, and keeping it out of the music library. The bytes are AAC in an MP4 container either way. That is also why RM to M4A produces a structurally identical file under a different name.

Sizing the File for a Long Recording

Spoken word does not need music bitrates, and audiobooks are usually long, so this is where the useful savings are. The arithmetic is straightforward — bitrate multiplied by duration — and it does not depend on content:

Two things to keep in mind. First, your source sets the ceiling: RealMedia audio was compressed for dial-up and early broadband, often at 20 to 64 kbps, so encoding it at 128 kbps stores the existing artifacts more faithfully rather than adding any quality. Match the source's rough bitrate rather than exceeding it. Second, choosing Mono in Audio Channel is usually worth more than any bitrate change on a lecture or sermon, because it removes half the data before the encoder starts.

Frequently Asked Questions

Why is AAC the only codec I can pick?

Because it is the only codec valid for this container in the option set — the .m4b extension is an MPEG-4 audio file, and among the codecs available here, AAC is the one it accepts. The Audio Codec group therefore shows a single entry, which the page opens on. This is not a limitation worth fighting: AAC is the codec every audiobook player expects to find inside an .m4b.

Will my M4B have chapter markers?

Not from this conversion. Chapter markers are a separate structure inside the MP4 container, and a RealMedia file does not carry anything that maps onto them, so the output has none — we confirmed the finished file contains no chapter entries. Most audiobook players fall back on remembering your playback position, which is the more useful feature anyway. If you need real chapters, add them afterwards in a tagging tool.

My M4B came out a fraction of a second long. What happened?

Your .rm almost certainly carries a RealAudio 1.0 (ra_144) audio track. We measured exactly this failure: the RealMedia demuxer gives every ra_144 packet a timestamp of zero, and the MP4 container .m4b uses requires increasing timestamps, so the track collapses. It is not a problem with your file and no setting on the page works around it. Convert the same source with RM to MP3 or RM to WAV instead — we confirmed both come out at full length from the identical source.

Is an M4B different from an M4A?

Only by name and by how software behaves toward it. Both are AAC in an MPEG-4 container, and our measured output even carries the M4A brand in its header, because there is no separate audiobook brand in the format. What the .m4b extension does is tell Apple Books and comparable players to treat the file as an audiobook: position memory, playback speed control, and separation from the music library. If your player does not care about that distinction, RM to M4A gives you the same audio.

What happens to the video in my RM file?

It is discarded. This is an audio conversion, so the picture is dropped and only the sound track is decoded and re-encoded. That is normally what you want with a lecture or interview recording, where the video was a slideshow or a static frame anyway. If you want to keep the picture, convert to a video format instead.

What if my RM file has no audio track at all?

The conversion fails and no file is produced. We confirmed the general behaviour on a video-only source: with no audio stream to read, the encoder has nothing to write, and the job errors out rather than handing back a silent file. Nothing in the pipeline generates silence to fill the gap. Open the file in VLC first and check whether an audio track is listed if you are unsure.

Should I raise the bitrate to improve an old RealAudio recording?

No. RealMedia audio was compressed hard for the connections of its day, frequently at 20 to 64 kbps, and everything discarded at that point is gone. Encoding the decoded result at 128 or 192 kbps simply stores the surviving artifacts more faithfully in a bigger file. Match the source's approximate bitrate, choose Mono if the material is speech, and spend your attention on the duration check instead.

Can I convert only part of a long recording?

Yes. Switch Trim from Unchanged to the time range and set a Start time and a Duration, in seconds or as HH:MM:SS.sss — the controls open at 0 and 10 respectively. Only that window is decoded and encoded. If you would rather split a long file into several separate pieces, run it through the Audio Cutter after converting, since that tool accepts .m4b directly.

How are my uploads handled and how long are they kept?

Your .rm is uploaded over an encrypted connection, decoded and re-encoded on our servers, and both the upload and the resulting .m4b are deleted automatically after a few hours. Files are never shared or made public, there is no watermark and no account is required. Because the processing runs server-side, the practical constraint on a many-hour recording is upload size and time rather than anything about your own machine.