HandBrake

Anybody know of a good program to combine mp4 files for free? Google's first result is mp4Joiner, but I've tried that program and the quality of the new mp4 file, both video and audio, both take a dive after the process is done. I see other programs, but they look sketchy.

I'm open to paid programs, if it's <$30, preferably <$20.

One question - are ALL of the MP4s created with identical video and audio codecs? That is, you don't have some with MP3 audio, another with multiple tracks in AAC and AC3, etc?

If their specs aren't identical, you will have to "make them match", by recoding them to a common format. And any time you recode the video and audio, you will experience loss of quality.

Is that what happens in mp4Joiner when the audio codecs aren't the same? If so, maybe I'll give the program another chance (unless you can recommend something better?).

According to their website, mp4joiner does not recode:

MP4Tools is a collection of cross-platform free tools to manipulate MP4 files. It contains following applications:

MP4Joiner is a free application that allows join multiple MP4 files into one without reencoding and without quality loss.
MP4Splitter is a free application that allows split a MP4 file in multiple files without reencoding and without quality loss

If the specs of the sources are not the same, though, that implies the combined file could "glitch" at the boundaries, as codecs change.

I think you could use ffmpeg to do this, but that's just a guess. ffmpeg can do a lot of things, if you know how to manipulate its command line. I don't.

I've considered ffmpeg, but it looks too complicated to me. I might have to take a whole course to understand the syntax and language.

Maybe, if the files are all of the same data types, use MakeMKV, and then something like Subler to convert the final file to mp4.

SC

There are two ways to do this: Concatenation, and Stream Processing.
Both go rather quickly.
Both are called "Muxing," regrettably.
Both will play, equally regrettably, but only Stream Process is clean, end to end.

The problem with concatenation, which is like a butt splice or crash edit, is that drops, freezes, skips, and sync errors can occur. The players may fudge over the hiccups well enough to go unnoticed, but encoders as well as professional NLEs can choke because they must first decode to raw bits, not just pass a stream of instructions mediated by time and ref-frames.

Most free "muxers" and many command lines concatenate, and thus are GOP-agnostic just like a DOS prompt ("join here, dummy").

A utility, such as Videoredo for Windows, a paid solution, will reprocess the IPB frame order, indexes, guess at a frame if corrupt, and resync when necessary, usually by dropping an audio frame.

I understand Subler does the same thing, but I have no hands-on knowledge.