QuickTime and Mac OS High Sierra

I just update my MacBook Pro to Mac OS High Sierra and I later found that QuickTime cannot open AVCHD file. What shall I do?

The basic problem here appears to be error trapping associated with the "pre-scan" of the file before opening the data in the QTX player app. (I.e., under High Sierra, it now appears that mono, 2-channel mono or stereo AC3 data in certain file containers is "error trapped" during a pre-loading scan of the file data.) Since AVCHD is basically AVC video muxed with stereo AC3 audio, the operating system refuses to allow the data be loaded to the QTX media player. (The same thing happens to my H.265 videos containing a mono, 2-channel mono or stereo audio track.)

You have a number of options here. You can complain to Apple in Feedback or Bug Reports and hope the issue is fixed sometime in the reasonably near future. Another, more immediate workaround, would be to convert the MTS files to MOV, MP4 or M4V files containing user targeted audio and video compression formats or, if you have the appropriate software available, you can copy the AVC video "as is" to a new file container while simultaneously transcoding the AC3 audio to AAC. E.g., an app like Subler (or Smart Converter Pro) can copy the original MTS AVC video (High@L4) to an M4V or MP4 (or MOV) file container while transcoding the AC3 48.0 KHz @ 256 Kbps audio to AAC audio. Be advised, however, that different apps may default to different AAC transcode settings. (I normally use one of the latter options to preserver the original video quality.)

I have the same issue with all my mp4 videos containing AC3 audio. It worked perfectly well before the last update of High Sierra. I've read the kind answer provided by Jon Walker below, but it seems quite an heavy and consuming task to convert all my videos for a codec issue. I hope there must be a way to make Quicktime recognize AC3 audio codec. All other video players do. But i'm stuck if i want to play my video on my Apple devices.

Thanks in advance for your support.