Quicktime cannot open or convert .mov file

Good and bad news. I downloaded StreamClip. Was not sure which format to export, so I tried DV. The new iMAC was able to read the DV file, except there are portions where either the sound cuts out, or gets out of the synch with the video.

Should I try exporting in a different format? Or try different parameters?

The source file is 45 minutes long, and it took streamclip over 5 hours to convert the file to DV.

Again, it is difficult to answer questions without knowing the specifics.

For instance, with regard to the audio problem is the original file audio "in sync" when you play it in the MPEG Streamclip player? Did you resample the audio to 16-bits while converting to DV? Which DV format did you use—e.g., muxed DV or DV/LPCM in MOV, DV-25 or DV-50, etc?

As to target formats, what are your requirements? If converting for archival storage as a modern master file and storage space is not an issue, I would likely use Apple ProRes 422 for video (now included on all recent MacOS versions) with LPCM (Uncompressed, AIFF, WAV, etc.) audio in a QuickTime (MOV) file container which is a more modern editing combination than DV and, like DV, is also QT7/QTX app compatible. On the other hand, if storage space is an issue, you'd probably prefer to create H.264/AAC MOV files with the video data rate unlimited and using the "Quality" slider to maintain output file size in your preferred optimum file size-to-resolution range with audio data rate limited as needed. If not making conversions for archival or intermediate editing use, then target conversions directly for your preferred target distribution compression formats and settings—in most cases H.264/AAC in MOV, MP4, or M4V file containers with data rates dictated for targeted combination of quality, resolution, and/or use (e.g., online viewing, computer playback, optical media playback, mobile device, etc.) as you see fit.

In many cases it may be best to simple try a number of compression format combinations and settings to see what suits your particular needs on a project by project basis. (I.e., pick a source file segment containing typical graphic complexity and lighting for the source video and then run a sample conversion for evaluation.) Generally speaking, there is no "one shoe fits all" solution here. However, with experience, you should find you can make some fairly good "best guesses" based on the content characteristics of your project files and make adjustments accordingly.

Good Luck!

Streamclip plays the file without any sync issues.

Once I have the .mov file created by streamclip, quicktime 7 on my older iMAC also plays it without any sync issues.

When I copy the file to the new iMAC and open it, the system 'converts' it, and the result of that conversion is out-of-sync.

The purpose of using MPEG Streamclip (HandBrake or other similar app) is to avoid having to perform multiple conversions. Why don't you try converting directly to H.264/AAC or ProRes422/Uncompressed compression formats using MPEG Streamclip and then playing or editing the new file data directly (without a second conversion) in QTX or iMovie on the new iMac?

I am running a brand new iMAC MACos 10.31.1

For the purpose of this response will assume you are referring to High Sierra here.

I have some old iMovie projects from 10 years ago when I used iDVD to burn DVD's, before youtube took off.

I want to incorporate those movies into some new projects.

There is usually a .mov file located under the movies->shared movies->iDVD directory. Which I am hoping is the original movie I burned onto the DVD.

Audio/Video data actually "burned" to DVD by iDVD should be in the form of "muxed" MPEG-2 video with PCM audio stored in VOB file containers. iDVD "authors" DVDs by converting other Audio/Video compression formats to the required MPEG-2/PCM VOB format for "burning" the data. Unfortunately, MOV sourced data could be stored using any audio and/or video compression format compatible with your system 10 years ago which, as you now know, may or may not be compatible with the current QTX playback and/or conversion structures embedded in MacOS versions since Mavericks. Without knowing the specific audio and video compression formats stored in the stored MOV files, it is impossible state with certainty what you should do at this point. For instance, VOB content can normally be converted to H.264/AAC MP4 or M4V files which would be playback and edit compatible with the QTX app using an app like HandBrake. In a similar manner, HandBrake can convert most MOV files containing common third-party, as well as, Apple "legacy" compression formats to those same MP4 or M4V QTX compatible file formats. On the other hand, many proprietary compression formats—including many Apple "Pro" compression formats would probably not be supported by HandBrake but would be supported by MPEG Streamclip if the "Pro" codec package is installed. In addition, use of MPEG Streamclip is often a better choice if you are converting "legacy" MOV content to a QTX editing compression format like Apple ProRes 422/LPCM.

However, when I double click on the file either Quicktime will say cannot open the file, or it will say converting the file but never finish - the progress bar does not move.

There are a number of possibilities here. Files used by iDVD can be "Reference" or "Standalone" files. Reference files for which the original "project" files are no longer available cannot be opened by either QTX or QT7 players. Files containing data created by unsupported playback audio and/or video codecs will not open in the QTX player app. Files containing unsupported playback data which is both recognized and conversion supported by "legacy" or third-party codecs will normally be converted to H.264/AAC MOV successfully. Unfortunately, on some occasions files may contain recognized compression formats which is not "fully" conversion compatible with the QTX Convert structure embed/system codec configuration, contains damaged atoms, is incompatible with security updates, etc. and either never completes the conversion process or converts improperly. Once again, knowing the type of file, type of compressed data, and/or addition support for which your system may be configured helps to determine what is normally to be expected here and how to "get around" certain potential problems.

I still have the older MAC and downloaded Quicktime 7 which could read the file.

But it looks like I need QuickTime Pro in order to export it into a newer format, but you can't get Pro anymore, so still stuck.

Again, if the MOV files are still playable in QT7, then they should be conversion compatible with MPEG Streamclip which uses the same "passive" system codec configuration that is available to the QT7 player app on the same platform. (I.e., "active" use of certain "legacy" codecs was turned off under Lion in preparation for a post-Mountain Lion release of Mavericks.)

I noticed every so often it would get out of sync, and found if I went backwards a few seconds and waited a few seconds the sound would be back in sync. I speculated the computer just could not always keep up and the sound fell behind.

So what I did was use streamclip to break the 45 minute file into shorter segments.

iMovie originally broke imported DV content into 8-9 minute segments automatically for similar reasons. Most "modern" video editors no longer have to do this as the DV data is usually split into DV video with resampled LPCM audio during the import/capture process or converted to a more modern video editing format. (That was why I asked about resampling the audio above.) In addition, most older DV camcorders were sold defaulted to 12-bit audio sampling while most early versions of iMove expected 16-bit audio—i.e., another common source of audio sync issues.

So what I did was use streamclip to break the 45 minute file into shorter segments.

Then in streamclip I did 'save as' instead of 'export', which was much faster.

On the new iMAC, after it does a conversion, I can now play the files with sound in sync.

Not sure if you are segmenting the "source" file here or the "DV" file you created using MPEG Streamclip. If the former, then it would imply your source file may have a "security" trapping issue with the file container and the data itself is actually QTX compatible and may not really need to be converted at all—just copied to a new container to avoid the security trap. (This is why I ask so many questions about the source file compression format and request posters upload a sample clip for examination.)

Basically, if you are satisfied with your current workflow—then well and good. On the other hand, if you wish to refine and optimize your workflow, then try one of the alternatives suggested above to determine which best suits your needs.