Ouch! You're at risk because each update option -- either way -- normally acts on all tags.
Therefore, for normal use, it is essential to consider ONE of the two locations as the MASTER set of tags. Change values in either the database or the media files, but not both.
Because the update process (either way) overwrites ALL tag values, if there are differences between the SET of tag values in the database and the media file, you have to decide which SET to retain, then use update to replace one SET of tag values with the other.
In your case, with desired BPM tag values in the files but not in the library, the only way to put them in the library is to "update library from tags". (I'm assuming your file BPM tag has the same name MC would use for the same data.)
BUT, if you also have edited the standard tag values in the library but NOT in the files, "update library from tags" will wipe them out.
Here's a "should work" way to preserve all your library values but let you update the library with just the BPM value that is in the files:
(DISCLAIMER: I did something similar years ago with MC to switch from the standard Rating/stars field since I find it dangerous to use, but I haven't done this type of manipulation lately which is why I say "probably".)
1. Using a suitable tag-editing tool, copy all your BPM settings to a new custom tag name that does not otherwise exist in the files or the MC library. You need a tool that can work directly on the files and copy one specified tag's values to the custom tag without touching other tags. (MC possibly can do this; see TAG EDITING WITH MC below.)
2. In MC choose "update tags from library". MC will overwrite ALL the tags in your files that match the library with the values of the same tags in the library. Therefore, this update will wipe out the values you added to the standard BPM tag in the files. But assuming the new custom tag name n your files does not exist in your library, MC's file update process should ignore it, thereby preserving the BPM values.
3. The result is that your files have all your library values, AND the files still have the custom tag that has your BPM values. (The real BPM tag will now have whatever value was in MC's BPM field, which presumably you don't care about.)
4. Next, add the same new custom tag name name to your library as a custom field. Now MC can look for and read it from your files.
5. Then "update library from tags", which will bring the values in the custom field into your MC library. Now your MC library has the standard BPM field with values you don't want, and the custom field with values you do want.
6. Then use MC's Copy action to copy all the values from your custom field into the standard BPM field. Once you are satisfied that all is well, you can delete the custom field from MC.
7. At this point, the same values are in MC and the media files. For all future tag editing, pick ONE data store (MC's library, presumably) as the only place you change tag values, and set MC to store tags in files so they are updated automatically when you change data in the library.
TAG EDITING WITH MC: For step #1 you can probably use MC as the tag editing tool. Create a temporary library (empty), and load it with all your files. MC will read in the tag values, and/or you can use "update library from tags". Then add the custom field to the library, and use MC's Copy action to copy the BPM values into it. Now you'll have an MC library with all your file tag values plus the BPM value duplicated in the custom field. Then "update tags from library" to put this new field into the file tags. Then close this library and open your main MC library and continue with step #2.
Where this can get hairy is if you also have other custom fields already in the tags and in your permanent library, therefore you need to add them to the temporary library so they are preserved through the two-way updates. A tag editing tool that could act only on the standard and custom BPM fields would eliminate this concern. I'm not aware that MC can be told to touch just certain fields, except through the laborious process of setting all the other library fields to read-only.
Where this method could all break down is if your particular media file format can't store all of MC's values, but that's another reason to treat MC
s database as the master and consider in-file tags as a form of tag value backup.
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Topic: Difference between update library from tags and update tags from library??? (Read 1539 times)