Seeking info on Tempo Markers

[dmitch57]

I'm trying to learn about Tempo Markers and their storage. Disclaimer: I'm not trying to solve a particular problem, I'm just trying to understand something that's not documented very well in the Logic Pro docs. 

Take a project which records in WAV format (technically BWF - Broadcast WAV Format - but we call it WAV). There is a gradual tempo change (an accelerando) in the area we're recording. Record an audio region in that section. Enable Flex Time view, turn on Flex & Follow for the new region. Look at the region in the audio editor and see a bunch of Tempo Markers (solid vertical blue lines). For simplicity the audio itself is silence. All we see is an array of Tempo Markers, like this:

 

The Finder window shows the creation time of the actual WAV file.

Now remove the tempo information from that audio file (right click on region > Tempo > Remove Tempo Information). Look at the region now:

Note two things. 

1. The Tempo Markers (solid blue) changed to Flex Time markers (white).
2. The mod date of the audio file did not change. This was a few minutes after the audio file was first created.

 

Now add the tempo info back into the audio file (right click on region > Tempo > Write Project Tempo to Audio File). Look at the file again:

The Flex Markers are back to being Tempo Markers. Cool. But the mod date of the audio file has not changed. And no tag edit/dump program I have shows any tags at all in that file. 

I conclude that the Tempo information that Logic Pro is claiming to write to the audio file is not really being written to the audio file, although the Audio File Editor acts as if it is. (E.g. you can't select and/or delete single Tempo Markers, like you can with Flex Markers.)

So. Is this tempo information just stored in the LP project file? Is it ever written to an audio file? How, and in what format (I sort of assume iXML but until I see it with my own eyes I won't be sure)? And an extra bonus question: if tempo info is written to an audio file, what's a good external program for viewing it?

Thanks.

[David Nahmani]

The tempo information is written to the audio file, and contained inside the wave file as meta data. If you import that audio file in a new project, the new project can read and import that tempo data. 

[dmitch57]

Can you explain why the mod date of the audio file isn't changing in the above cases? Also - do you know a good external tool to view the tempo info in an audio file? Thanks.

[wonshu]

It should change, but we can't rely on Logic actually doing that. I've seen this behaviour as well...

[dmitch57]

Thanks for the replies.

I'm also interested learning about the format of the tempo information stored in WAV (and, presumably, also in AIFF) files. This is partly to determine interoperability with other DAWs. I mean, like the DAWs my friends have. I've been surprised at the minimal amount of info out there on the web on this subject.

[dmitch57]

Hah hah. The file size in bytes does change after an "add to" or "remove from" tempo info op...

 

With no tempo info

 

Rambler.local Audio Files> ls -l
-rw-r--r--@ 1 dmitch  staff  1633858 Aug 17 09:48 audio wav#01.wav

With tempo info

 

Rambler.local Audio Files> ls -l
-rw-r--r--@ 1 dmitch  staff  1634730 Aug 17 09:48 audio wav#01.wav

So that file mod date is pretty much meaningless. At least it's not trustworthy. Interesting.

Still, the tag dump programs I have (Fission, Metadatics, JRiver, iTunes) don't show any tag info at all in this file when it has tempo info. I'd love to find one that does...

 

 

 

 

[danisalat]

On 8/17/2023 at 6:15 PM, David Nahmani said:

The tempo information is written to the audio file, and contained inside the wave file as meta data. If you import that audio file in a new project, the new project can read and import that tempo data. 

Is there a way to set Logic so it doesn't write tempo information into files when recording, bouncing, etc? I always end up having to delete the info and also have trouble importing it into other DAWs, and I find no benefits for having it embedded.

[David Nahmani]

2 minutes ago, danisalat said:

Is there a way to set Logic so it doesn't write tempo information into files when recording, bouncing, etc?

When bouncing a project or section, there's a checkbox called "Include Tempo Information" that you can disable. That's the only option to do that that I know of.