Hi there,

So, when encoding a FFAVIN stream into a tiled dash, I have this warning message:

`[FFAVIn] Unknown frame rate, will use 25 fps - use `:#FPS=VAL` to force frame rate signaling`

However, when looking at the filters documentation either [here](https://github.com/gpac/gpac/wiki/ffavin) or `gpac -hx ffavin `, nothing seem to be corresponding to this warning message.

1. What is the relevant option?
2. Is there a way to make the warning message more explicit?

----
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

Unclear warning FFAVIN message
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Unclear warning FFAVIN message
Issue date:

--- start body ---
Hi there,

So, when encoding a FFAVIN stream into a tiled dash, I have this warning message:

`[FFAVIn] Unknown frame rate, will use 25 fps - use `:#FPS=VAL` to force frame rate signaling`

However, when looking at the filters documentation either [here](https://github.com/gpac/gpac/wiki/ffavin) or `gpac -hx ffavin `, nothing seem to be corresponding to this warning message.

1. What is the relevant option?
2. Is there a way to make the warning message more explicit?

----
Thanks for reporting your issue. Please make sure these boxes are checked before submitting your issue - thank you!

- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). I can share files anonymously with this dropbox: https://www.mediafire.com/filedrop/filedrop_hosted.php?drop=eec9e058a9486fe4e99c33021481d9e1826ca9dbc242a6cfaab0fe95da5e5d95

Detailed guidelines: http://gpac.io/2013/07/16/how-to-file-a-bug-properly/

--- end body ---
