Automatic conversion of aac to m4a

[bat guano]

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> If anybody feels like testing it, I've put up a fork on my github account, https://github.com/nickludlam/get_iplayer
>
> Cheers,
> Nick
>
Well, I tested your version with this command:-
get_iplayer --get --type=radio --force --m4a --mp4box=/usr/bin/MP4Box --pid=b00zd8f3

Bells on Sunday.
My MP4Box is in /usr/bin (not /usr/local/bin)

The aac file downloaded ok, but it didn't use MP4Box.
It gave this error:-
Can't use an undefined value as an ARRAY reference at /usr/bin/get_iplayer line 4715.

I'm using Ubuntu.