Add functions to change compression level and bitrate mode.

[ytya]

Added interface to change compression level and bitrate mode as described in #390.
I was able to test it on Windows, but not on other platforms.

[bastibe]

Very cool, thank you very much! But I think the write function is the wrong place for this.

[ytya]

Thank you for the review. I've moved them to parameter of SoundFile. Is that what you meant?

[bastibe]

Thank you for the changes. This looks better already.

But I think there's still a misunderstanding. As far as I can tell, we can't change the bitrate or compression of an already-opened file. Thus, we should make these items constructor arguments, and otherwise read-only.

But please do correct me if I'm misunderstanding this.

Additionally, SoundFile is still predominantly used with uncompressed files, where these settings make no sense. They should probably be hidden from the repr where they're not applicable.

[sammlapp]

ah I had just finally started implementing the bitrate in a branch myself, glad to see this is already nearing completion

[sammlapp]

also, I tested with OPUS as well as MP3 and am happy to see that setting the compression level in this way works across formats. This will resolve my request in #252 for writing OPUS with variable bit rates as well as resolving #390

[ytya]

@bastibe
I followed your advice and changed the code. As for __repr__, i thought there was no problem with the operation as None was entered before the change, but we modified it from the point of view of not displaying extra information.
Please feel free to point out if this is not what you intended.

@sammlapp
Thanks for testing. I also tested opus and flac and it worked.

[bastibe]

This looks really good, thank you very much!

Just one more comment: If I'm reading this correctly, the compression/bitrate information is only available if it was passed to the constructor. But it's None if we opened a pre-existing file, or did not specify the default arguments.

Is there a reasonable way of populating the two properties automatically if they were not given by the user? (Or perhaps I'm misunderstanding the code?)

[ytya]

Firstly, if no arguments are specified, this is the default behaviour of libsndfile. This is the same behaviour as the current soundfile, so there should be no problem.

Secondly, if you open pre-existing file, I don't know how to set these parameters. The libsndfile does not have a GET_COMPRESSION_LEVEL command, so I can't get the compression level. There is a GET_BITRATE_MODE command, so I should be able to get it for bitrate mode, but getting just that is not very useful.

The sound quality will deteriorate as the lossy compression is repeated, so I personally think that the case of saving a file with the same compression as the opened file is not important. This function should generally be used when converting formats, or when saving edited sound files as a last step.

[bastibe]

I see. Well, without a GET_COMPRESSION_LEVEL command, we can't really display anything. In that case, the current state is good. Thank you!

Just one more note before we merge: the test is currently broken on Python < 3.9, because it's using numpy.concat, which was not yet available. Could you fix that test?

[ytya]

Sorry, not enough testing. This should be fine now.

[bastibe]

Thank you very, very much for your contribution!

[sammlapp]

sf.write('max_compression.mp3', data, samplerate, bitrate_mode='VARIABLE', compression_level=1)
gives an error (works fine if 0<=compression_level<1.0), is this expected?

[ytya]

It's the libmp3lame specification used by libsndfile. As reported here, setting compression_level=1 with VBR causes libsndfile to return an error.

[sammlapp]

Ok, perhaps this could be clarified in the docstring. Also, I'm adding a separate PR that just provides a demo of this functionality in the readme, @bastibe feel free to accept or reject, no offense taken if you reject :)