I just custom-compiled Ardour 5.3.0

I know an acquaintance, whose name I will protect, who uses “Garage Band” on his Mac, but who has a hard time imagining that there exist many, many different programs like it, for other platforms, and that there must exist such, in order for professional musicians also to have access to a great array of such tools.

Of greater relevance is the fact, that such software exists under Linux as well – not just on Macs or PCs – as well as under Android.

And there is one observation which I would like to add, about what form this takes if users and artists wish to do audio work using Free, Open-Source applications.

Typically, we can access applications that do most of the work that polished, commercial examples offer. But one area in which the free applications do lag behind, is in the availability of sample packs – aka loops – which some artists will use to construct songs.

If Linux developers were to offer those, they would probably also need to ask for money.

Further, Garage Band has it as a specific advantage, that if such loops are simply dropped into the project, this program has the tempo stored, with which that loop was playing by default, in addition to which all DAWs have the tempo of the project set and available. Garage Band will automatically time-stretch the loop, to adapt to the project tempo. Most of the DAW programs I know, do not do this automatically.

A common ability the open-source applications offer though, is to time-stretch the sample manually after importing it, which can be as easy as shift-clicking on one of the edges of the sample and dragging it.

In order for this to be intuitive, it is helpful if the sample has first been processed with a Beat Slicer, so that the exact size of the rectangle will also snap into place with the timing marks on the project view, and the sample-tempo will match the project-tempo.

Shuriken_Klystr_1

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libsamplerate

In This Posting, I gave much thought, to how the ‘Digital Audio Workstation’ named QTractor might hypothetically do a sample-rate conversion.

I thought of several combinations, of “Half-Band Filters” that are based on the Sinc Function, and ‘Polynomial Smoothing’. The latter possibility would have often caused a computational penalty. But there was one, simpler combination of methods, which I did not think of.

QTractor uses a GPL Linux library named ‘libsamplerate‘. Its premise starts out with the idea, that a number of Half-Band Filters can be applied in correct sequences with 2x oversampling or 2x down-sampling, to achieve a variety of effects.

But then, ‘libsamplerate‘ does something ingenious in its simplicity: A Linear Interpolation! Linear interpolation will not offer as clean a spectrum as polynomial smoothing will in one step. But then, this library makes up for that, by just offering a finer resolution of oversampling, if the client application chooses it.

This library offers three quality levels:

  1. SRC_SINC_FASTEST
  2. SRC_SINC_MEDIUM_QUALITY
  3. SRC_SINC_BEST_QUALITY

 

Now, in This Posting, I identified an additional issue which arises, when we are doing an “Arbitrary Re-Sampling” and down-sampling. This issue was, that the source stream contains frequency components that are higher than the output stream Nyquist Frequency, and which need to be eliminated, even though the output stream is not in sync with the source stream.

To the best of my understanding, this problem can be solved, by making a temporary output stream 2x as fast as the final output stream, and then down-sampling by a factor of 2 again…

Sincerely,

Dirk

(Edit 07/21/2016 : ) The ‘GPL’ requires that this library be kept as free software, because it is in the nature of the GPL license, that any work derived from the code must also stay GPL, which stands of the “General Public License”.

But, because the possibility exists of some commercial exploitation being sought after, the Open-Source Software movement allows for a type of license, which is called the ‘LGPL’, which stands for the “Lesser General Public License”. The LGPL will allow for some software to be derived from the original code, which can be migrated into the private domain, so that the author of the derived code may close their source-code and sell their product for profit.

There exists a library similar to this one, that is named ‘libresample‘, with the express purpose that that one be LGPL code.

Yet, the authors of ‘libsamplerate‘ believe that this GPL version of the library is the superior one, which they would therefore have kept in the public domain.


 

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