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fre:ac Developer Blog

Welcome to the fre:ac developer blog. I will post status updates and other information about fre:ac development here.

fre:ac development status update 12/2018 Print
Written by Robert   
Saturday, 05 January 2019 23:18

Hi and welcome to the latest fre:ac development status update! I did not write a report for November as I was busy preparing the latest alpha release around the end of the month, so this report covers both, November and December of 2018.

BeGeistert 2018

On the 3rd and 4th of November I attended the BeGeistert 2018 Haiku developers conference which was held in my hometown of Hamburg, Germany. I met great people from the Haiku project, got to work on the fre:ac port for Haiku and ported Monkey's Audio over.

It was a great event and a nice experience and I'm already looking forward to attending future BeGeistert meetings in the years to come.

Flatpak (and work on Snap)

The big news for December certainly is fre:ac finally being available as a Flatpak. This makes it easier than ever for Linux users to discover and try out fre:ac.

The fre:ac Flatpak is available from Flathub, the largest application repository for Flatpak distribution.

Now that the Flatpak is ready, I'm shifting my attention towards building a Snap package as well. There already is some progress, but I did not get to work on the project a lot during the holiday season. Thus I now expect the Snap to be ready around mid January.


The December release of fre:ac added a notification component that can play a customizable sound or display a message box upon finished conversions. This was requested by several users doing very large conversions that might take anywhere from several minutes to hours. They often leave fre:ac running unattended, so an audible notification upon job completion can be useful.

Additionally, the December release now supports accent colors on macOS Mojave and improves scaling on HiDPI displays, especially with radio buttons and with edit fields where text was sometimes cut off in scaled mode.

Minor enhancements added in November and December include an option to switch stereo channels that was added to the channel converter DSP component and support for drag & drop in the tag editor. The latter did not make it into the December release though and will debut in the next alpha or beta.

Various fixes

In addition to the above changes, many bugs have been fixed in the past two months:

  • Fixed MP4 files not being created when <directory> placeholder is used in output filename pattern
  • Fixed tag editor being unable to open some file types on case-sensitive file systems
  • Fixed freaccmd issues when input and output files are the same
  • Fixed crash when converting mono to stereo
  • Fixed encoding mono MP3s

That's all for this issue. The report for January should be out in early February, so make sure to check the site for updates.

fre:ac development status update 10/2018 Print
Written by Robert   
Saturday, 03 November 2018 09:35

Hi all, this is the fre:ac development status update for October 2018. I have a lot of interesting things to share.

Haiku packages and fixes

fre:ac running on HaikuFirst and foremost, I created fre:ac packages for the Haiku operating system. The Haiku project just released their first beta in the end of September and fre:ac is now available via their HaikuDepot package manager. Just search for it and you are one click away from installing fre:ac.

I also fixed the Fraunhofer AAC encoder and updated the LAME MP3 encoder package for Haiku while at it. I plan to also port Monkey's Audio to this interesting OS.

But while it's great that fre:ac now runs on Haiku, the 20180913 release is still far from perfect. I implemented a lot of fixes in October to make the next alpha run much better:

  • Fixed window titles becoming inactive when a menu, dropdown or tooltip is shown
  • Fixed scrollbars not working when moving the cursor outside the window
  • Improved image downscaling using a weighted average box filter
  • Added support for font scaling (limited HiDPI mode)
  • Fixed application signature to avoid warning on startup
  • Fixed resource compilation to simplify package script

A new alpha with these fixes and more will probably be out in mid-November.

Notarization of macOS version

Apple recently introduced notarization for apps distributed outside the app store. It's basically an automated malware and best-practices check for applications that when passing will display a "this app was checked by Apple" message on macOS Mojave.

It took me a weekend to adapt the fre:ac package for this new security feature, but now it's functioning very well. Starting with the next release, fre:ac for macOS will be notarized by Apple.

Other fixes

Some other fixes have been implemented in October:

  • Linux HiDPI fixes
    The Linux version of fre:ac will now evaluate the GDK_SCALE environment variable in order to automatically scale the font and UI element size on HiDPI displays.
  • freaccmd fixes for non-Windows systems
    The fre:ac command line interface had some issues with spaces in file names. Namely, the current version does not work with the standard Unix shell space escaping (which is putting a backslash in front of each space character). This will be fixed in the next release.
  • Fixed issue with enabling "Write to input folder" and "Delete original files after encoding" options
    When enabling the "Write to input folder" and "Delete original files after encoding" options at the same time and the output filename equals the input filename, the current version of fre:ac can delete both, the existing input and the new output file after conversion, leaving you with none of the two. The next release will fix this and will be brought forward one or two weeks for this.

Work place changes

Finally, after more than 15 years at the previous company, I changed my job on 1st of October. I'm now working for PreSonus Software Ltd. in Hamburg, Germany on tasks much closer to what I'm already doing in my free time with fre:ac.

I will mainly work on the Notion music notation software, but also look into improving Studio One's audio file export and conversion capabilities and much more.

I'm happy to be part of the PreSonus team and really looking forward to great things coming in the future.

That's all for this month. Be sure to come back next month for another update.

fre:ac development status update 09/2018 Print
Written by Robert   
Sunday, 07 October 2018 17:47

Hi all, this is the fre:ac development status update for September 2018.

New alpha release

I published a new alpha last month, integrating the SuperFast encoding mode and freaccmd's dynamic arguments support that I wrote about in last month's issue. With these features added, the new alpha is now almost feature complete with respect to what is planned for fre:ac 1.1 beta.

Improved tags interoperability

Several changes have been implemented to improve compatibility with tags written by other applications (most notably foobar2000). These changes enable fre:ac to extract more information from files containing such tags than previously possible. The individual improvements are:

  • Support reading and writing ID3v2 tags from/to .wav files
  • Support reading cue sheets from ID3v2 tags
  • Use album artist for artist field if only the former is set

One example where these will be useful is when ripping a CD to a single .wav file with foobar2000. Upcoming fre:ac releases will be able to list the individual tracks contained in the .wav file even if no separate .cue file is available.

Improved error handling

Despite fre:ac 1.1 alpha releases having supported parallel conversions for some time, error handling is still designed for single threaded operation. When a conversion fails, the affected thread shows an error message and the whole conversion process stops at that point.

This will be much improved in the next release. An error in one conversion will not stop the whole process any longer. Instead, error messages will be collected and displayed all in one dialog at the end of the conversion process.

In other words, when the 100th file in a 1000 files conversion job causes an error, fre:ac will continue to process the other 900 files instead of stopping the whole job. No more worrying that large unattended conversion jobs stop early without you noticing.

HiDPI improvements

Last but not least, I've also implemented some improvements for the placement of tool windows and dialogs in HiDPI mode. Dropdown lists, popup menus, tooltips and dialogs should now be displayed where you would expect them when running in scaled mode.

This concludes this month's issue. Be sure to come back in a month for the next report.

fre:ac development status update 08/2018 Print
Written by Robert   
Monday, 10 September 2018 21:59

So I'm really late for the monthly status update this time. Will try to post earlier next month.

There are only two things to be reported this time, but those surely are great news:

SuperFast codecs merged into mainline

The SuperFast versions of the LAME, AAC (FAAC, FDK-AAC and Core Audio), Opus and Speex codecs have finally been merged into mainline fre:ac. You still have to enable SuperFast mode manually on the Resources configuration page to make full use of them, but that option is set to become enabled by default before the fre:ac 1.1 final release.

Enabling SuperFast mode
SuperFast mode is marked experimental for now and has to be enabled manually

Read here and here for more details on fre:ac's SuperFast encoding technology.

Dynamic arguments feature merged

I wrote about the dynamic arguments feature for freaccmd in last month's issue. Now that code has been merged, making it the second big feature completed this month.

While not visible for users of the graphical UI, this will make things much easier for those who would like to script and/or automate conversions with the command line interface.

There will be a new alpha release with these changes in the next few days. Keep watching for it and stay tuned for the next update in about a month.

fre:ac development status update 07/2018 Print
Written by Robert   
Sunday, 05 August 2018 19:39

Hi all, here is the July 2018 update on fre:ac development.

New alphas and SuperFast preview

There were two new alpha releases last month providing all the new features talked about in previous development status reports and fixing several issues. Unlike previous alphas, the current version is now recommended for everyone to try out. If you find any issues, please report them to

There also is a new preview of the SuperFast conversion technology, this time including SuperFast LAME. If you missed the article on how SuperFast LAME is made possible, check it out here: SuperFast LAME technical details

The SuperFast technology will be included in the next regular alpha release and will then be available for everyone to use.

Dynamic encoder arguments for freaccmd

An important item on the feature list for fre:ac 1.1 beta is support for command line configuration arguments for all codecs added during fre:ac 1.1 development. Currently, freaccmd supports arguments for a very limited set of codecs only. The code to change that lives in the dynamic-arguments branches of fre:ac and BoCA and I made good progress towards merging it with the master branches in the past month.

With the new code, codecs can specify which command line arguments they support and freaccmd will make them available without the need for any codec specific code in the command line frontend itself.

Other changes

  • Improved handling of album artist in tag editor
    Handling of album artists in the tag editor has been improved to fix issues when the field is changed in album editing mode. Prior to this change this could result in the album artist information getting lost for some or all of the relevant tracks.
  • Improved adjustment of dialogs to text sizes
    Several dialogs have been reworked to adjust dynamically to the size of translated texts. The width of labels displayed in a dialog can vary greatly in different languages, so dynamic adjustment is necessary. This is not completed yet, so work on this will continue in the next few months.

That's all for this issue. Make sure to come back next month for the August status update.

SuperFast LAME technical details Print
Written by Robert   
Friday, 20 July 2018 19:27

So I finished the SuperFast LAME multi-threaded MP3 encoder last week and it's time to write about some technical aspects of it.

tl;dr: Implementing SuperFast LAME required some additional work to handle certain features of the MP3 format. You can download a preview release of fre:ac with SuperFast LAME support from GitHub.

The challenge

SuperFast LAME is significantly more complex than the SuperFast components for AAC, Opus and Speex, mostly because of technical peculiarities of the MP3 format.

The main difficulty is that while most other formats have discrete frames of audio samples in their bitstreams, MP3 frames can overlap each other:

In this example, the average frame size is 4 blocks of data. The individual frame lengths are 4, 3, 4, 3, 1, 5, 5 and 7 blocks. In an AAC bitstream, each frame will simply have a length matching the number of data blocks required for that frame and the frames will neatly come one after another. In an MP3 bitstream, however, (at least for CBR files, VBR is more complicated) frames have a fixed size and when there is space left in a frame after all samples have been encoded, that space can be used by the following frames. This space available to following frames is called bit reservoir and allows the codec to maintain a set target quality in most cases, even when frame sizes are fixed and audio complexity changes.

Have a look at the example. The 5th frame is only one data block long and that data block fits completely into the 4th frame. It even leaves some space, so the first data block of the 6th frame starts in the 4th frame as well. Looking at only the 5th and 6th frame, their layout in the bitstream looks like this:

Here the frame headers come after the data and in case of frame #5, there even is data of another frame (#6) between its data and its header. In real world MP3 streams, the situation can be even more intricate.

Basic SuperFast operation

So this is a problem when implementing the SuperFast technology for MP3. SuperFast works by passing chunks of audio data to separate encoder instances and later joining the encoded data blocks back together in the right order. This requires the frames to be available in discrete form in order to deal with overlap and joining the frames correctly. The SuperFast encoding loop usually looks like this (click to jump to example source code):

  1. Find next worker thread and wait until it is ready
  2. Check if worker thread has encoded frames
    a. Skip overlap frames
    b. Write other frames to output stream
  3. Pass next chunk of audio to worker thread

MP3 difficulties

When dealing with MP3, multiple issues arise from the peculiarities around the bit reservoir:

  • The encoder might not return all encoded frames after processing a chunk of data as some frames might still be waiting for additional data to put in the bit reservoir.
  • Frames are not available in discrete form, but may be overlapping each other.
  • After dealing with the above, frames need to be put back into an MP3 compatible bitstream after joining.
  • Frames might require more reservoir than is available after joining with frames coming from other codec instances.

Previous attempts to create multi-threaded MP3 encoders dealt with these issues in a very simple way: They completely disabled the bit reservoir to get nicely laid out frames with no overlapping data. This solution cuts into the resulting MP3's quality, though, which is why such encoders never really gained traction.

So let's see how we can handle these issues more adequately.

Unraveling it

The first one is relatively simple. After encoding a chunk of data, we call lame_encode_flush_no_gap to force the encoder to return all encoded frames even if they are not completely filled yet. This makes sure we can operate with all the relevant frames in the next steps.

The second issue is handled by a bitstream unpacker that parses the data returned by the encoder and extracts discrete frames from the bitstream. After this step all frames will be laid out as a frame header followed by the complete data belonging to that frame. No more intermixing with other frames' headers or data.

After unpacking, we are ready to perform overlap skipping and ordering of data chunks from different encoder instances.

When writing the ordered frames to the output stream, we now need to make sure to repack them back into an MP3 compatible bitstream. The repacker deals with frame sizes and the bit reservoir and tries to pack frames in the most compact way.

Sometimes, though, a frame requires more reservoir than is currently available and the repacker needs to find a way to fit it in. It basically has two options to accomplish this: If only a few extra bits are needed, the repacker can add padding to a frame. This will add an additional byte and sometimes this is enough to provide the required reservoir. In cases where it is not sufficient, the repacker can enlarge one or more previous frames to a bigger frame size. This usually allows to provide enough reservoir, but requires all affected frames to be repacked again.

However, even this might not be enough when issue number 4 comes into play. In some rare cases, a frame requires so much reservoir that it is simply not possible to fit it into the bitstream. This can happen because one encoder instance cannot know how much reservoir will be left over by the instance encoding the preceding chunk. In cases where the preceding instance has to deal with a difficult to encode signal, it might leave next to no reservoir available to the next encoder.

Dealing with this was difficult. While there are some simple options like forcing the encoder to use a lower bitrate, these might potentially result in audible quality drops. So I tried to find another way to handle this.

Basically, the SuperFast algorithm will try to re-encode the audio part starting with the non-fitting frame and repeat this until it fits. To work around situations where it might never fit using this strategy, each time it fails, we try to put some more pressure on the bit reservoir by prepending a few frames of difficult to encode dummy data. These dummy frames force the encoder to spend some reservoir on them and lead to using less reservoir for our previously non-fitting frame, eventually allowing us to fit the frame into the bitstream.

The result

With all these additional steps, the process for SuperFast LAME now looks like this (click to jump to source code):

  1. Get next worker thread and wait until it is ready
  2. Check if worker thread has encoded frames
    a. Unpack frames
    b. Skip overlap frames
    c. Repack other frames
    i. If successful, continue with 2d
    ii. Write completed frames to output stream
    iii. Put increasing pressure on the reservoir
    iv. Re-encode starting from failed frame
    v. Continue with 2a
    d. Write repacked frames to output stream
  3. Pass next chunk of audio to worker thread

Arriving at this point took several months of work, but was absolutely worth it. The SuperFast LAME encoder scales well with the number of CPU cores and can provide a 3.5x speedup on a quad-core processor. On my 8 core, 16 thread CPU, I was able to achieve up to 12x speed increase with it.

Unlike previous attempts to speed up MP3 encoding, SuperFast LAME does this while still using the MP3 format's bit reservoir feature and uses an unmodified encoder library - the necessary changes are all implemented in the frontend application and could be used with alternative MP3 encoders as well.

I plan to implement this technology on top of the command line LAME frontend in the future. For now, my priority is on releasing fre:ac 1.1 beta and final versions, though. But keep watching this blog for future annoucements about a SuperFast enabled stand-alone LAME version.


SuperFast LAME is now in testing and included in the SuperFast Preview Release 3 available at GitHub.

Source code

Check out the SuperFast repository on GitHub if you would like to learn more or build the code yourself. The SuperFast LAME implementation can be found in the components/lame folder.

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