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Sonnox Fraunhofer Pro‑Codec

Real‑time Data Compression Plug‑in By Mike Senior
Published August 2011

Sonnox's clever plug‑in allows you to hear exactly what different MP3 and AAC encoder settings do to your precious mixes — in a true blind test, if you like!

The Fraunhofer Pro‑Codec plug‑in's main real‑time comparison mode.The Fraunhofer Pro‑Codec plug‑in's main real‑time comparison mode.

With every passing year, it seems that, despite the earnest hand‑wringing of audiophiles the world over, more and more of the music that consumers hear has been mangled to some degree by data‑compression algorithms. As such, more conscientious mixing and mastering engineers have got into the habit of rendering and auditioning their work in a variety of data‑compressed formats, to alert themselves to the strengths and weaknesses of different compression algorithms — either so that they can reduce encoding side‑effects with pre‑emptive processing, or so that they can select and configure the most suitable encoding.

Pragmatic though this approach may be, it does suffer from the fact that data‑compression algorithms tend to be offline processes. In other words, you have to render your mix/master, then render a data‑compressed version, and then import both files back into your studio DAW system for comparison. Fortunately, though, respected plug‑in developers Sonnox have now teamed up with Fraunhofer IIS, the original inventors of MP3, to develop the first real‑time data‑compression plug‑in, Fraunhofer Pro‑Codec, thereby providing a means of streamlining this tediously iterative quality‑control exercise.

Real‑time Audio Data Compression

The software is available for both Windows XP/7 and Mac OS 10.5 or above, and comes in AU, RTAS and VST formats. Authorisation is via an iLok dongle and you can choose mono, stereo or 5.1 surround modes. Three tabs along the top of the window switch between its three operating modes: online (real‑time) encoding, offline encoding, and offline decoding. To start with, let me focus on the first of these, since it accounts for the lion's share of the feature set. The basic concept is that you insert up to five different data‑compression codecs into the Codec List slots, and then use the little speaker‑icon Monitor buttons to listen to the effect of each on the audio that is passing through the plug‑in. Alternatively, you can hit the Master In button to compare the currently selected codec with the unprocessed input signal, or listen to the difference between the input and encoded signals (in other words, the audio that the data compression is chucking out) by clicking on the Diff button.

A Lissajous display and dual vectorscopes provide detailed visual feedback about the impact of processing artifacts in the stereo field.A Lissajous display and dual vectorscopes provide detailed visual feedback about the impact of processing artifacts in the stereo field.Although part of what audio data‑compression schemes do is completely lossless (in other words, removing data redundancy on a purely mathematical level), this typically only cuts file sizes roughly in half. To carve anything else away, you need to start reducing the resolution of the audio information, which inevitably adds noise and/or distortion to the signal. The purpose of psychoacoustic encoding is to funnel these potentially undesirable artifacts into areas of the frequency spectrum where the human hearing system is least likely to perceive them, and it's because this bit of smoke and mirrors is so effective that typical MP3 files can be 10-12 times smaller than their uncompressed PCM source files. However, the more you try to squish the data, the more the processing artifacts, as showcased in the Difference signal, start to impinge unmusically on the listening experience. The aim of this plug‑in is to help you decide which codec produces the most acceptable trade‑off between audio enjoyment and file size.

While final quality judgements should clearly be made by ear, Fraunhofer Pro‑Codec does, nonetheless, provide a great deal of supplementary visual information to support the auditioning process. Central to this is the multi‑function display area that dominates the centre of the GUI. By default, this shows a zoomable, real‑time FFT frequency analysis (the yellow line) of either a single selected input channel or the mono sum of all of them. The red shaded region beneath the yellow line represents the spectral content of the Difference signal, while a set of green lines above it indicates the Noise to Mask Ratio (NMR), a theoretical measurement of how effectively any data-compression nasties are being concealed or 'masked' by the spectral energy of the music itself. The higher up the display the NMR line reaches in any given spectral band, the more likely it is that the listener will detect processing artifacts in that frequency region. Although Sonnox admit that a given individual's subjective impression of artifact levels inevitably won't correlate exactly to the calculated NMR readings, the dancing green line is still useful for focusing your attention towards the most likely spectral 'danger areas', as far as encoding quality is concerned.

When you're working with a stereo input, the Display Type button accesses no fewer than three circular meters. The left‑hand one is a Lissajous display with pink and yellow traces, allowing comparison of the input and output stereo envelopes respectively. The central meter is a faster‑acting stereo vectorscope, which shows the input signal in green against the Difference in red, while the right‑hand meter magnifies the Difference signal to enable closer inspection of the stereo characteristics of any encoding artifacts.

Further information about the encoded files can be gleaned from the three little tabbed pages to the right‑hand side of the Codec List. The first of these (Trim) reveals, for each codec in the list, what is likely to happen to the audio peak level when the encoded file is decoded for playback purposes. It's not uncommon for the peak level of data‑compressed decoded files to increase because of the way each codec's internal filtering routines affect the audio waveform, especially if you're working with mixes that have been heavily loudness‑enhanced at the mastering stage. Therefore this facility can help you reduce the risk of down‑the‑line clipping in the end user's playback device, by alerting you to the predicted degree of overshoot and by providing you with compensatory pre‑encoder gain control. The second tab (Comp) is simple enough, indicating the approximate data‑compression ratio per codec, while the third tab (AB or ABX) provides some nifty tools for ratifying your listening preferences in concrete terms (see 'Blind Testing' box for details).

Recording & Offline Processing

If you fancy blind‑testing your preferences while auditioning different codecs, try the simple AB facility (left) or, if you're really hardcore, the statistically measured ABX testing function (right).If you fancy blind‑testing your preferences while auditioning different codecs, try the simple AB facility (left) or, if you're really hardcore, the statistically measured ABX testing function (right).Sonnox Fraunhofer Pro‑Codec

When using the real‑time mode, you can record encoded audio from any or all of the entries in the Codec List, using the five Arm switches in conjunction with the master Record button. If your DAW is already playing back, the Record button engages recording immediately, but if not, the plug‑in obligingly waits until you start playback, to avoid any leading 'dead air' on the output files. Generating encoded files off‑line is also a piece of cake: select the plug‑in's Offline Encode tab and then drag the source file from your computer's OS onto the plug‑in window. You can then arm any of the Codec List entries, as before, and click the Encode button to zap all the files in one go. The Offline Decode tab works similarly, accepting any of the supported encoded formats and translating them into WAV or AIFF files at 16‑ or 24‑bit resolution.

However you decide to create encoded files, the plug‑in can be programmed to add up to six different suffixes to the filenames so that you know which version has been encoded in which manner — a very thoughtful inclusion. That said, there is currently only limited support for adding ID3 metadata to MP3 files, and no option at all for adding AAC metadata.

In Use

Once I'd spent 10 minutes reading up on what the various controls and displays do, Fraunhofer Pro‑Codec was pretty straightforward to use. Everything seemed to work as it should, and the switching was smooth and glitch‑free. The plug‑in's CPU munch varied a great deal depending on the number and type of codecs I inserted, and although normal processor drain measured around the five percent mark, at one point a single plug‑in instance was taking up more than 20 percent of my quad‑core's muscle while I was comparing the different variable bit‑rate options of MP3 in surround. So if your computer still has a floppy drive, you can probably forget about using this software.

The ability to solo and compare the side‑effects of different encoding routines is quite simply a revelation, especially given the additional analysis and metering facilities the plug‑in provides, and it's also interesting to discover some of the processing artifacts that can creep in if the lossless algorithms are fed with unsuitable material. Being able to statistically test subjective impressions about the codecs also provides an excellent perspective — and good ear‑training, too!

Here you can see Fraunhofer Pro‑Codec's offline encoding in action, converting a single, uncompressed source file into five different data‑compressed formats.Here you can see Fraunhofer Pro‑Codec's offline encoding in action, converting a single, uncompressed source file into five different data‑compressed formats.Despite an overwhelmingly positive experience with the plug‑in, I did experience a few twinges of concern. The first was that because the plug‑in relies so heavily on Fraunhofer's know‑how, there are quite a lot of popular alternative compression codecs not available for comparison — Windows Media, MP2, Ogg Vorbis and FLAC, to name a few. Furthermore, even if you own an offline codec for any of these formats, there's no way to plumb the encoded audio render into Fraunhofer Pro‑Codec's neat and effective ABX test system, which is a disappointment.

I was also peeved that there was no way to record the uncompressed input signal alongside the compressed formats. Most DAW systems will let you bounce the output of your master output channel alongside, but that does seem a little inelegant. You could also argue that including a lossless algorithm in the Codec List could achieve something tantamount to this, but that feels pretty kludgy too. Finally, I was surprised to find that the off‑line encode and decode facilities would only process one source file at a time, rather than allowing batch‑processing of a queue of several. Dragging and dropping a few albums' worth of files one by one is far from exhilarating!

Squeeze Me!

But I don't want to end this review on a sour note, because none of these niggles stops Fraunhofer Pro‑Codec from being an excellent product concept at heart, and one aimed squarely at a wide‑open market niche. Mixing and mastering engineers alike stand to learn a great deal from what this plug‑in reveals, as well as saving a chunk of time into the bargain. While it's not exactly cheap, it does something so useful and unique that Sonnox would probably have had to charge the same again to dissuade me from casting my own plastic vote.  

Blind Testing

Not sure you can tell the difference? Pro‑Codec can set your mind at rest with its two blind‑testing modes: AB and ABX. The first simply lets you toggle between auditioning any pair of codecs (or any codec and the input signal) from a single button. Normally the plug‑in's GUI makes it visually clear which signal you're listening to, but if you don't want that distraction, the Hide button banishes all visual cues. The ABX mode provides a more formal extension of this idea, offering a proper ABX testing scheme which will inform you in concrete terms whether your preference for one of any pair of the available signals is statistically significant. I love this feature — so much so, in fact, that I wish Sonnox would issue it on its own as a stand‑alone plug‑in, so that I could test other plug‑in processes with it too!

Supported Codecs

There are six basic codec types available within Fraunhofer Pro‑Codec: MP3, MP3 HD, AAC‑LC, HE‑AAC, HE‑AAC v2 and HD‑AAC. The supplied PDF manual goes on at length about the exact differences between them, but for those in a hurry, here's a 'back of an envelope' overview.

All the codecs support mono, stereo, and 5.1 encoding/decoding, with the exception of the stereo‑only HE‑AAC v2. Session sample rates of 32kHz, 44.1kHz and 48kHz are supported across the board too, but HE‑AAC will also handle projects at doubled sample rates for surround work, while HD‑AAC will accomodate up to 192kHz. Where the codec doesn't support your session sample rate, the plug‑in downsamples the codec's input to an appropriate supported rate. It's also worth mentioning that proper real‑time auditioning of the highest‑quality MP3, AAC‑LC and HE‑AAC codecs within the plug‑in isn't possible in those cases where resampling is required to achieve the target bit rate, in which case a lower‑quality algorithm is used for auditioning than is actually used to encode files for output purposes. I don't think either of these issues are likely to cramp the style of most users, but the Codec List entry is helpfully coloured red to ensure you're aware of what's happening nonetheless.

Both variable and constant bit rates are available for the lossy codecs, although the range of constant bit rates available depends on the plug‑in's channel configuration and your DAW's session sample rate. Beyond these general points, here are some specifics about each codec:

  • MP3: The ubiquitous web download format. Provides constant bit rates of 40‑320kbps, with compression ratios roughly in the 6:1‑20:1 range.
  • MP3 HD: A 'high definition' lossless MP3 codec for 16‑bit audio. Compression ratio of roughly 2:1.
  • AAC‑LC: The 'low complexity' codec most commonly encountered via Apple's iTunes. Provides constant bit rates of 32‑800kbps, with compression ratios roughly in the 6:1‑20:1 range.
  • HE‑AAC: A 'high efficiency' codec for mobile devices, streaming media, and digital radio (most notably Digital Radio Mondiale). Provides constant bit rates of 12‑640kbps, with compression ratios roughly in the 16:1‑80:1 range.
  • HE‑AAC v2: Another 'high efficiency' codec, much like HE‑AAC, but incorporating refinements dedicated to low‑bit‑rate stereo applications such as DAB+ digital radio and Internet streaming to mobile devices. Provides 12‑56kbps constant bit rates, with compression ratios roughly in the 32:1‑80:1 range.
  • HD‑AAC: A 'high definition' lossless MP3 codec capable of 24‑bit resolution. Compression ratio of roughly 2:1.

MP3 Licensing

Something I strongly suspect most studio users are unaware of is that both the MP3 and AAC compression routines are based on Fraunhofer patents, and as such they are legally entitled to royalty payments from anyone using MP3 or AAC codecs for commercial purposes — a fact spelt out in both Fraunhofer Pro‑Codec's PDF manual and the End User Licence Agreement. The collection of these payments is coordinated by their licensing representative, a company called Thomson, and full information of the scale of these payments is provided on their site ( At present, no royalties are required for AAC codec usage, and any enterprise turning over less than $100,000 per year also appears to be exempt from royalty payments for the MP3 codecs too. While this means that this licensing issue is unlikely to cause most users any sleepless nights at present, there is no guarantee that these conditions might not change in the future.


  • A neat, quick, and easy‑to‑use tool for comparing some of the most popular data‑compressed audio formats.
  • Real‑time and off‑line encoding/decoding with up to five different codecs simultaneously.
  • Excellent visual analysis tools and a great little ABX statistical test machine onboard.


  • Some popular alternative data‑compression codecs, such as Windows Media, MP2, Ogg Vorbis and FLAC, are excluded.
  • No off‑line batch processing. You'll just have to convert one source file at a time, I'm afraid.


Despite its limited reach in terms of the codecs available for comparison, this is a product that can provide mixing and mastering engineers with an extremely valuable insight into the impact of data compression on their work.

Test Spec

  • Sonnox Fraunhofer Pro‑Codec v1.0.0.4723.
  • Rain Recording Solstice O3 PC with AMD Phenom II X4 810 quad‑core 2.61GHz processor, 4GB DDR2 RAM, running Windows XP Pro SP3.
  • Tested with Steinberg Cubase v6.0.0 and Cockos Reaper v3.75.