48kbps AAC public test

In the lossy audio coding field, competition is nowadays focused on two application fields: multichannel encodings and low bitrate encodings. This test is focusing on the second one, the low bitrates area. The purpose of this test is to establish the quality that state of the art MPEG audio encoders can provide. The flagship natural audio format in this family is AAC. During the last years, low bitrate AAC has been enhanced by two new tools:

A tool designed to reproduce higher frequencies based the lower frequency part and some parametric information. AAC-LC (Low Complexity) with added SBR is called HE-AAC (High Efficiency), or HE-AAC v1.

A tool designed to artificially reproduce the stereophonic sound field. Audio data is encoded as a monophonic stream, and some additionnal parametric information is embedded into the stream to allow re-creation of stereo. HE-AAC with added PS is called HE-AAC v2.

Those two tools are parametric tools, clearly geared toward low bitrate encoding. When increasing bitrate, they become less interesting, and altough they shine at very low bitrates, from a given point can even reduce quality. At 48kbps it is not yet clearly known if HE-AAC v2 still provides more benefits than drawbacks against HE-AAC v1.

Test results

The test is now closed. Results are available here[8].

Which encoders and settings are tested?


Fixed point encoder version 6.3.0, available as source code from 3GPP[6]. 48 kbps constant bitrate.

Integrated into Winamp 5.2 beta 393, 48 kbps constant bitrate.

Integrated into Winamp 5.2 beta 393, 48 kbps constant bitrate.

Pre-version (not yet released), 48 kbps average bitrate

Pre-version (not yet released), 48 kbps average bitrate


Anchors are not real contenders of the test, but are featured to provide a proper scaling of test results.

High anchor. Version 3.97b2, used with "-V5". This is a variable bitrate setting, providing about 130kbps on average (i.e. when encoding a lot of files, bitrate on individual files can vary)

Low Anchor. Version 6.0.2, 48 kbps constant bitrate.

Who should take the test?

Anyone interested in audio quality at very low bitrates. Or people that have no interest but would like to help making this test better.

How do I take the test?

When is the test running?

The public 48kbps aac test will start on the 20th of February, 2006. It will end on the 19th of March, 2006.


No, even a single tested sample is interesting, so you can send your results. Of course, the more the better.

Let the sliders at 5.0 for the contenders that you are not able to discern from the reference. Results are still valid.

Yes, ABC/HR is in Java, and Antoine Pitrou submitted a .sh script to uncompress the samples. Note that you will need binaries of Faad, Madplay, Flac and Ssrc[7] to be able to uncompress the samples.


If you have any other question, please get in touch.

Thank you very much.