Codec listening test
A codec listening test is a scientific study designed to compare two or more lossy audio codecs, usually with respect to perceived fidelity or compression efficiency.
Most tests take the form of a double-blind comparison. Commonly used methods are known as "ABX" or "ABC/HR" or "MUSHRA". There are various software packages available for individuals to perform this type of testing themselves with minimal assistance.
Testing methods
ABX test
In an ABX test, the listener has to identify an unknown sample X as being A or B, with A and B available for reference. The outcome of a test must be statistically significant. This setup ensures that the listener is not biased by his/her expectations, and that the outcome is not likely to be the result of chance. If sample X cannot be determined reliably with a low p-value in a predetermined number of trials, then the null hypothesis cannot be rejected and it cannot be proved that there is a perceptible difference between samples A and B. This usually indicates that the encoded version will actually be transparent to the listener.ABC/HR test
In an ABC/HR test, C is the original which is always available for reference. A and B are the original and the encoded version in randomized order. The listener must first distinguish the encoded version from the original, prior to assigning a score as a subjective judgment of the quality. Different encoded versions can be compared against each other using these scores.MUSHRA
In MUSHRA, the listener is presented with the reference, a certain number of test samples, a hidden version of the reference and one or more anchors. The purpose of the anchor is to make the scale be closer to an "absolute scale", making sure that minor artifacts are not rated as having very bad quality.Results
Many double-blind music listening tests have been carried out. The following table lists the results of several listening tests that have been published online. To obtain meaningful results, listening tests must compare codecs' performance at similar or identical bitrates, since the audio quality produced by any lossy encoder will be trivially improved by increasing the bitrate. If listeners cannot consistently distinguish a lossy encoder's output from the uncompressed original audio, then it may be concluded that the codec has achieved transparency.Popular formats compared in these tests include MP3, AAC, Vorbis, Musepack, and WMA. The RealAudio Gecko, ATRAC3, QDesign, and mp3PRO formats appear in some tests, despite much lower adoption. Many encoder and decoder implementations exist for some formats, such as MP3, which is the oldest and best-known format still in widespread use today.
| Source | Dates | Formats | Bitrate | Codecs | Musical genres | Samples | Listeners | Best Result | Comments |
| 2001 | multiple | ~128 |
| 1 | 16 | Musepack and AAC | |||
| October 2001 - January 2002 | multiple | ~128 | Various | 3 | 25-28 | Musepack or Vorbis | |||
| July 2002 | multiple | ~64 |
| Various | 12 | 24-41 | mp3PRO | Both Vorbis variants were a close second. | |
| June 2003 | AAC | 128 CBR | Various | 10 | 11-18 | QuickTime | |||
| July 2003 | multiple | ~128 | Various | 12 | 14-24 | Musepack | AAC, WMA, and Vorbis tied for close second | ||
| September 2003 | multiple | ~64 | Various | 12 | 30-43 | Nero HE-AAC | This test showed that listeners preferred 128 kbit/s MP3 audio encoded by LAME to all the tested codecs at 64 kbit/s, with greater than 99% confidence: "No codec delivers the marketing plot of same quality as MP3 at half the bitrates." | ||
| January 2004 | MP3 | ~128 |
| Various | 12 | 11-22 | LAME | The author noted that the results may have been affected by the use of an outdated version of the Xing encoder and non-optimal settings for ITunes. | |
| February 2004 | AAC | ~128 | Various | 12 | 19-29 | iTunes | Open-source FAAC codec improved greatly since previous test | ||
| May 2004 | multiple | ~128 | Various | 18 | 12-27 | aoTuV and Musepack | |||
| June 2004 | multiple | 32 CBR | Various | 18 | 47-77 | Nero HE-AAC | |||
| July 2004 | multiple | ~175 |
| Classical | 18 | 1 | Musepack | ||
| August 2005 | multiple | ~180 | Classical | 18 | 1 | aoTuV | The author reflects on substantial improvements in Vorbis encoding since his previous test :"Vorbis is now –thanks to Aoyumi – an excellent audio format for 180 kbit/s encodings." | ||
| August 2005 | multiple | ~96 |
| Classic, various | 150 classical, 35 various | 1 | aoTuV and AAC tied, aoTuV | The author selected each participating encoder by pitting multiple encoders against one another in an initial "Darwinian phase." For example, LAME was chosen as the representative MP3 encoder because it clearly outperformed four other MP3 encoders on a subset of the full sample corpus. | |
| December 2005 | multiple | ~140 | Various | 18 | 18-30 | 4-way tie | "I think this test shows that with the current encoders, the quality at 128 kbit/s is very good... It's time to move to bitrates like 96 kbit/s or even lower." | ||
| March 2006 | AAC | 48 | Various | 18 | 10-20 | 5-way tie | "... it seems that overall, plain HE-AAC might be better than HE-AAC v2 at this bitrate, but a lot more samples would be needed to be able to draw definitive conclusions regarding this. | ||
| November 2006 | multiple | ~48 |
| Various | 20 | 22-34 | Nero HE-AAC | WMA Professional and aoTuV tied for second | |
| July 2007 | multiple | ~64 |
| Various | 18 | 21-33 | Nero Digital and WMA Professional | ||
| October 2008 | MP3 | ~128 | Various | 14 | 26-39 | 5-way tie | "The quality at 128 kbps is very good and MP3 encoders improved a lot since the last test." Also notes that Fraunhofer and Helix codecs are several times faster at encoding than LAME, although virtually identical in terms of perceived audio quality. | ||
| March 2011 | multiple | ~64 |
| Various | 30 | 25-13 | CELT / Opus | In , CELT is referred to as Opus, its name when later standardized. | |
| July/August 2011 | LC-AAC | ~96 | Various | 20 | 25 | Apple QuickTime | |||
| May 2013 | MP3 | ~224 | Various | 25 | 1 | 4-way tie | Most impairment grades rated between 4 and 5. Both speech samples transparent except for the low anchor. | ||
| July - September 2014 | multiple | ~96 |
| Various | 40 | 33 | Opus | In Opus is clear winner, Apple AAC is second, Ogg Vorbis and higher-bitrate LAME MP3 are statistically tied in joint third place. FAAC, known to be inferior in advance, was used to discard bad results and as quality scale anchor. | |
| Cunningham and McGregor | February 2019 | multiple | 192 - 1411 | Pop | 10 | 100 | 5-way tie | Participants reported no perceived differences between the uncompressed, MP3, AAC, ACER high quality, and ACER medium quality compressed audio in terms of noise and distortions but that the ACER low quality format was perceived as being of lower quality. However, in terms of participants’ perceptions of the stereo field, all formats under test performed as well as each other, with no statistically significant differences. | |
| Source | Dates | Formats | Bitrate | Codecs | Musical genres | Samples | Listeners | Best Result | Comments |