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Codec listening test

Scientific study designed to compare two or more lossy audio codecs

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

Main article: ABX test

In an ABX test, the listener has to identify an unknown sample X as being A or B, with A (usually the original) and B (usually the encoded version) available for reference. The outcome of a test must be statistically significant. This setup ensures that the listener is not biased by their 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 (which is the Hidden Reference that the "HR" in ABC/HR stands for), 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

Main article: MUSHRA

In MUSHRA (MUltiple Stimuli with Hidden Reference and Anchor), the listener is presented with the reference (labeled as such), a certain number of test samples, a hidden version of the reference and one or more anchors. The purpose of the anchor(s) 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 (and extensions), 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 (both proprietary and open source) exist for some formats, such as MP3, which is the oldest and best-known format still in widespread use today.

Source	Dates	Formats	Bitrate (kbit/s)	Musical genres	Samples	Listeners	Best Result	Comments
ff123	2001	multiple	~128		1	16	Musepack and AAC
ff123	2001 October - 2002 January	multiple	~128	Various	3	25-28	Musepack
or Vorbis
ff123	2002 July	multiple	~64	Various	12	24-41	mp3PRO	Both Vorbis variants were a close second.
Roberto Amorim	2003 June	AAC	128 CBR	Various	10	11-18	QuickTime
Roberto Amorim	2003 July	multiple	~128	Various	12	14-24	Musepack	AAC, WMA, and Vorbis tied for close second
Roberto Amorim	2003 September	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:
Roberto Amorim	2004 January	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.
Roberto Amorim	2004 February	AAC	~128	Various	12	19-29	iTunes	Open-source FAAC codec improved greatly since previous test
Roberto Amorim	2004 May	multiple	~128	Various	18	12-27	aoTuV (Vorbis) and Musepack
Roberto Amorim	2004 June	multiple	32 CBR	Various	18	47-77	Nero
HE-AAC
HydrogenAudio user "guruboolez"	2004 July	multiple	~175	Classical	18	1	Musepack
HydrogenAudio user "guruboolez"	2005 August	multiple	~180	Classical	18	1	aoTuV (Vorbis)	The author reflects on substantial improvements in Vorbis encoding since his previous test (above):
gURuBoOleZZ	2005 August	multiple	~96	Classic, various	150 classical, 35 various	1	aoTuV and AAC tied (classical), aoTuV (various)	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.
Sebastian Mares	2005 December	multiple	~140 (nominal 128)	Various	18	18-30	4-way tie (all except Shine)	"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 (64 kbit/s)."
Mp3-tech.org	2006 March	AAC	48	Various	18	10-20	5-way tie
(all except anchors)	"... 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.
Sebastian Mares	2006 November	multiple	~48	Various	20	22-34	Nero
HE-AAC	WMA Professional and aoTuV tied for second
Sebastian Mares	2007 July	multiple	~64	Various	18	21-33	Nero Digital and WMA Professional
Sebastian Mares	2008 October	MP3	~128	Various	14	26-39	5-way tie
(all except L3enc)	"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.
HydrogenAudio user IgorC (March/April 2011)	2011 March	multiple	~64	Various	30	25-13	CELT / Opus	In results, CELT is referred to as Opus, its name when later standardized.
HydrogenAudio user IgorC (July - August 2011)	2011 July/August	LC-AAC	~96	Various	20	25	Apple QuickTime
HydrogenAudio user "Kamedo2"	2013 May	MP3	~224	Various	25	1	4-way tie
(all except BladeEnc
low anchor)	Most impairment grades rated between 4 (perceptible but not annoying) and 5 (imperceptible). Both speech samples transparent (p
HydrogenAudio user Kamedo2 (July/September 2014)	2014 July - September	multiple	~96	Various	40	33	Opus	In results 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	2019 February	multiple	192 - 1411	Pop	10	100	5-way tie (WAV, MP3, AAC, ACER HQ, ACER MQ)	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.

References

(2019). "Subjective Evaluation of Music Compressed with the ACER Codec Compared to AAC, MP3, and Uncompressed PCM". International Journal of Digital Multimedia Broadcasting.

Info: Wikipedia Source

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