Results of the public multiformat listening test @ 64 kbps (March/April 2011)


These are the summary results of the multiformat listening test @ 64 kbps.

You can download a ZIP file containing all results for all samples.

Encryption keys can be downloaded from here and here.

How to interpret the plots: Each plot is drawn with 5 codecs on the X axis and the rating given (1.0 to 5.0) on the Y axis. The 95% confidence intervals are given on each plot. The mean rating given to each codec is indicated by the middle point of each vertical line segment. Each vertical line segment represents the 95% confidence interval (using ANOVA analysis) for each codec. This analysis is identical to the one used in previous listening tests.

One codec can be said to be better than another with greater than 95% confidence if the bottom of its segment is at or above the top of the competing codec's line segment. Note that this is an approximate analysis with some assumptions, and the confidence is far greater in most cases. An almost assumption-free analysis (bootstrap) is below.

Note that CELT is referred to as Opus as this will be the standardized name.

Important note: These plots represent group preferences (for the particular group of people who participated in the test). Individual preferences vary somewhat. The best codec for a person is dependent on his own preferences and the type of music he prefers.


Plot of the complete result (30 samples, 531 results):

Full plot

Closeup of the interesting results (30 samples, 531 results):

Zoomed plot

Per-sample results

A page with graphics for each sample individually is here.

Bitrate table

The codecs and settings were calibrated to provide ~64kbps on a large variety of music.

These are the bitrates used by the codes for the samples in the test:

Codec bitrates

Bootstrap analysis:

	Read 5 treatments, 531 samples => 10 comparisons
	Means:
	      Vorbis   Nero_HE-AAC  Apple_HE-AAC          Opus    AAC-LC@48k 
	       3.513         3.547         3.817         3.999         1.656 

	Unadjusted p-values:
		  Nero_HE-AAC   Apple_HE-AAC  Opus          AAC-LC@48k   
	Vorbis        0.488         0.000*        0.000*        0.000*       
	Nero_HE-AAC   -             0.000*        0.000*        0.000*       
	Apple_HE-AAC  -             -             0.000*        0.000*       
	Opus          -             -             -             0.000*       

	Apple_HE-AAC is better than Vorbis (p=0.000)
	Apple_HE-AAC is better than Nero_HE-AAC (p=0.000)
	Opus is better than Vorbis (p=0.000)
	Opus is better than Nero_HE-AAC (p=0.000)
	Opus is better than Apple_HE-AAC (p=0.000)
	AAC-LC@48k is worse than Vorbis (p=0.000)
	AAC-LC@48k is worse than Nero_HE-AAC (p=0.000)
	AAC-LC@48k is worse than Apple_HE-AAC (p=0.000)
	AAC-LC@48k is worse than Opus (p=0.000)

	p-values adjusted for multiple comparison:
		  Nero_HE-AAC   Apple_HE-AAC  Opus          AAC-LC@48k   
	Vorbis        0.490         0.000*        0.000*        0.000*       
	Nero_HE-AAC   -             0.000*        0.000*        0.000*       
	Apple_HE-AAC  -             -             0.000*        0.000*       
	Opus          -             -             -             0.000*       

	Apple_HE-AAC is better than Vorbis (p=0.000)
	Apple_HE-AAC is better than Nero_HE-AAC (p=0.000)
	Opus is better than Vorbis (p=0.000)
	Opus is better than Nero_HE-AAC (p=0.000)
	Opus is better than Apple_HE-AAC (p=0.000)
	AAC-LC@48k is worse than Vorbis (p=0.000)
	AAC-LC@48k is worse than Nero_HE-AAC (p=0.000)
	AAC-LC@48k is worse than Apple_HE-AAC (p=0.000)
	AAC-LC@48k is worse than Opus (p=0.000)    
    

ANOVA analysis:

        FRIEDMAN version 1.24 (Jan 17, 2002) http://ff123.net/
	Blocked ANOVA analysis

	Number of listeners: 531
	Critical significance:  0.05
	Significance of data: 0.00E+00 (highly significant)
	---------------------------------------------------------------
	ANOVA Table for Randomized Block Designs Using Ratings

	Source of         Degrees     Sum of    Mean
	variation         of Freedom  squares   Square    F      p

	Total             2654        4521.67
	Testers (blocks)   530        1498.18
	Codecs eval'd        4        1893.65  473.41   888.29  0.00E+00
	Error             2120        1129.85    0.53
	---------------------------------------------------------------
	Fisher's protected LSD for ANOVA:   0.088

	Means:

	Opus     Apple_HE Nero_HE- Vorbis   AAC-LC@4 
	  4.00     3.82     3.55     3.51     1.66   

	---------------------------- p-value Matrix ---------------------------

		 Apple_HE Nero_HE- Vorbis   AAC-LC@4 
	Opus     0.000*   0.000*   0.000*   0.000*   
	Apple_HE          0.000*   0.000*   0.000*   
	Nero_HE-                   0.439    0.000*   
	Vorbis                              0.000*   
	-----------------------------------------------------------------------

	Opus is better than Apple_HE-AAC, Nero_HE-AAC, Vorbis, AAC-LC@48k
	Apple_HE-AAC is better than Nero_HE-AAC, Vorbis, AAC-LC@48k
	Nero_HE-AAC is better than AAC-LC@48k
	Vorbis is better than AAC-LC@48k
    

Notes:

The graphs are a simple ANOVA analysis over all submitted and valid results. This is compatible with the graphs of previous listening tests, but should only be considered as a visual support for the real analysis.

For a correct calculation of the statistical probability, and to see if one can safely make any conclusions, one has to refer to the bootstrap output. You can see that the results are highly significant for all but one comparison (Vorbis vs. Nero HE-AAC).

When crosschecking against the old friedman/ANOVA utility one can see that the results are almost identical, which is expected as there are many results and the codecs were mostly not transparent, so the results are reasonably normally distributed in this test.

A potential issue is that not every sample was tested by the same amount of listeners, and that notably, the first few samples got more submissions than later ones. Preliminary analsysis (by only including the listeners that tested all samples) shows that this makes no difference to the conclusions.

Post-screening:

Invalid results were discarded according to the following criteria, which were made public at the beginning of the test:

Contact

IgorC: igoruso@gmail.com