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Chirp0to24kHz and its noisy variations, Killersamples for many lossy codecs
post Apr 7 2013, 22:30
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Here is a linear chirp (sine with increasing frequency) from 1 Hz to 24 kHz with amplitude 0.8 and its variations with brown, pink and white noise added (noise amplitude is 0.02). NeroAAC, Musepack and Opus 1.0.2 + 1.1 alpha failed on it even at their best quality. Vorbis and MP3 not failed. A lot of files are at these links (encoded samples + spectrograms):
Opus 1.0.2
Opus 1.1 alpha
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Attached File  Chirp0to24kHz.zip ( 10.51MB ) Number of downloads: 138
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post Apr 8 2013, 17:03
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The use of artificial samples to rate codecs is ok as long as ABX hearing tests are still involved. Rating deviations in a Time-frequency diagram as audible/inaudible without ABX is risky. The example softrunner showed for Opus was "obvious enough"(the line was cut = silent parts in the uncompressed WAV), but the deviations you're showing may not be audible at all. This is completely independent from the choice to use musical/unmusical samples.

With spectograms you are still allowed to draw conclusions about the technical nature of a encoder (algorithms used, size of transforms, etc.). For example I would expect transform codecs like AAC Vorbis MP3 to show different anomalies than nontransform codecs like ADPCM, or pure speech codecs. Conclusions about the technical nature of a codec are only interesting if they are not already published, which is unlikely.

This post has been edited by Primius: Apr 8 2013, 17:26
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