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Opus Audio Transparency?, At what bitrate may Opus-encoded audio be considered transparent?
post Nov 26 2012, 05:46
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Before I say anything else, let me just note that I am totally new here at Hydrogenaudio and this is my first post. If a moderator feels this thread is posted in the wrong section, by all means move it. I'm honestly a bit overwhelmed, as I feel like this topic could be related to several areas, but I think I've found a good place for the time being. Also, a disclaimer: None of this post is meant to violate TOS #8. I am not attempting to claim that any one codec or bitrate is better or higher quality or more efficient in ANY way. I only mean to solicit opinions and possibly conjecture from others.
TL;DR I'm new and I don't believe I am in violation of TOS #8.

With all the buzz about Opus, the newly standardized audio codec on Hydrogenaudio, I thought I might conduct some tests of the transparency of Opus when encoding music. Specifically, I am interested in converting my existing FLAC collection to Opus for use on an MP3 player with Rockbox. Basically, I scoured the web trying to find a standard or recommended bitrate that would give me at least near-transparent audio quality. I could find none, although it appears that the Hydrogenaudio Wiki article on Transparency recommends ~144kbps for "complex audio" with Opus. I was somewhat skeptical of this number, mostly because I haven't seen it anywhere else, and I didn't think it would be possible to do much better than HE-AAC at around 150.

I figured a good way to judge transparency would be to take a generally accepted number for another format (HA Wiki says most people find ~192kbps MP3 to be transparent in most cases) and compare various Opus bitrates to that. My way of comparing is not at all scientific: just looking at spectrograms of various audio files. I do not expect to use spectrograms to prove anything one way or another, but I thought my results were interesting and worth sharing, so here they are.

NOTE: I used LAME VBR quality V2 to test against because the Wiki said it was generally considered to be transparent/near transparent.

ABBA - Take a Chance On Me | FLAC | LAME V2 | Opus 144kbps
ADELE - Rumor Has It | FLAC | LAME V2 | Opus 144kbps
DEADMAU5 FEAT. IMOGEN HEAP - Transmiscommunications | FLAC | LAME V2 | Opus 144kbps
ENYA - Orinoco Flow | FLAC | LAME V2 | Opus 144kbps
LADY GAGA - Bloody Mary | FLAC | LAME V2 | Opus 144kbps
NICKI MINAJ - Roman Holiday | FLAC | LAME V2 | Opus 144kbps
PINK MARTINI - Hey Eugene! | FLAC | LAME V2 | Opus 144kbps

I would love to hear a more experienced analysis of these results. Feel free to sound off! Do you think that Opus is transparent or *nearly transparent* at ~144kbps VBR, like the HA Wiki says? Does the speckled "noise" at the top of Opus graphs degrade the audio quality, or help hint at information that was present in the FLAC files? If an Opus file does not appear to be transparent at this bitrate, do you think it is fair to rate it higher quality than its LAME V2 counterpart?

My response: After extensive listening, I have found differences between the Opus files and their FLAC parents to be present, but difficult to distinguish. LAME performed well also, but I was able to pick out those files from the FLAC more often. I think Opus is at least *nearly transparent* at 144kbps, but I don't have very solid proof for it, other than my own ears.

Thanks so much for your comments! Again, any help is appreciated. What do you think would be the best way to measure the "rate of transparency" for an Opus file short of an organized listening test? Any other bitrates you think I should try?

~ Ethan

This post has been edited by ethan234098: Nov 26 2012, 05:55
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post Nov 26 2012, 18:41
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I feel like people have generally misunderstood the purpose of my post. I'm not trying to show superiority of Opus to MP3 or anything like that. My purpose was to explore the rate at which "complex audio" can be considered transparent when encoded with a particular codec. The spectrograms are relevant because they help show the loss of audio data in particular frequencies. The FLAC files held much higher concentrations of data at higher frequencies than either of the two alternatives. The images are simply a means of illustration of this. After all, transparency is the ideal result of lossy data compression, and the particular frequencies of data an encoder chooses to throw away can make a great deal of difference.

The spectrograms illustrate the differences between data retention between two different encoders and different bitrates. I'm not trying to imply that one is better than the other, just show that there is a difference and discuss whether or not one sample might be similar in comparison to another.

I do not think my original question to be unreasonable. I'm just exploring transparency in a codec for which there exists little, if any, evidence regarding bitrates and transparency. I don't believe that any of my tests or samples or images constitute anything scientific or solid, but I do think they are interesting.

This post has been edited by ethan234098: Nov 26 2012, 18:41
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post Nov 26 2012, 21:09
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QUOTE (ethan234098 @ Nov 26 2012, 12:41) *
...After all, transparency is the ideal result of lossy data compression, and the particular frequencies of data an encoder chooses to throw away can make a great deal of difference...

Sorry you went to the trouble to make graphs but they are not relevant. Your test methodology is based on invalid assumptions. Lossy encoders are not trying to preserve frequencies unless they can be heard.

Throwing away frequencies that are not audible (because masked by other frequencies occurring at the same time) is mostly how lossy encoders are able to compress the data. You have to use your ears to determine if the data has been audibly changed. This is why TOS#8 specifically rules out graphs.
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Posts in this topic
- ethan234098   Opus Audio Transparency?   Nov 26 2012, 05:46
- - Garf   QUOTE (ethan234098 @ Nov 26 2012, 05:46) ...   Nov 26 2012, 08:52
- - IgorC   The claims of transparency at a given bitrate are ...   Nov 26 2012, 14:18
- - saratoga   QUOTE (ethan234098 @ Nov 26 2012, 00:46) ...   Nov 26 2012, 17:30
- - ethan234098   I feel like people have generally misunderstood th...   Nov 26 2012, 18:41
|- - rick.hughes   QUOTE (ethan234098 @ Nov 26 2012, 12:41) ...   Nov 26 2012, 21:09
|- - Kohlrabi   QUOTE (rick.hughes @ Nov 26 2012, 22:09) ...   Nov 26 2012, 23:05
- - Destroid   First of all, you should have taken a bit more tim...   Nov 26 2012, 18:49
- - yourlord   A spectrogram comparison tells us virtually NOTHIN...   Nov 26 2012, 18:54
- - nu774   You have to learn more about how perceptual coders...   Nov 26 2012, 19:16
- - greynol   There is a reason why Garf answered in the way tha...   Nov 26 2012, 23:30
- - ethan234098   Clearly I have some reading to do! Thank you a...   Nov 27 2012, 00:45
|- - DonP   QUOTE (ethan234098 @ Nov 26 2012, 19:45) ...   Nov 27 2012, 01:25
- - yourlord   yep.. looks like opus at 144kbps is transparent to...   Nov 27 2012, 03:35
- - ethan234098   QUOTE (DonP @ Nov 26 2012, 17:25) QUOTE (...   Nov 27 2012, 16:36
|- - zerowalker   got any results:)?   Jan 5 2013, 14:24
- - Oliver   QUOTE (ethan234098 @ Nov 27 2012, 10:36) ...   Mar 8 2013, 03:53
- - wswartzendruber   Perhaps he could try testing the new alphas.   Mar 8 2013, 06:09
|- - RobertM   QUOTE (wswartzendruber @ Mar 8 2013, 15:0...   Mar 8 2013, 07:49
- - azaqiel   QUOTE (RobertM @ Mar 8 2013, 00:49) provi...   Mar 8 2013, 17:18
- - db1989   QUOTE (ethan234098 @ Nov 26 2012, 04:46) ...   Mar 8 2013, 18:36

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