Opus Audio Transparency?, At what bitrate may Opus-encoded audio be considered transparent? Options

[ethan234098]

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

[nu774]

You have to learn more about how perceptual coders work.
They are based on human audio perception (psychoacoustics), and can do anything to the waveform to spare bits so long as the difference is not audible, and give more bits to more important, sensible parts to human ear.
Therefore, you cannot simply and naively compare the quality of perceptual coders by looking at waveform or spectrogram.