Heya!
I wanted to find out how Opus behaves over many generations of transcodes/re-encodes of the same 30s sample. So I used this simple bash command to create 9999 generations of encodes:
for i in {0001..9999}; do opusdec --force-wav gens/$(printf "%0*d\n" 4 $[$i-1]).opus - | opusenc --vbr --bitrate 128 - gens/$(printf "%0*d\n" 4 $i).opus; done
To get the obvious out of the way: Generation 9999 sounds like crap! ... but I was also kind of impressed that the song was still recognizable and it seems to me as if the degradation seems to plateau more and more (less and less differences). To illustrate here's the audio differences between generation 0001 and 5000:
... and the differences bettween 5000 and 9999:
Maybe someone else with some time on their hands wants to continue this for (much) longer, to see how long it takes until it becomes entirely garbled.
Just a quick technical comparison of generation 0001.opus and 9999.opus via opusinfo:
0001.opus
Opus stream 1:
Pre-skip: 312
Playback gain: 0 dB
Channels: 2
Original sample rate: 48000Hz
Packet duration: 20.0ms (max), 20.0ms (avg), 20.0ms (min)
Page duration: 1000.0ms (max), 971.0ms (avg), 100.0ms (min)
Total data length: 563445 bytes (overhead: 1.18%)
Playback length: 0m:30.082s
Average bitrate: 149.8 kb/s, w/o overhead: 148.1 kb/s
Logical stream 1 ended
9999.opus:
Opus stream 1:
Pre-skip: 312
Playback gain: 0 dB
Channels: 2
Original sample rate: 48000Hz
Packet duration: 20.0ms (max), 20.0ms (avg), 20.0ms (min)
Page duration: 1000.0ms (max), 971.0ms (avg), 100.0ms (min)
Total data length: 515641 bytes (overhead: 0.921%)
Playback length: 0m:30.082s
Average bitrate: 137.1 kb/s, w/o overhead: 135.9 kb/s
Logical stream 1 ended
The average bitrate has decreased significantly at this point, but is still higher than I would've expected.
As an aside: When listening to 'older' generations it's interesting to me that instead of degradation the biggest 'annoyance' is the introduction of additional artifacts on top of the original audio that seem to get louder and louder (successively adding on top of each other?) over time.
The spectrals also look - superficially - much better than I would have expected, but there's some very clear 'bald spots' visible in the higher frequency range:
Don't let Spectograms and Waveforms fool you, it sounds more terrible than it looks (there's a lesson in here somewhere about not relying on waveforms to judge sound). I'll try to attach generations 0001 and 9999 to this post so you can have a listen for yourself.
For anyone who wants to look at the whole progression, here's the whole archive: gens.tar.xz (watch out, this extracts to ~5GB of files)
In conclusion: Don't transcode lossy files, but if you absolutely have to, Opus seems to do okay in the scenario, especially during the very first few generations (it's hard to tell if there's any audible difference at all, but I suck at ABXing).
Not sure if this is of interest to anyone else, but I just decided to share...
Topic: The slow and painful death of an opus file (Read 7953 times)
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