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Topic: How bad is transcoding ... really? (Read 22902 times) previous topic - next topic
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How bad is transcoding ... really?

Reply #25 –
It seems to depend on the interaction between the source and destination codecs. I did a few tests this morning and it seems that WMA and Vorbis don't get on well at all:

WMA 96kBps -> Vorbis 128kBps
ABX Vorbis from CD 10/10 (very easy)
ABX Vorbis from WMA 10/10 (not as easy, but still not difficult)
The transcode adds a strangely variable quality. At some points it sounds fine and at others it sounds as though Seether are playing in a metal pipe. Cymbal clashes and the singer's voice are the most obviously effected.

MP3 96kBps -> Vorbis 128kBps
ABX Vorbis from CD 10/10 (fairly easy, about what I would expect from 96k mp3)
ABX Vorbis from MP3 8/10 (not conclusive - another try without the morning traffic noise would be better)
This one doesn't have the obvious artifacts of the WMA->Vorbis conversion. The final ABX is pretty hard as it became a matter of playing "hunt the artifact" rather than an obvious change in sound.

How bad is transcoding ... really?

Reply #26 –
cabbagerat, what settings did you use for Vorbis? If you used bitrate management at 128kbps, Vorbis is NOT a CBR codec and bitrate management produces suboptimal quality. Try a quality setting instead.

Here comes some ABX results. Note that I don't have golden ears. Hearing damage from being a musician has made my ears probably representative of the average casual listener when I listen hard. I'm careful to wear hearing protection always, but it just creeps up on you. =(

I'm trying to duplicate the conditions of the original poster. Encoded a FLAC rip using LAME 3.90.3 --alt-preset standard and Vorbis 1.1 RC1 -q0, then transcoded the MP3 to OGG using the same settings.

The transcode is slightly smaller in file size (1 kbps less average bitrate). ABX results follow. Note that the ABX was *completely* blind, total score is only revealed at the end of the test.

Original vs. Normal Ogg
5 out of 5, p = .031
Piece of cake. HF boost gives it completely away

Original vs. Transcoded Ogg
5 out of 5, p =.031
Ditto. I thought this was easier to ABX than the first, but based on the next test I guess it was my imagination. I have seen the placebo effect first hand!

Normal vs. Transcoded
9 out of 18, p = 0.593
Sometimes, I swear that X is neither A or B. Well, my ears can't tell the difference. I say go for it!

For portable/casual listening, I go all the way down to Vorbis Q -1. About 40 kbps average. I love Vorbis because the quality degrades so nicely, there are no real distracting artifacts even at absolutely tiny bitrates, it just sounds generically lower-fi.

Anyway, conclusion: Transcoding from --alt-preset standard should be okay, especially for portable use.

How bad is transcoding ... really?

Reply #27 –
I tried mp3 APS to vorbis .85 with aotuv b2 with about 70kbps. This was listenable but easily a/b'd but I didn't use an abx tool.

Q 2.5 gave me around 100kbps and this wasvery nice for portable use for me.

How bad is transcoding ... really?

Reply #28 –
Quote
    cabbagerat, what settings did you use for Vorbis? If you used bitrate management at 128kbps, Vorbis is NOT a CBR codec and bitrate management produces suboptimal quality. Try a quality setting instead.

I used -q3 (the default), which on this sample made an average bitrate of ~128kBps.

There is an interesting difference in our results. I take a low quality original (96kBps) and transcoded it at higher quality (128kBps) - something that many traders do to "gain extra quality". Analogy took a high quality original (LAME -ps) and transcoded to a lower quality (vorbis -q0). I got easily ABXable results and analogy got unABXable results. This suggests (but does not prove) an interesting result: that low->high transcoding does more damage to quality than high->low transcoding.

Having said that, I don't think I could ABX my samples through a pair of earbuds with traffic noise in the background - the typical portable listening environment. It seems to me that high->low transcoding for portable use is perfectly acceptable. Low->high transcoding in an attempt to gain quality is (as most knew already) stupid.

How bad is transcoding ... really?

Reply #29 –
Then again, we have different ears, and were using different samples. What's un-ABXable for me might be easily ABXable for you. I might not have been using a particularly difficult sample, too. And remember, I'm a hearing-damaged musician (hearing stops at about 18 khz).