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Topic: Transcoding one lossy format to another. Will this result in a different file? (Read 1775 times) previous topic - next topic
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Transcoding one lossy format to another. Will this result in a different file?

I’m discussing the merits of using compressed lossless formats with bluetooth and am under the impression that for listening, that’s one area where using lossless could actually have benefits because for example if I was to listen to an MP3 and it was again compressed to AAC; there may be audible loss of fidelity or may not be but it would at least be possible. Also, converting me lossy format to another should degrade quality. Am I correct? I’ve heard the argument that for example taking a 320 kbps MP3 and converting it to 256 KBPS AAC would not result in quality loss because the AAC conversion wouldn’t redundantly remove high and low frequencies again and the algorithm wouldn’t need to compress more than it already has. That sounds very wrong to me.

Re: Transcoding one lossy format to another. Will this result in a different file?

Reply #1 –
Transcoding from one lossy format to another will always result in loss of quality as the encoder has no way to determine whether it is processing 'noise' from the first encoding or true audio content. Whether you can hear a difference is an entirely different question. ;)

From limited personal testing, you probably need to go through several generations before you notice particularly offensive artefacts.

Re: Transcoding one lossy format to another. Will this result in a different file?

Reply #2 –
Quote from: john33 on
Transcoding from one lossy format to another will always result in loss of quality as the encoder has no way to determine whether it is processing 'noise' from the first encoding or true audio content. Whether you can hear a difference is an entirely different question. ;)

From limited personal testing, you probably need to go through several generations before you notice particularly offensive artefacts.

That’s what I thought and if that difference is audible of course would be dependant on various circumstances and dependent on the audio sample in question as well, but my personal opinion and experience is the same - that it probably takes a few generations for myself anyways anecdotally.

Different compression algorithms are different, so I can’t see any other conclusion as you’d still be applying a doffed star type of compression regardless that would be cutting out even more information. If not audible, there still must be a file difference. Does this still apply to transcoding again with the same format and settings though?

Re: Transcoding one lossy format to another. Will this result in a different file?

Reply #3 –
On the basis that different encoders in different formats will apply different algorithms, I think it is generally accepted that encoding between different formats is probably the preferred option.

Re: Transcoding one lossy format to another. Will this result in a different file?

Reply #4 –
Quote
aking a 320 kbps MP3 and converting it to 256 KBPS AAC would not result in quality loss because the AAC conversion wouldn’t redundantly remove high and low frequencies again and the algorithm wouldn’t need to compress more than it already has. That sounds very wrong to me.
It's quite as simple as throwing-away frequencies you (hopefully) can't hear. 

With MP3 "damage" accumulates with every generation of re-compression, and MP3 is actually one of the worst formats for this!     AAC is pretty-much immune from accumulated damage.  

But, I don't know if transcoding from MP3 to AAC is better than compressing MP3 twice...   I don't know if you get the best of both or the worst of both.

Nine different codecs 100-pass recompression test
 

Re: Transcoding one lossy format to another. Will this result in a different file?

Reply #5 –
Quote from: DVDdoug on
Quote
aking a 320 kbps MP3 and converting it to 256 KBPS AAC would not result in quality loss because the AAC conversion wouldn’t redundantly remove high and low frequencies again and the algorithm wouldn’t need to compress more than it already has. That sounds very wrong to me.
It's quite as simple as throwing-away frequencies you (hopefully) can't hear. 

With MP3 "damage" accumulates with every generation of re-compression, and MP3 is actually one of the worst formats for this!     AAC is pretty-much immune from accumulated damage.  

But, I don't know if transcoding from MP3 to AAC is better than compressing MP3 twice...   I don't know if you get the best of both or the worst of both.

Nine different codecs 100-pass recompression test

Thanks a lot for that link. I’d be interested to see it done on more passages, but of course I should be the one to do it then. I had no idea about AAC being immune to multiple transcodes. It’s little consequence to me as when I’m listening wireless, high fidelity isn’t the point anyways but it seems that it’s likely that it may not make a difference anyways. MP3 to AAC is I’ll bet how most people who listen to Bluetooth audio on Apple devices do it.

Re: Transcoding one lossy format to another. Will this result in a different file?

Reply #6 –
Quote
MP3 to AAC is I’ll bet how most people who listen to Bluetooth audio on Apple devices do it.

I wouldn't be surprised if a fair amount of those people are listening to an MP3 that was uploaded to Youtube by someone else, converted there to AAC, then downloaded and converted to MP3 again (because it's the codec everyone knows & thinks they want), and then played on their iDevice and converted to AAC over Bluetooth.