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Topic: Some questions on encoding (Read 7513 times) previous topic - next topic
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Some questions on encoding

1. Are MP3 encoders used in resizing bit rates of MP3 audio? Or something else?

2. Let's say I want to convert 192kbps MP3 files to VBR. Will the sound quality of the output audio be acceptable compared to the output of higher-bitrate MP3s (or of lossless formats)?

Thanks.

Some questions on encoding

Reply #1 –
It's generally not advisable to transcode between lossy codecs. You will incur generational losses. They may or may not be perceptible, but the changes you are after are rarely worth the risk. A simple change from CBR to VBR will gain you almost nothing and likely cost some audio quality.

When you have lossy files it's best to consider them as final versions. If you want another bitrate or codec then you should encode from a lossless source.

Some questions on encoding

Reply #2 –
Quote
1. Are MP3 encoders used in resizing bit rates of MP3 audio? Or something else?
What???  If you re-encode at a different bitrate, yes.

Quote
2. Let's say I want to convert 192kbps MP3 files to VBR. Will the sound quality of the output audio be acceptable compared to the output of higher-bitrate MP3s (or of lossless formats)?
You theoretically loose quality every time you encode or re-encode to a lossy format.  If you take a 192kbps file, decode to wave, then re-encode to 192kbps, it will be degraded.  If you re-encode to 320kbps it will also be degraded.  You may or may not notice the difference.

Data is thrown-away during the lossy compression process and you can't get it back...  It's something like copying a Blu-Ray disc to VHS and then copying it back to Blu-Ray.... Copying a VHS tape to Blu-Ray doesn't give you high-definition video even though it's in a hi-def format.

If you re-encode to lossless, the file is not degraded.    A 192kHz MP3 that's decompressed on-the-fly by the player software is the same as an MP3 file that's decompressed to a lossless format and played later.

Some questions on encoding

Reply #4 –
Quote from: greynol on
Sigh.  For the n-teenth time, going from lossy to lossless can also result in degradation.

Huh? If you decode a lossy file and then compress it with a lossless codec, the lossless file would be the same as the input lossy file. That's the whole point of lossless codecs. The lossless file would be larger and have no more and no less quality than the lossy file, thus making the process counterproductive (having extra size for no extra quality), but the quality certainly couldn't go down (unless there were bugs in the decoder used to decompress the lossy file prior to feeding it to the lossless encoder, but that's something of a separate problem). Am I missing something?

Some questions on encoding

Reply #5 –
Quote from: Aleron Ives on
Quote from: greynol on
Sigh.  For the n-teenth time, going from lossy to lossless can also result in degradation.

Huh? If you decode a lossy file and then compress it with a lossless codec, the lossless file would be the same as the input lossy file. That's the whole point of lossless codecs.


"Lossless is lossless" is in fact wrong as a general statement; it requires the target format to be able to contain signal from the source file. For example, FLAC does not support floating-point; converting a floating-point file to FLAC is therefore not lossless (except by coincidence in special cases - there will always be signals which are perfectly encodable both ways, like complete silence).

mp3 files can cross "digital zero", and if not done properly, converting to a fixed bit format can clip.

Some questions on encoding

Reply #7 –
They may well be clipped when played normally, too.

Some questions on encoding

Reply #9 –
Quote from: yourlord on
It's generally not advisable to transcode between lossy codecs. You will incur generational losses. They may or may not be perceptible, but the changes you are after are rarely worth the risk. A simple change from CBR to VBR will gain you almost nothing and likely cost some audio quality.

When you have lossy files it's best to consider them as final versions. If you want another bitrate or codec then you should encode from a lossless source.

OK, thanks for the advice!

Quote from: DVDdoug on
You theoretically loose quality every time you encode or re-encode to a lossy format. If you take a 192kbps file, decode to wave, then re-encode to 192kbps, it will be degraded. If you re-encode to 320kbps it will also be degraded. You may or may not notice the difference.

Data is thrown-away during the lossy compression process and you can't get it back... It's something like copying a Blu-Ray disc to VHS and then copying it back to Blu-Ray.... Copying a VHS tape to Blu-Ray doesn't give you high-definition video even though it's in a hi-def format.

If you re-encode to lossless, the file is not degraded. A 192kHz MP3 that's decompressed on-the-fly by the player software is the same as an MP3 file that's decompressed to a lossless format and played later.

How about from 320kbps to lower bit rates?

Some questions on encoding

Reply #10 –
Quote from: fizzwhiz on
How about from 320kbps to lower bit rates?


Like DVDdoug said, it happens everytime. 

If its enough that you notice the difference is another question.

Some questions on encoding

Reply #11 –
But I'd like to keep the bit rate of my MP3s consistent, and MP3 files are the only type of audio I can work with (I don't have any CDs to rip songs from). Any suggestions, anyone?

Some questions on encoding

Reply #12 –
If your target is fairly low bitrate mp3s, such as for mobile use, then just go ahead and reencode anything that is significantly higher in bitrate than your target. There will be some loss in quality but the final quality probably won't be that much lower than if you had started with a lossless source. I wouldn't reencode if the source is just a little bit higher bitrate than your target, just keep it as-is.

Some questions on encoding

Reply #14 –
Quote from: fizzwhiz on
But I'd like to keep the bit rate of my MP3s consistent


Why?

You would be better served to simply seek a consistent minimum quality level. If some of your files exceed that level then just keep em. My library is largely FLAC, and the bitrates of those files vary from 400-1400kbps. They are all the same quality (perfect) but different bitrates. Likewise, the only way to keep your lossy files at a consistent bitrate is to encode using CBR mode which from a quality standpoint is generally inefficient. When I transcode my library for mobile use I transcode to a desired quality level and let the encoder choose the appropriate bitrate to attain that quality. Sometimes the files may average 80kbps, sometimes they may average 128kbps, but the quality level is consistent across all the lossy files. I have a few albums laying around in 256-320kbps mp3 I bought from online stores back in the day (I only buy lossless sources now) and those go on my mobile devices as is. I also have some old albums whose disks are long lost/destroyed which I ripped 10 or so years ago to q6 Ogg Vorbis which results in much higher bitrates than I currently transcode to. Those also go onto my mobile devices as is.

I don't understand a need for consistent bitrates, especially one so dire as to risk quality degradation. I promise your player doesn't care if all the files are 192kbps or not!

Some questions on encoding

Reply #15 –
Quote from: fizzwhiz on
Let's say I want to convert 192kbps MP3 files to VBR.


Quote from: fizzwhiz on
I'd like to keep the bit rate of my MP3s consistent


I'm having trouble reconciling these statements.

Which do you want? Consistent quality and variable bitrate... or variable quality and consistent bitrate? That's the choice. But actually, this is only if you are encoding MP3s from the original WAV or AIFF files. If you are talking about transcoding MP3 to MP3, the quality can't go anywhere but down. Just don't do it. (And this is notwithstanding the fact that an ABX test would probably show that you can't even tell the difference between bitrates above 160, regardless of whether CBR or VBR...)

If you really want to switch existing CBR MP3s to VBR, use mp3repacker. It can make your CBR files become VBR, without actually changing the audio at all. It's pointless, other than to save a small amount of space. You won't be getting the consistent quality that you'd get if you were encoding to VBR MP3 from the original WAV or AIFF file.

Some questions on encoding

Reply #16 –
Quote from: mjb2006 on
I'm having trouble reconciling these statements.

Which do you want? Consistent quality and variable bitrate... or variable quality and consistent bitrate? That's the choice. But actually, this is only if you are encoding MP3s from the original WAV or AIFF files. If you are talking about transcoding MP3 to MP3, the quality can't go anywhere but down. Just don't do it. (And this is notwithstanding the fact that an ABX test would probably show that you can't even tell the difference between bitrates above 160, regardless of whether CBR or VBR...)

If you really want to switch existing CBR MP3s to VBR, use mp3repacker. It can make your CBR files become VBR, without actually changing the audio at all. It's pointless, other than to save a small amount of space. You won't be getting the consistent quality that you'd get if you were encoding to VBR MP3 from the original WAV or AIFF file.

I'm sorry for the confusion caused. I meant consistent quality and variable bitrate. At the moment all of my MP3 files are at 192kbps CBR, and I am planning to convert all of them to VBR. Anyway, thanks for the helpful advice. Now I don't think I'll be re-encoding any of my MP3s anymore.

Some questions on encoding

Reply #17 –
For question no. 2, you will definitely lost the quality of mp3.

Some questions on encoding

Reply #18 –
Quote from: alastairbrian on
For question no. 2, you will definitely lost the quality of mp3.

Please be careful about your wording. There will definitely be loss of information, but this does not tell us if there will be an audible difference between them.