Quick question: I've got a bunch of audio files in 320kbps aac that I need in 192kbps vbr aac. Since I'm staying with the same codec and only going to a lower bitrate, is it okay to convert directly from the 320kbps files to 192 kbps? Or do I need to start from the original cd/lossless files?

If you have the luxury of being able to re-rip the CDs then you will get a better result by one step lossless > vbr AAC. If you transcode from MP3 320 CBR to vbr AAC then that's two steps and will lose more information.

Thanks, I do actually have the original CDs, so I can go from lossless. But the 320kbps files I have are aac, and I'd be going to 192kbps aac vbr. Does the fact that I'm staying with the same codec (rather than going from mp3 to aac or something) change much? Am I still losing extra information by transcoding from 320kbps aac to 192kbps aac than if I'd started from lossless, and how much of a difference does it really make? Just curious.

Yes you would lose more quality. How much difference is hard to say, but you could do a few ABX tests to see if you can tell the difference between the 320 kbps and 192 kbps AAC files.

They should transcode without obvious issues using 320k aac sources. Even 256k sources do well.

If you forced me to place a bet, I'd put my money on "no audible difference" between the two methods. In fact, you may not hear any difference between the CD (or WAV rip) and the 192kbps copies (in a blind ABS test).

The rule-of-thumb is "avoid transcoding between lossy formats", and I try to follow that whenever possible. But it's not always that bad.

As a side note, the Holy Grail of lossy-to-lossier transcoding is 'bit peeling', a feature that Vorbis attempted to implement. To date, it has been an extremely difficult problem. It doesn't work right, yet.

http://en.wikipedia.org/wiki/Bitrate_peeling
http://en.wikipedia.org/wiki/Vorbis#Bitrate_peeling

Along those lines (not to thread-hijack):

I have some source files that only exist as 256 MP3/AAC (Amazon & iTunes) -- I want to trim them down to -V5 or -V4 with lame to save space on my mobile devices (I keep the best quality ones separate for squeezecenter) ... are there any other settings I should consider when re-converting them ? Or just send them through lame as I normally would?

Thanks!

Nope, just encode as usual.

I've got some AAC 256 Kbps iTunes songs. I'll run some tests in a near future but do you think that I'll be able to hear the difference between the source and the transcoded file (from AAC CBR 256 to Lame 3.98.2 V3) using the radio of my car?

Thank you

I transcode all the time from 256k mp3 sources without a problem, mainly from vinyl rips I did years ago and where I deleted my wav source files to save disk space, but also just because the mp3 is say closer to hand. I also transcode all the time from iTunes aac to mp3 without a problem.

As others have mentions an ABX will answer your question definitively but I wouldn't bother. Just enjoy the music!