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jeijkie
2007-08-24, 07:51
Since getting a bit better equipment down the chain I have noticed a noise which seems to follow the music, mostly the bass when it's only bass or not too much other high frequencies in the music. It's a metallic noise, sort of like the veil you can hear on bad mp3s.

I have tried a number of different setups and formats to single out the problem:

- CDP playing original cd. =No noise
- WAV and FLAC ripped from original cd and played through SB3 =No noise
- MP3 ripped with Windows Media Player from original cd, played through SB3 =Noise
- MP3 ripped with EAC/Lame from original cd, played through SB3 =Noise
- Burned a cd from the ripped mp3-file and played it on the CDP =No noise.

So, it only seem to occur with mp3s on the SB3. FLAC works fine. Music through CDP works fine. It seems that something's wrong with the decoding of the mp3files perhaps?

Does anyone have a clue or similar experiences? Thankful for all help.

Regards,
Jacob

point7
2007-08-25, 16:46
What bitrate do you use for MP3 ?

Do you use the analogue output or digital ?

CatBus
2007-08-25, 23:18
A "metallic noise" is a pretty common artifact on over-compressed MP3s--I suspect that's what you're hearing (what bitrate are you using?). The only clue that doesn't fit is the re-burned CD, and I'm wondering if there was a flaw in that test or even a flaw in the player that masks this particular artifact.

An EAC/LAME encoded MP3 should start exhibiting pretty obvious artifacting at anything below 128K. Do not use Windows Media Player to encode MP3s at all. Hope that helps.

funkstar
2007-08-26, 01:59
Here is some reading on the subject of compression artifacts and MP3s

http://en.wikipedia.org/wiki/Mp3#Audio_quality

and linking onto:

http://en.wikipedia.org/wiki/Compression_artifact#Compression_artifacts_in_audi o_coding

Hope this is useful :)

jeijkie
2007-08-26, 03:58
Thank you for your answers. Some additional info:
- I have tried different bitrates, up to 320
- I use the analog output, no separate DAC (although I am considering it)
- I only used WMP to see if it was a problem with EAC and the problem is there with mp3s 'acquired elsewhere'

I know how overcompressed mp3s sound like and the noise resembles that, but with overcompressed mp3s you hear the artifacts with cymbals for example but for me the noise follows bass frequencies, not the high. It's sort of a faint buzzing metallic noise.

The cd player I use is a Naim CD5x so I don't think the player is unable to show the artifact. I can hear it through my speakers but only very faint. It is most apparent with my headphones (Grado RS1 and AKG K1000), either directly from the cd/SB3 to my headamp or through the speaker amp. I think I have ruled out that the problem stems from other gear, especially since FLACs work good.

Any other clues?

CatBus
2007-08-26, 07:49
The audio pathway from the SB3->amp->speakers is clean, as you demonstrated with your FLAC test. There are no audible artifacts on a 320kbps MP3, so you've demonstrated it's not artifacting.

The only possible problem I can imagine is at the MP3 encoder/decoder level. If you disable built-in MP3 decoding in SlimServer, and the problem goes away, that will tell you that the firmware decoder is buggy (and that the server decoder is fine). I seriously doubt that but maybe you've got some special MP3s.

If it's a firmware decoder bug, file a bug. If it's not...sheesh, I dunno.

jeijkie
2007-09-01, 11:50
I'm not sure if i understand. If I disable the built-in mp3 decoding the mp3 files won't play at all since there's no other option for mp3 files in the server settings.

Is there a way to use another (external) decoder for mp3 files?

Edit: Ooh... and I don't think that it's just the mp3s I have decoded myself, the artifacts can be heard on all mp3 files (as long as they don't have too much high frequency information which make the artifact hard to discern. And you need a firm bass frequency too create the artifact as well).

Again, this is not just with my own mp3s or with one single amplification or speaker/headphone.