Source of HF in MP3?

[SouthSIDE Glen]

I just received a sample MP3 from a friend of mine who does a lot of VSTi synth work. His setup is rather basic, with one MIDI/Synth PC feeding a second Cubase DAW PC via analog EMU 404 sound cards (I know, I know; but those cards are actually not completely awful for the price. They're still sound cards, but far superior than your average SoundBlaster-Upper card.)

Anyway he sent me a 128k MP3 of an unfinished work in progress of his, all VSTis of analog synths with some drum samples for rhythm, just so I could hear his progress.

There are technical problems all over the recording, including about 16dB of offset due to a high-amplitude 23Hz sub-tone that runs through the whole thing that I don't believe is supposed to be there (though I'm waiting for confirmation from him on that - he is playing with analog synth emulations, after all.)

But what has me scratching my head is that the track, being am MP3, and a 128k one at that, has the usual bottomless cliff at 15kHz where the MP3 compression just cuts everything off. Except here quite a bit of >15k noise all the way up the spectrum remains *on the MP3*. It does have a bit of a periodic component to it (depending on the FFT analyzer I use), but it does not really have the normal profile of anti-aliasing noise like one might expect if that were the source of it. Add to that the fact that it made it past (or maybe was caused by?) the MP3 compression, and I'm stumped as to just how it got there. I've never seen anything quite like it before.

Has anyone experienced anything like this with their MP3s that could give an idea as to the actual source: the MP3 encoder itself, sound card converter artifacting, bad VSTi emulation, or something else?

G.

 

[mixmkr]

It's the EMU card. He should use something professional like the Soundblaster card

 

[Massive Master]

It's called "Flying Monkey Schmutz" -- Periodic piles of low-level HF distortion that comes from nowhere. Just part of MP3 for the most part... Some are worse than others (and 128's on LAME are covered with them).

 

[SouthSIDE Glen]

Just part of MP3 for the most part... Some are worse than others (and 128's on LAME are covered with them).

As I slept on it last night, the more the only thing that made any sense to me for the HF problem was the MP3 encoder. I'll have to check with him and see which one he's using, and get him to start using a grown man's encoder . Then it's get him off the VSTis with the awful offset....

Thanks, John.

EDIT: BTW, it turns out my buddy is no longer using the sound cards to connect the machines, but is now transferring between them via S/PDIF.

G.

 

[mixmkr]

and the problem with VSTi is the offset? I don't have any such problem and such monsters like Omnisphere, etc are pretty impressive. I like hardware synths because of just hooking them up and "go"...but the world of VSTi is pretty amazing to me.
I want to think some of the current "hardware" synths are just in plastic boxes while the VSTi is living in your computer...no big difference.

 

[SouthSIDE Glen]

and the problem with VSTi is the offset? I don't have any such problem and such monsters like Omnisphere, etc are pretty impressive. I like hardware synths because of just hooking them up and "go"...but the world of VSTi is pretty amazing to me.
I want to think some of the current "hardware" synths are just in plastic boxes while the VSTi is living in your computer...no big difference.

Well, I have to apologize for misspeaking a bit; he's using stand-alone synth software and not VSTi plugs. I'm not sure what his whole inventory is, but most of it is in the form of Arturia analog synth emulators (ARP, moog, etc.). He's found a few technical problems with some of the emulations like reversed sawtooth waves, incorrect waveform voltages, etc. that just aren't true to the originals and (in some cases) just don't really make sense.

I think that's what happening with his "offset". Looking at it further It's not offset in the classic sense;i.e. the rest voltage is still 0DC, but at least one of his bass fundamental waves, if not more than that, are not properly balanced between positive and negative voltages. I think this is a problem with the waveform generation in at least one of his synths.

None of this is chiseled in stone yet, but it's my best guess so far.

G.