[vorbis-dev] Spectral band replication

[Simon Hosie]

On Thu, 14 Jun 2001, Stefan T. Böttner wrote:
> Harmonic frequencies are integer multiples of the fundamental frequency.
> That means if you take all frequencies in a particular block (say up to 11
> KHz for a medium bitrate encoding), double them and add them to the
> original at a lower volume, then this results in frequencies up to 22 KHz.

Curious.  Along this line of thinking, have you done any checking to see how
well the higher frequencies encode if you subtract harmonics of the lower
frequencies first (and add them back in the same ratios when decoding)? 
Actually, it's not that simple, is it.  Bother.  Have you done it anyway?

To be honest I don't mean to get actively involved in this sort of thing,
but the very first post I read after subscribing here relates to one of my
current pet projects (at least in my mind).

> I already tried this with music that I downsampled to 11 KHz, i.e. 
> frequencies up to 5.5 KHz, and it did sound much better when adding the
> harmonics x2, x3, x4 or even more.

My understanding is that one of the qualities of valve amps that gets
audiophiles so excited is the presence of the lower harmonics.  That's
basically what you're doing here.  You'll find lots of chatter about the
qualities of valve amps and what they do that improves (or fails to degrade)
music if you look on the net.

I'm currently in the process of collecting schematics and datasheets to
write my own valve amp simulator.  I wasn't planning on doing an accurate
simulation, or anything, I was just trying to mess the signal up a bit to
see if I could see what the big deal was.

What kind of percentages did you use?

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