[linux-audio-dev] Paper on dynamic range compression

Erik de Castro Lopo mle+la at mega-nerd.com
Wed Oct 18 11:20:57 UTC 2006


John Rigg wrote:

> On Wed, Oct 18, 2006 at 06:50:39PM +1000, Erik de Castro Lopo wrote:
>
> > But this should only be happening on transients and the clipping
> > should stop as soon as the attack portion of the compression
> > process kicks in.
> 
> Yes but my point is that the resulting aliasing sounds worse than the
> harmonic distortion.

The aliasing is present for 10s of milliseconds. It blurs into the 
instrument attack. Have a look at the attack transient of an acoustic
guitar or a piano. There a bunch of stuff in the first 30 odd 
milliseconds that bears no relationship with the fundamental note
frequencey.

> True, but those are removed by the anti-aliasing filters in
> the ADCs.

I talking about the stuff below half the sample rate.

> > Ok, lets say we're sampling at 44.1kHz, which makes the highest
> > 3rd harmonic we can represent is 22.0/3 kHz which is about 7kHz.
> > Do you really listen to many instruments where the fundamental
> > is at 7kHz?????
> 
> A lead guitarist will often deliberately play a harmonic `squeal'
> in a lead solo (particularly in rock and metal). As a guitarist myself,
> I fairly frequently generate fundamentals of around 4 kHz.

Thats a bit less than an octave below what I'm talking about.

> Deliberate
> use of higher frequencies than that might not be very common, but
> the equipment should at least deal with it gracefully.

I agree completely.

> The fact remains that a lot of high end professional users consider many
> of the free software plugins to be "nearly unusable" (see Ben Loftis'
> earlier post in this thread).

I have no problem whatsoever with the possibility that all current free
software compressors are poorly implemented, but that doesn't prove that
its not possible to implement a good compressor without upsampling.

All the best and most famous compressors were implemented using flawed
1960s and 1970s technology. The Joe Meek compressor uses light depend 
resistors!!!! In order to make a good digital compressor, we need to 
remember that a compressor is lo-tech. Any benchmark DSP implementation 
should also be relatively lo-tech. Once you have a good working lo-tech
DSP implementation then you can start looking to improve it using 
whatever far out DSP techniques you can think of.

Erik
-- 
+-----------------------------------------------------------+
  Erik de Castro Lopo
+-----------------------------------------------------------+
"Indeed, I am impressed that Google runs an 8,000 node Linux
cluster, 5 data centers, an extensive network, and a rapidly
evolving application all with a staff of 12."
  -- http://research.microsoft.com/~gray/papers/FAAMs_HPTS.doc



More information about the Linux-audio-dev mailing list