And Now For Something Completely Different

The last two days I spent a considerable amount of time thinking about acoustic processors. Not sure this really fits here, but maybe some consider it interesting nevertheless.

An acoustic processor is normally a box you integrate into your stereo to improve the sound. Specifically, by adjusting the audio signal in such a manner as to balance anomalies created by your hearing room and/or your equipment.

Now acoustic processors aren't terribly popular, so far. There are probably several reasons for that. For one, high end audio purists are generally sceptical about any modifications to the audio signal. Also, such a box isn't easy to integrate: For several reasons (price, calibration, distortion), such a processor can feasibly only work on digital signal. However, while CDs have been around for quite a while now, only recently most other audio sources are becoming digital. And even with digital sources like CD players, the signal is usually already converted in the source and passed on as analog. No home for the poor little acoustic processor.

Last but not least, the price tag: Such a box is all but simple, and consequently all but cheap. Those who need it most -- with cheap equipment and inappropriate hearing rooms -- can't afford it; those who could, have fairly little need.

Being one of those who'd need it but can't afford (my equipment isn't bad, but far from perfect in the low bass area; and my current hearing room is terrible), I've been playing for quite a while with the idea of going a route even I could afford: Use software to do the acousting processing offline. Grab all my CDs, torture them in the offline acousic processor, and burn the result again -- producing CDs that will sound terrible in any other setup, but should be perfect with my equipment and hearing room.

So, how does such an acoustic processor work? Well, the simplest variant is just an equalizer with a lot of bands (128 or so), and an auto-calibration system. (Measurement microphone in conjuction with a program to run a test and adjust the parameters.)

However, this simplistic variants do not work terribly well. The problem is that just adjusting absolute volumes doesn't help too much. Temporal effects play a big role: For one, if the speakers or the hearing room generate resonances, the effective volume may depend on the length of the sound. Even more importantly, psychoacoustic effects make sounds prolonged due tue resonances/reverberation seem relatively louder.

(Furthermore, it's desirable to correct phase discrepancies between channels and frequency bands, for improved positioning and naturalness; to correct dynamics for improved vitality and resolution; and so forth... But that's definitely beyond my amateur means.)

So what we want to do is adjust the volumes of individual frequency bands depending on the signal levels. When a sound sets in, the relevant frequency band's volume is adjusted by the stored volume factor for short sounds in this band; when it persists for a longer time, we successively move to the factors for longer sounds. Well, at least that's my idea on how it should work.

My major problem is my very limited knowledge of acoustics, psychoacoustics and digital signal processing. As a layman, I believe we first need a frequency analyzer, continuously tracking the signal level per frequency band in the input signal over time. This frequency analyzer needs to have similar properties to our hearing, I guess. Now using some function involving the different adjustment factors and the signal level history, we can determine the necessary current volume adjustment for each band. These levels are perpetually fed into an equalizer, processing the input audio signal.

Well, so much for the theory; now if someone could tell me exactly how to implement this...

Another complication is that having no measurement equipement, I'm trying to determine all the necessary volume adjustment factors by hand/ear, using various test sound. So far, my experiments were rather discouraging; but I still have hope... (For the first time in my life, I'm considering a wireless keyboard.)

And well, once remastering all my CDs in this manner, I'd like to use the occasion also to fix some evident recording errors... Most notably, undo this abominable moronic dynamic compression most CDs are fucked up with. How do we do that, again?...

No comments: