[sdiy] active fluctuating treble boost

[Eric Brombaugh]

Spencer Johnson wrote:
> run an analog tone into a 9400 converter in F/V mode, so as to create
> a fluctuating voltage at its output. this voltage is then converted to
> a fluct. current and fed into an opto-isolator. then you'd have a
> fluctuating resistance, which could be able to control any res. value
> in any component, but my main idea for its use would be treble
> boosting. that resistance would be used to change the amount of treble
> boost in a basic active treble boosting circuit so that when the input
> received a lower frequency, the boosting would be greater, whereas
> when the input was a higher frequency, the boosting would be less
> noticeable. the purpose of the circuit would be to clean up the
> muddiness of lower frequency signals lacking intonation, without
> increasing the treble of already crisp high frequencies, avoiding a
> piercing tone.
This sounds like the inverse of the National Semiconductor "Dynamic 
Noise Reduction" technique. This is a non-complementary noise reduction 
processor that basically creates a bandwidth-sensitive lowpass filter. 
When a narrow-band signal goes through, the filter gets narrower and 
when a wideband signal is presented, the filter opens up. The idea being 
that you cut off low-level HF noise when there's no main signal there to 
mask it.

Your idea is the opposite: detect a signal with little energy in the 
high end and boost it. Don't boost so much when there is more energy in 
the high end.

Look up the datasheet for the National parts in this family. Google for 
"Dynamic Noise Reduction", or start with the LM1894 chip. You might be 
able to 'bend' these circuits to produce the inverse.

Eric