Noise gate pedal from salvaged Dolby NE645B IC

[iiimonfire]

Hi everybody! I'm interested in creating a noise gate/noise reduction pedal using an IC from a salvaged stereo system. The IC is a Dolby NE645B - the datasheet can be found here: http://www.soucastky.eu/files/products/NE645.pdf

I've got two questions about this data sheet:
1) There's a table on page 4 - I'm not sure I understand the terminology. Am I to assume that the data represents the amount of noise reduction, given single-frequency tones (at the given frequency values in the leftmost column) at the indicated input levels (the topmost row)? I ask because values are shown as positive dB values, whereas the input levels are in negative dB. Basically, the motivation here is I'm trying to import this data to a spreadsheet where I can visualize and assess this IC's performance (IO curve, frequency response, etc.) before I begin invest time in building/testing/debugging/etc.



2) The test circuit on page 5 - Which resistor would I have to go about replacing with a potentiometer if I wanted to have a variable threshold for noise reduction? (assuming that this is doable)



I will take the time to say how much I love this forum! I've been about a year and a half into this hobby, and it only keeps getting more and more interesting. The assistance from the community on here is second to none!  <3

[EBK]

I'm guessing the chart shows pre-emphasis of higher frequency signals -- a stronger boost as frequency increases.  Also, some compression -- lower-level inputs are getting more boost.

Again, just a guess.

[EBK]

Looking at this more closely, what you have is half of a noise reduction system.  As I said previously, you have a pre-emphasis filter and a compressor.  These, by themselves, do not reduce noise.  Rather, they make your signal louder and more trebly, with the assumption that subsequent noise sources that will impact your signal will be in that trebly region (e.g., tape hiss). Making the signal louder in advance gives a signal-to-noise ratio benefit. That way, when you later restore the signal to normal levels (through a de-emphasis filter and expander that reduces the treble and turns down the volume), you will effectively be turning down the noise by the same amount.

Basically, this won't help you get rid of noise that is already there.  It is designed to reduce noise that comes in later in your signal chain.

This sort of thing is actually used at the input in some analog delay pedals, to combat the noise added by the BBD.  Again, the counterpart processes (de-emphasis and expansion) would be done at the output of the pedal.

[iiimonfire]

Thanks for the super-quick response @EBK!

I see. So I guess my options at this point are:

1. Search the original circuit board for the coupled IC that performs the de-emphasis filtering + expansion (assuming that such an IC does exist)?
2. Build a treble booster or exciter?
3. See if I can add it to an analog delay or reverb circuit, maybe to clean up the delay lines for crisper echoes?
4. Something else?
5. Nothing. Kick my feet up, spark up a doob, and enjoy my weekend. 

[Mark Hammer]

I have a 646B chip in my parts bin.  I suspect it was obtaioned the same way, by cannibalizing it from an unused or otherwise nonfunctional tape deck.  Still waiting to figure out what to do with it, though perhaps implementation as a noise-control system in some sort of modulation pedal might be the ticket.

[iiimonfire]

I have a 646B chip in my parts bin.  I suspect it was obtaioned the same way, by cannibalizing it from an unused or otherwise nonfunctional tape deck.  Still waiting to figure out what to do with it, though perhaps implementation as a noise-control system in some sort of modulation pedal might be the ticket.

I hadn't thought of that. So maybe (option 6) a daughterboard spliced at the input of a noisy chorus pedal (I've heard some complaints about the MXR Analog Chorus, for example)?

[EBK]

Looks like the same chip is used as either an encoder or decoder, depending on how you hook it up.

[Mark Hammer]

That's also true of the equally-traditional NE570/571/572 compander chip, used for the same purpose.