Noise Gate

[col]

In my Babani book of Advanced Projects for the Electric Guitar there is a noise reducer for the delay circuit. Could this be used in isolation for a chain of pedals and is there another IC that can be used instead of the NE571 which I am having difficulty in locating.

Col

[puretube]

http://pdf.alldatasheet.co.kr/datasheet-pdf/view/18878/PHILIPS/NE571.html

the SA... is the same as the NE...

however it is a compander,
not neccessarily suitable per se as a noisegate.

[col]

Thanks, the book says that the ic is a compander. I don't know what a compander does but the info o the NE571 says it is suitable for use in telephone systems etc. I don't have a web page so I can't scan the page and put it up, I just wondered if the circuit would work in isolation. It has in input and output, 9v and earth so I don't see any reason why it shouldn't,

Col

[Paul Perry (Frostwave)]

It's possible to make a noise gate from one, but it takes a bit of work.
The midwest-analog site has gone, but thanks to google cache, here are a few leads:
http://64.233.179.104/search?q=cache:oT2Tbb05FAAJ:www.midwest-analog.com/diychips.html+noise+gate+NE571&hl=en

[Mark Hammer]

There was a project in E&MM that used a 570/571 for a noise gate.  This may well have been it.  Probably a Penfold project if my instincts are right.

The other thing is that there is a difference between a noise reducer and a noise gate.

A noise gate operates on the principle that when accompanied by signal, whatever ongoing noise is mixed in with the signal will be barely noticeable and indeed "masked" by the signal.  Once the signal level goes below a certain minimum level, however, the noise becomes very noticeable.  Consequently, the circuit completely shuts/blocks the signal from passing through when it goes below a certain preset level, and opens up again (i.e., lets signal flow through) when the signal once again reaches the required threshold.

If there is a huge level difference between the actual noise floor and the "true" signal, and if the signal envelope is just right, this approach works relatively well.  Sadly, not everything in life is a mic'd kick drum (where the signal comes and goes in short, high-contrast, bursts), and this approach can be a problem for things like controlling hum from a single-coil guitar for someone who likes to play like Mark Knopfler and has many notes that need to sustain for a while or are lightly picked.  A noise gate will either let far too much noise through in this instance, or if you set it to eliminate the noise, it will chop off the beginnings and tails of notes in an irritating way.

This leads to a slightly improved approach, which is what's officially called "downward expansion".  This is, in a way this complete opposite of what a limiter does.  Where a limiter works to *reduce* any volume differences between signals above a certain level (i.e., the dynamics of everything UP TO that level are not affected; only what is above that level), an expander works to *exaggerate* volume differences of everything below a certain level.  So, it doesn't really shut anything off or on, but as the signal level subsides at the end of a note, and the note+noise goes below a certain threshold, the circuit makes it sound MUCH lower than it is, and the extent to which it exaggerates the volume drop increases as you go even lower.  This manages to preserve the start and end of softer passages a bit better than a gate, though it still isn't perfect.

A third strategy is a "noise filter".  This operates on the premise that the irritating noise accumulated in the signal path is going to be primarily hiss.  Liek a noise gate, whent he signal level is high enough, you don't really "hear" the hiss, but as the signal level drops, the hiss becomes more irritating.  To combat the hiss, but retain the signal dynamics and not forfeit the onset and tail of the notes, a noise filter uses a variation of an "autowah" circuit to trim the upper treble off as the note decays.  So, it applies a treble rolloff during softer passages to make the hiss less noticeable.

More complex versions of this combine the one-two punch of a downward expander and a noise filter to both downplay the parts where the "signal" consists primarily of diverse noise sources, and turn the treble down.  This is actually a bit better, since the noise can be both hum AND hiss.  The noise filter is fine for rolling off hiss, but since it starts from the top of the frequency spectrum, it does nothing for the hum way down in the lower regions of the spectrum.  The expander/filter combo helps to attack all sources of unwanted noise throughout the spectrum.

It is quite possible that the circuit in the book you refer to is simply a downward expander, since that would be easily achievable with a 570/571.  This is not quite as good as the expander/filter combo, but likely to be more usable across a broader range of situations than a simple gate.

[StephenGiles]

I have a circuit for the Bel Noise Reduction which uses a 571 - probably in pile 6D, but I have seen it in the last week or so, which seems to be a sort of DBX style unit. I built and used it with my Teac 3340 way back. So the 571 can be forced into behaving in a hi fi sort of way - it's on my list to dig out.

Ah Patti Loveless is on the wireless - wonderful!!
Stephen