Frequency to Pitch voltage conversion?

[Lostgallifreyan]

Didn't do any searching for you but these words may help: Compressor,
Expandor, Log Amplifier, Antilog Amplifier, Differential Log Amplifier,
Log Comverter, Data Compressor, Log Ratio Circuit.
The basic circuit looks like 4 op-amps so there must be some one chip
soln around.
Best

I've done most of those searches before. The problem with log amps is
most are specifed for RF. I want DC, and the string DC is NOT a good key
for use in a search engine, unfortunately... It doesn't help at all, to get
useful results. This is one of those problems where you really need to
search people's experience, hence posting here to try to get suggestions
for a specific part from those who have used it.

The circuit (as standard) can be done with just two op-amp stages, but it
uses a pair of matched transistors and a custom thermistor (usually made by
or for a synthesizer company like Moog). I still hope that there is a
single IC made that solves the thermal inaccuracy and makes the log amp as
easy to use as a linear op-amp.

 

[Lostgallifreyan]

TI have a log amp I believe. 100th of a semitone over 5 octaves
requires a very accurate system...

look at www.ti.com and search for logarithmic.

The small micro doing frequency detection and conversion to DC is
about the most accurate, flexible, integrated system for this app,
IMHO

-Andrew M

Thankyou. I'd quoted points you made in two other posts but never thanked
you directly yet.

This being the subthread where people have grasped what I want, I'll
restate my request in non music-related terms, as doing that has caused a
LOT of confusion..

I need a fast DC log converter that will do base 2 log output with accuracy
to within 5mV/V on its output, over a range of 5 doublings of input volts.
It needs to be a single IC, preferably an 8-pin DIL, with very low thermal
drift, and to be as simple to use as an op-amp.

I'll look at the log-amp search suggestions I've been given, but as this is
a problem that goes far deeper than a newby with a search engine is likely
to handle, I'm still hoping that someone who uses these devices might have
some parts suggestions to make things easier.

If there is a standard part equally simple to use, that takes a regular
pulse train input to make a frequency-to-log-converted-voltage digitally,
outputting a voltage with 14 or 16 bit accuracy, this will also be an ideal
answer. This part has to have very low thermal drift.

 

[Jamie]

I'm not trying to analyse a complex signal. I keep saying that I only want
a conversion of voltage scales.

All I'm looking for is a single IC to make a base 2 log conversion of
frequency to pitch. V/Hz, to V/Octave. I don't mind if it takes its input
as a train of regular pulses or as a voltage derived from a train of pulses
(as produced by the frequency to voltage converters I'm already aware of,
if the voltage output of the PLL I'm using doesn't help). This IC should
preferably be an 8-pin DIL, very easy to use.

so what i gather here is you want a fixed output frequency
with in a given window of input freq/Voltage? for example
if you had a 100..200 hz input you would like the output
to stay at 150 hz for example? and then step to the next
level for example 201..300 input to give you a 300 hz out ?
is this correct? this is what i call a stair case output.
anyways, i am just about to give up on this..
but from what i gathered, you mite want the output to
to do this with a Voltage input that you can vary and
the output will simply stay with in the case window until
the input range gets high enough to switch to the next level?

sometimes called an Octive generator for tuning instruments.
but normally they use a switch the select the output.

 

[flexten]

Looked at some of my old (1976) analog books. The AD755N is one such
animal, also look at 752, 751 and 756. Don't know if Analog Devices
still exists, but those parts sure show up on search. AD755P in the PNP
version. My 1976 book list price AD755N (1-9) $55, (10-24) $49, hope
the price didn't inflate!

Best

 

[Lostgallifreyan]

Looked at some of my old (1976) analog books. The AD755N is one such
animal, also look at 752, 751 and 756. Don't know if Analog Devices
still exists, but those parts sure show up on search. AD755P in the PNP
version. My 1976 book list price AD755N (1-9) $55, (10-24) $49, hope
the price didn't inflate!

Best

Great stuff.

Looking for 755N or 755P got me some leads. Those things look like they
might be old hybrid modules, maybe not made now. (I also wondered about AD,
having read something somewhere. I hope they haven't demised).

I found this page:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/3611
It's probably going to go quickly beyond my grasp, but I'll read closely,
it looks like a good one.

I also came across a Texas Instruments 'LOG100'. I don't know enough to
make a good choice though, so any input still welcome.

I was wrong about my accuracy requirement, btw, is actually 12 times more,
I was thinking of semitones not the whole octave, so I need within 400µV/V.

What amazes me is that this doesn't seem to be a standard part. The 741 and
many other op-amps are, and are made by many firms, so there's no chance of
easily losing access to new ones. I'd thought (and hoped) that log amps
would be almost as easy to find as linear ones.

 

[flexten]

http://www.analog.com/UploadedFiles...rm=computation&fterm=Analog computation&la=en

 

[flexten]

Yeah, these slow chips seem to be called "computation chips" nowadays.
Everything i see is mostly RF stuff, GHz. Anyway at AD they have a
section on log amps and one on computation chips. The 755 was nice in
it's day but may be hard to find, yes, probably LSI. Digikey has
catalog of Analog Devices with names behind part numbers, the AD site
is impossible to find stuff.

Best

 

[flexten]

One final post for me. So maybe the AD538, LOG100 or MAX4206. I see the
LOG100 is stopped production yet looks good. I like the MAX part also.


Best

 

[Lostgallifreyan]

One final post for me. So maybe the AD538, LOG100 or MAX4206. I see the
LOG100 is stopped production yet looks good. I like the MAX part also.


Best

Thanks for that.

I'll look at those three. This stopping of production bothers me..

Maybe there might be something digital entirely self-contained with 16 bit
I/O, seen by the user as voltages in and out. If there isn't, there might
at least be some cause for it. If these chips were as easy to use in
analog circuits as op-amps are, I think the general uptake might keep them
in use. Still, I've learned that there is enough use in light measurment
for it to be relatively easy to find something.

That MAX4206 is still a lot harder to use than an op-amp. If I could get
simple volts in, volts out with high resistance input, low resistance
output, I'd be happier.

Anyone got a cheap PICBASIC setup? I might just have to get into that to
solve this thing, if a PIC with inbuilt 16 bit DAC exists.

 

[Lostgallifreyan]

One final post for me. So maybe the AD538, LOG100 or MAX4206. I see the
LOG100 is stopped production yet looks good. I like the MAX part also.


Best

I found a datasheet for an AD8307.
8-pin DIL. Looks like it takes volts in directly, not current, and puts
volts out.

It might be ideal, but I have NO idea if it is. They say that it has a
bandwidth from DC to 1 GHz, and that it can be used for DC purposes, but
that datasheet reads like an Open University program on advanced RF and AC
systems. How this is going to be as easy to use as an op-amp, I don't know.
Op-amp's seem to have some very simple starting points to grasp, but this
thing seems to start in exalted levels of complexity that leave me cold.
There's nothing there that gives me any idea of how it will work on DC
signals, let alone whether it will give me the accuracy I want for 100
cent tuning accuracy in voltage control over 5 octaves (I'd settle for
three).

 

[Lionel]

Did I say 'simple', and 'accurate'? It needs to be accurate, as Andrew M
said, to get to within 100th of a semitone over 5 octaves. The 'standard'
method uses the nonlinearity of one of a pair of matched transistors in a
single package, but it's neither easy or accurately controllable. These are
all problems which I am certain are best solved on a single chip substrate
as are the accurate designs of op-amps and such, and I'm hoping someone
here can name such an IC, preferably an 8-pin DIL for easiest use.

If I can't get a single tiny analog IC to solve this, I'll have to consider
the answer that Andrew M gave, as I already have a 'ninja on speed' fast
tracking squarewave that would be ideal input for a frequency detector that
fed a digital conversion to analog output in V/Octave scale. Again, if
anyone can recommend an existing IC I can buy and use as easily as using a
standard op-amp, please name it.

Please understand that I am NOT into reinventing the wheel, or solving this
as a fun excercise alone. This is surely a problem that 40 years of IC
manufacturing have solved cleanly!

I doubt it. Your application - especially with the accuracy
constraints you've quoted - is spectacularly obscure, & is the type of
function that's usually embedded within electronic musical instruments
in the form of a chunk of DSP code.
I'd be astounded if it were possible to meet your requirements with a
reasonably sized & priced analog circuit, & doubly so if there was a
standard chip meeting your spec's.
If it were me, I'd use the smallest single-chip embedded controller I
could find that has an A2D & a D2A converter (possibly one of the
smaller PICs) & do the whole thing in software.

 

[Lionel]

I thought of a PIC chip, but that's not the simple answer I want.

Sadly, I strongly suspect that it's the simplest answer you're going
to get.

I'm
hoping there's a self-contained 8-pin DIL as standardised and easy to use
as a general purpose op-amp is. I need 12 bits or more of accuracy anyway,
and I'd surely do better with a small analog IC.

12 bits of accuracy is extremely difficult to achieve in a completely
analog, DC-coupled signal chain - especially if you need to perform
transfer functions of the sort you've described. Even single opamps
with that sort of accuracy are somewhat exotic, & I can't think of any
complex function analog chips that even come close.

You seem to be pretty determined to go with an all analog solution,
but I really think that it'll prove to be a blind alley, & you'll end
up having to either give up your project or go digital to get a
workable solution.

 

[Lostgallifreyan]

I doubt it. Your application - especially with the accuracy
constraints you've quoted - is spectacularly obscure, & is the type of
function that's usually embedded within electronic musical instruments
in the form of a chunk of DSP code.
I'd be astounded if it were possible to meet your requirements with a
reasonably sized & priced analog circuit, & doubly so if there was a
standard chip meeting your spec's.
If it were me, I'd use the smallest single-chip embedded controller I
could find that has an A2D & a D2A converter (possibly one of the
smaller PICs) & do the whole thing in software.

ok I think I need to find a cheap copy of PICbasic, hopefully from
someone who's decided they don't want to use theirs..

While the market for a ready-made IC as you describe might be small, there
is definitely enough of a market to make a bit of money from it. I might
ask Dieter Döepfer if he's considered stocking a few for hobbyists who want
to adapt his or other maker's modular gear.

 

[Lostgallifreyan]

Sadly, I strongly suspect that it's the simplest answer you're going
to get.


12 bits of accuracy is extremely difficult to achieve in a completely
analog, DC-coupled signal chain - especially if you need to perform
transfer functions of the sort you've described. Even single opamps
with that sort of accuracy are somewhat exotic, & I can't think of any
complex function analog chips that even come close.

You seem to be pretty determined to go with an all analog solution,
but I really think that it'll prove to be a blind alley, & you'll end
up having to either give up your project or go digital to get a
workable solution.

Ok, I agree. I wanted to see what other's thought so I pushed at this the
best way I knew, ask instead of trying to invent intractable wheels.

As a last grasp at an all-analog idea, I have one question:

Could the AD8307 by worth playing with as a DC log amp for three octaves
worth? For some uses I can relax accuracy restrictions, in one case it only
has to be close enough that whistling a note be tunable to the right pitch
by listening to the final output through headphones. So long as it's not
off-tune by a semitone or more, it might be possible to make up for errors
by practising with it. So long as the thing makes a consistent DC log scale
conversion, it might do. I can't tell from its data sheet because it's all
about AC and RF performance, and I'd learn faster if someone who knows
about that IC could tell me.

 

[Lostgallifreyan]

I decided to look at PIC chips. It won't be quick but it will get me there,
and get me to other things too.

As a last grasp at an all-analog idea, I have one question: (repeat of
earlier unanswered post..)

Could the AD8307 by worth playing with as a DC log amp for three octaves
worth?

For some uses I can relax accuracy restrictions, in one case it only
has to be close enough that whistling a note be tunable to the right pitch
by listening to the final output through headphones. So long as it's not
off-tune by a semitone or more, it might be possible to make up for errors
by practising with it. So long as the thing makes a consistent DC log scale
conversion, it might do. I can't tell from its data sheet because it's all
about AC and RF performance, and I'd learn faster if someone who knows
about that IC could tell me.