Simple audio signal to level convertor?

K

Kasterborus

Guest
Hi,

I'm looking for a circuit that can read a standard line level audio
signal and output a continous voltage 0-5v proportional to the
amplitude of the input.

This in turn would be read by an ADC/microprocessor to modulate some
flashing lights.

As always - any help is greatly appreciated.

Dave
 
On Tue, 1 Jul 2008 10:15:55 -0700 (PDT), Kasterborus
<kasterborus@yahoo.com> wrote:

Hi,

I'm looking for a circuit that can read a standard line level audio
signal and output a continous voltage 0-5v proportional to the
amplitude of the input.

This in turn would be read by an ADC/microprocessor to modulate some
flashing lights.

As always - any help is greatly appreciated.

Dave
Click to expand...
Google for "colour organ" (or "color organ") or "Light organ"



--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
 
Bob Eld wrote:

"Eeyore" wrote
Bob Eld wrote:

Insure that the max peak of the audio signal is within
the ADC range.

Diode clamp it !

Graham

Yeah you can clamp it but then you loose that and higher values as peaks.
The whole idea is to convert the peaks to numbers not to clip them off.
Click to expand...
You can actively diode clamp. Just don't want to hurt the poor ADC 'just in
case'. I doubt that for flickering lights, losing anything below say 0x240 will
hurt any


There's really no need to clamp it. The processor input will have internal
"diodes" that will keep the voltage from exceeding the rail voltage plus a
little.
Click to expand...
A discussion we've had many a time before here !


Likewise the input voltage won't go below ground.
Click to expand...
It WON'T ? In what ideal world is this ?


A series resistance should be added to limit the input current and a shunt
resistance
added to keep the DC level from shifting up due to rectification by these
clamps.
Click to expand...
Uh ?

Graham
 
"Bob Eld" <nsmontassoc@yahoo.com> wrote in message
news:0zabk.16122$Ri.4973@flpi146.ffdc.sbc.com...
"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:486C4867.F6DAF534@hotmail.com...


Bob Eld wrote:

"Eeyore" wrote
Bob Eld wrote:

Insure that the max peak of the audio signal is within
the ADC range.

Diode clamp it !

Graham

Yeah you can clamp it but then you loose that and higher values as
peaks.
The whole idea is to convert the peaks to numbers not to clip them off.

You can actively diode clamp. Just don't want to hurt the poor ADC 'just
in
case'. I doubt that for flickering lights, losing anything below say
0x240
will
hurt any


There's really no need to clamp it. The processor input will have
internal
"diodes" that will keep the voltage from exceeding the rail voltage
plus a
little.

A discussion we've had many a time before here !


Likewise the input voltage won't go below ground.

It WON'T ? In what ideal world is this ?


A series resistance should be added to limit the input current and a
shunt
resistance
added to keep the DC level from shifting up due to rectification by
these
clamps.

Uh ?

Graham

Well, if there is an internal diode from the input to the negative rail,
it
will keep the voltage on the input from going below ground, ignoring the
diode drop, of course.

Now, if you capacitively couple to the input, this diode will rectify the
negative going portion of the audio signal and charge the capacitor,
positive towards the input. This will offset the zero base line in the
positive direction and screw up the peak detection. A judicious resistance
to ground with an appropriate series resistance will limit the current and
drain off this offsetting charge. It ain't rocket surgery!
Click to expand...

One thing to remember is that cmos inputs are usually prone to latch-up
disease. So, if the output goes higher than the positive voltage input of
the IC, you can end up with a triggered SCR between Vcc and ground. Not
good.

So, having a way to prevent this is a good thing, and generally consists of
a diode from the input to the positive rail. For digital inputs, I believe
most microprocessors use an internal diode. However, I'm not sure about
analog inputs, and would not want to risk it, particularly when a cap from
an external input can easily cause this situation, and the solution is so
simple.

See this link for more information:

http://ww1.microchip.com/downloads/en/AppNotes/00763b.pdf

Regards,
Bob Monsen
 
"Bob Eld" <nsmontassoc@yahoo.com> wrote in message
news:D7wbk.3604$vn7.1702@flpi147.ffdc.sbc.com...
"Bob Monsen" <rcmonsen@gmail.com> wrote in message
news:NCubk.6440$LG4.3212@nlpi065.nbdc.sbc.com...
"Bob Eld" <nsmontassoc@yahoo.com> wrote in message
news:0zabk.16122$Ri.4973@flpi146.ffdc.sbc.com...

"Eeyore" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:486C4867.F6DAF534@hotmail.com...


Bob Eld wrote:

"Eeyore" wrote
Bob Eld wrote:

Insure that the max peak of the audio signal is within
the ADC range.

Diode clamp it !

Graham

Yeah you can clamp it but then you loose that and higher values as
peaks.
The whole idea is to convert the peaks to numbers not to clip them
off.

You can actively diode clamp. Just don't want to hurt the poor ADC
'just
in
case'. I doubt that for flickering lights, losing anything below say
0x240
will
hurt any


There's really no need to clamp it. The processor input will have
internal
"diodes" that will keep the voltage from exceeding the rail voltage
plus a
little.

A discussion we've had many a time before here !


Likewise the input voltage won't go below ground.

It WON'T ? In what ideal world is this ?


A series resistance should be added to limit the input current and a
shunt
resistance
added to keep the DC level from shifting up due to rectification by
these
clamps.

Uh ?

Graham

Well, if there is an internal diode from the input to the negative
rail,
it
will keep the voltage on the input from going below ground, ignoring
the
diode drop, of course.

Now, if you capacitively couple to the input, this diode will rectify
the
negative going portion of the audio signal and charge the capacitor,
positive towards the input. This will offset the zero base line in the
positive direction and screw up the peak detection. A judicious
resistance
to ground with an appropriate series resistance will limit the current
and
drain off this offsetting charge. It ain't rocket surgery!




One thing to remember is that cmos inputs are usually prone to latch-up
disease. So, if the output goes higher than the positive voltage input of
the IC, you can end up with a triggered SCR between Vcc and ground. Not
good.

So, having a way to prevent this is a good thing, and generally consists
of
a diode from the input to the positive rail. For digital inputs, I
believe
most microprocessors use an internal diode. However, I'm not sure about
analog inputs, and would not want to risk it, particularly when a cap
from
an external input can easily cause this situation, and the solution is so
simple.

See this link for more information:

http://ww1.microchip.com/downloads/en/AppNotes/00763b.pdf

Regards,
Bob Monsen

The PIC stuff has internal protection diodes on the inputs. As far as I
know
all of their processors are so protected on the IO ports whether
configured
analog or digital. For a typical A/D input, see the PIC16F676 Data Sheet
page 47. There should be no trouble driving a few milliamps into these
diodes clamping off a signal without any external diodes.

I've never yet seen one latch up doing this. That's probably why the
diodes
are there.
Click to expand...

That is a microchip appnote, so somebody there appears to believe it can
happen, at least in some cases. Their summary suggests that it'll be a
problem if the input in question is driven higher than Vcc at startup, which
is a real possibility with an audio input which is driven externally.

On the other hand, I just spent 20 minutes trying to get a PIC12F675 to
latch up, and was not able to make it happen. So, maybe it isn't such a big
deal. The appnote I posted pointed to a couple of circumstances, like
inductive loads, driving power mosfets. Perhaps the spike has to be at
exactly the right time.

Regards,
Bob Monsen
 
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