Simple audio signal to level convertor?

[Kasterborus]

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

 

[neon]

the input is what? never say just the output 0-5v how is somebody going to determine what you want? i don't think you know. forget your micro you are making it complex for no reason. you want a sound modulator. work on that.

 

[Bob Eld]

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

Since you are going to ADC it anyway, simply capacitivly couple the audio
signal into the ADC. Insure that the max peak of the audio signal is within
the ADC range.

Convert to digital then detect and hold the maximum values for a determined
length of time. Use the numerical value in this hold register to control
what ever.

Each converted value is compared with the previously held value and updates
the held value if larger. A timer starts or restarts on each hold update.
The register is cleared on the expiration of the timer. This creates a
running peak of the audio signal.

The peaks values of most audio signals will be emphasized in the bass
region. This can be tailored with simple filtering if needed which isn't
likely.

 

[Eeyore]

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

Diode clamp it !

Graham

 

[Bob Eld]

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.

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. Likewise the input voltage won't go below ground. 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.

 

[Eeyore]

"Eeyore" wrote

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

 

[Bob Eld]

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



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



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



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!

 

[Bob Monsen]

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

 

[Bob Eld]

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.

 

[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.

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