Signal Rectification For Mixer VU Display

[MathsDude69]

Hey Guys,

Im currently designing a primitive audio mixer unit using opamp circuits for simple filters/crossover networks. Prior to the signal being run through a power amp circuit I am looking to pass the signal to an ATmega328 MCU which has an ADC with a range from 0 to 5 Volts outputting to a digital range of 0 - 1023. This range is then remapped to fit the range 0 - 10 and the MCU outputs this visually to an 10 segement LED bargraph. The issue I am having is the the initial audio signal is an AC waveform. Thus half of the time the ADC would be sampling a negative voltage :-( Naturally if I bridge rectify this signal the magnitude of the waveform will be minus 2 diode potential barriers (-1.4 volts assuming silicon diodes are used) or even worse the magnitude of the original signal will be less than the forward voltage of the two diodes in which case no signal will pass. Does anyone know a solution to this problem??

 

[berkeman]

Hey Guys,

Im currently designing a primitive audio mixer unit using opamp circuits for simple filters/crossover networks. Prior to the signal being run through a power amp circuit I am looking to pass the signal to an ATmega328 MCU which has an ADC with a range from 0 to 5 Volts outputting to a digital range of 0 - 1023. This range is then remapped to fit the range 0 - 10 and the MCU outputs this visually to an 10 segement LED bargraph. The issue I am having is the the initial audio signal is an AC waveform. Thus half of the time the ADC would be sampling a negative voltage :-( Naturally if I bridge rectify this signal the magnitude of the waveform will be minus 2 diode potential barriers (-1.4 volts assuming silicon diodes are used) or even worse the magnitude of the original signal will be less than the forward voltage of the two diodes in which case no signal will pass. Does anyone know a solution to this problem??

What is the amplitude range of the input signal? You say it is symmetric about 0V, so just adjust the amplitude and offset of the signal to match the 0-5V input range of the ADC. Amplitude and offset adjustments can be done with opamp circuits...

 

[schip666!]

It's called an "Envelope Follower", or in simpler times a bridge rectifier with a capacitor filter. Basically you want to turn your AC into a slowly varying DC, just like a power-supply, usually using diodes. You can use an opamp to get around the .7v diode drop. I used to have a nice circuit someplace, but you should be able to find one online.

It seems a bit of an overkill to use an ATMEGA chip as a VU meter...but if you got them...

 

[berkeman]

Good point about the VU meter -- Hey MathsDude, are you going to convert the signal strength into the LED bargraph display according to the Volume Units standard of VU meters?

http://en.wikipedia.org/wiki/VU_meter

.

 

[Averagesupernova]

Absolute value circuit is what you want. It is a non-linear op-amp circuit that works just like a full wave rectifier without the lost .7 volts. Google it and I'm sure you will come up with a schematic.

 

[gnurf]

Maybe the AC-to-DC converter in figure 4 is a solution to OP's problem?

http://www.freeweb.hu/kredi/elek/telj/acdcpr.pdf

 

[skeptic2]

If you are trying to duplicate a VU meter, this link may be of help.
http://sound.westhost.com/project55.htm