LapTop BreadBoarding

Step 4 First Circuit

A simple triangle/square wave oscillator will be the laptop's first circuit. A low supply current RRIO OpAmp was used in the breadboard. (In this case a LM6132). The output at node "OUT1" will go very close from zero to 5V. But since the audio input port for a MacBook only accepts audio level inputs, R5 and R6 are used to reduce the signal level. Op Amp OPA2 buffers the triangle wave and it needs to be attenuated too.

Comments

[Unit042]

I apologize if this quesstion has been answered before somewhere, but what exactly is the standard audio level for PC machines? I'm guessing from 0-1v, right?

I'm asking because I have run into a project that needs a cheap oscilloscope, and without the ability to capture the signal and not damage my computer, I cannot get the timing stuff right. (I can provide a link to it if you're interested)

[IronManMC]

Never ask an engineer a simple question ;)

Much audio technology comes to us courtesy of Ma Bell. "Line level" is telephone line level. Signal strength is measured with an S meter. 0 dB= 0.775 volts @ 600 ohms. Anything @ +3 dB (1.55 volts) or higher can produce crosstalk on other lines, so +3 dB is considered to be an absolute maximum. Acceptable telephone signals are generally around -10 dB, 0.0775 volts.

I have an audio mixer that labels line, instrument, and microphone levels as -20 dB, -35 dB, and -50 dB respectively. In case you're not into logarithms, -3 dB = half power; +3 dB= double, -10 dB= 1/10th, and +10 dB= 10 times.

So assuming that the dweebs that designed your sound card knew the specs and STUCK TO THEM, yes, your guess is close enough. If you exceed it, the input amp will eventually enter a nonlinear region. If it's vacuum tubes (stop laughing you fool - I used to work with computers that used vacuum tubes - and relays; my $3 digital watch has more processing power :D ), peaks begin to softly flatten out, producing a distortion that's rich in lower harmonics and pleasing to the ear. If it's solid state, the clipping is hard, harsh, and as pleasing as fingernails on the blackboard. Interestingly, the harmonic content is similar - higher odd overtones.

But you're building a 'scope, so you should be able to see this.

Line input is 600 ohms, not good for a 'scope. I'd buffer it with an op amp voltage follower.

There used to be plans and kits for converting (all tube) TV's to a big screen oscilloscope. I never tried one, nor do I know anyone who did.

Good luck with your project.


-Mike

[dsauer]

Thank you for the information. I worked in the audio IC design field, and all the customers seemed to have a set of audio signal definitions that I never got the full story on. It is nice to hear about the origin of things. Are all these audio specs written down someplace?

[dsauer]

 In my example, the RRIO Op Amp is going from 0 to 5 volts and it is seeing a gain reduction of 1/100. This puts the input at +/-25mV.  Standard audio used to be in the 100mV rms range with 10-20dB allowance for head room. I have not yet, but  plan on finding out the clipping level and input impedance at least for my MacBook Pro. But some great new resources like SciLab and Arduino have been drawing away my attention. Hope to use these resources in some fun future labtop circuits. 

Any resources that can capture sound on your PC can be used. The Mac includes a line in port. SciLab provides a way to get at the captured audio frequency signal. See my web site at www.idea2ic.com .