I understand the output from PC sound cards swings positive and negative (push-pull) with respect to the audio ground.
My objective is to create a complex audio signal in a graphical editor, such as CoolEdit or Adobe Audition, which provides an adjustable DC offset. I would then adjust the wave on the PC screen to fall entirely below the DC line.
My question is, will the above method result in a signal that is wholely negative in voltage with respect to the ground terminal of the stereo output of the card?
I would like to be sure before we purchase the software.
A good one can if it's DC coupled all the away, but don't depend on that. I think you need to get a USB to DAC converter maybe or a cheap USB sound dongle and enhance it..
On a sunny day (Sat, 12 Jul 2014 08:28:33 +1000) it happened David King wrote in :
Audio is likely not DC coupled, but it could be. If you had a VGA output you got 3 DC coupled channels of 8 bits... But lose your video. Extra VGA card? Bandwidt is then much biger too. Programming is more Diffi Cult.
The first generation Ipod Shuffle used a bridged amplifier output with no coupling capacitor, rather than a single ended output. You might be able to get one of those to reproduce a DC offset when connected to a line cable rather than headphones, unless there's some other limiting factor.
why purchase software? You can do EVERYTHING with a soundcard using free PC tools. Including driectly drive them with C/C++ code.
Both free LTspice AND octave are capable of making .wav format files which sound cards run.
Octave, you can do any waveform, bits, speed anything you want. Plot look at it look at the spectrum ect.
Now for the DC offset: soundcards usually have a rather husky output Z, supposed to be 600 ohms, but some are 560 ohms. So that means you can drive a 1:1 audio isolation transformer with one leg attached to a DC power supply, THEN you can make certain the waveform is as complex as you want and stay completely negative.
Parbly use the 1:1 600:600 1/4 inch cube 'red' transformer, costs around $10 last time I bought one. [it's an old telco wet line transformer, so it can take 100mA, I'd use the primary as the DC/output path. They have 50/70 ohm winding resistance and 1H core so.] you should be able to get your complex waveform through pretty well. The transformer gives you very 'safe' protection for everything. Or, you could simply use an OpAmp circuit with two inputs, one AC coupled for the soundcard output and the other DC coupled to what ever. Unless you're careful, I'd shy away from capacitive coupling where you need a big cap, which can store enough charge to accidently zap a component.
You sounded like you just wanted everything to be completely negative, not EXACTLY negative, or any other restriction, just negative. So if you don't mind dialing up the DC the set up should work well and be FREE. well almost.
Found the specs! maybe website still exists: Stancor PCT-77, at 3mA of DC??, or the much larger Stancor TTPC-6, TTPC-13 or 14, which can take more like 80+mA
You can really abuse the little transformer and it'll work well, but if you want no hassle go for the bigger Xfrmr
What exactly are you trying to achieve, force the speaker into the back stop of the coil movement or blow out to voice coil and cone, depending on the way speaker cables are connected ?
Speakers usually do not like large DC offsets, so you either AC couple the speaker to the amplifier with a clumsy big capacitor or with high quality high power amplifier use a relay in series with the speaker. In the latter case the protection electronics disconnect the relay if there is a big DC offset. The speaker is also disconnected at amplifier power up, until the power supply voltages have stabilized and the output is close to ground potential, to avoid interference from or damaging of the speaker.
It would be nice what he plans on doing with the final wave form, and at least what would be he highest frequency involved? For example, if only one channel is needed, he could use the other channel to modulate a simple PLL type circuit or a passive HI/LO filter to make a +/- reference he can mix in with FM.
You could use a variation of the video DC-restorer...
Video_DC_Restorer_Discrete_NPNs.pdf
on the S.E.D/Schematics page of my website. ...Jim Thompson
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I love to cook with wine. Sometimes I even put it in the food.
There was a circuit idea for design back in the April edition of ED (or was it EDN?) from an Indian gent. He used one of the stereo soundcard outputs for the desired waveform and the second output pwmed to generate the dc offset. Even had VB code download available from the magazine site.
So, wire a battery in series with the output. Or, use an op amp summing circuit. The exact circuit properties of consumer sound cards are not something you should rely on.
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