Oscilloscope interference and laptop outputs for newbie

[edy]

Hi,

I just acquired a couple of oscilloscopes and have been putting them through some basic testing with Lissajous patterns, Youscope, Oscillofun and so on. One thing I am noticing (and wanted to confirm with others who are more experienced) is the great deal of interference/inaccuracy on the outputs from my laptop "wave generator" software using the 1/8" phono plug.

Let me explain... I am using VA (Visual Analyzer here) http://www.sillanumsoft.org/

I have left and right outputs to a 1/8" phone plug which I cut on the other end (from a pair of cheap headphones). I connect both probe grounds to the single common bare wire, and tips of probes to left and right wires. I have my laptop "unplugged" from the wall just in case, to avoid grounding issues through the scope.

When I look at my waves individually, there is a lot of "cross-talk" or interference. That is, if I am looking at the sine on my left channel (A) but have the right channel (B) off, it looks ok. As soon as I turn up the volume on the other channel (which I am not looking at) the sine wave which I am viewing starts to show signs of the other one on top of it. So if the waves are similar in frequency, I start to see "beats" appearing.

Same thing happens when I reverse the situation. I believe this is screwing up some of the clarity of my Youscope and Oscillofun demos, and also the symmetry of my Lissajous patterns is not perfect (although to the casual eye it would look fine).

I take this to be caused by using cheap headphone wires that have no shielding between left and right channels, and a common ground. That is why using a laptop for producing more than 1 wave at a time is a bad idea.

The other thing I noticed is the square wave coming off the laptop from this software has a "ringing". Meaning it rides up, overshoots, then oscillates a bit until it gets flat, and same thing on the way down. In contrast, when I check against my scope calibration 0.5V 1kHz reference it is a beautiful clean signal.

Is there any way to clean up the laptop output better so it can be used for some basic demos, or is it just not designed properly for a clean signal out of a phone jack? I can't see any way to isolate it unless it had 2 separate jacks for left/right (like RCA type) that could have physically separate cables. Unless I found a 1/8" phone jack and just wired up 2 separate shielded cables so they don't physically touch each other along the way.

[AndyC_772]

The problem here is most likely your laptop, which would appear to have poor left/right separation and some ringing at frequencies above the audio band. I doubt it's anything to do with the cable.

In a moment of boredom I tried the Youscope demo myself a couple of weeks ago, and with my laptop it was awful - just a blur, lots of ringing, and the 'spot' was actually a short 45 degree diagonal line caused by high frequency noise common to both channels.

Then, I tried playing it via S/PDIF into a very high quality hi-fi DAC, and the difference was night and day - the image was sharp and clear, with the common noise completely gone. There was still some ringing until I changed one of the filter options on the DAC, and then that went away too, leaving an image that was quite square and with well defined corners.

Do bear in mind that demos like these are just a bit of fun, and certainly not a test of your scope in any way. They're a mildly amusing test of your PC sound card, nothing more.

[AndrejaKo]

I too think that laptop is the problem here. In general computer sound cards aren't all that good. I've seen some which can't produce frequencies above 16 kHz (Creative, I'm looking at you here!), some which clip rather hard when volume is above 1/2, some on which channels have different phases.

Then there's the quality control as well. I've seen some laptop soundcards which in some revisions for example have stereo microphone inputs and in other have mono microphone inputs.

My advice is not to expect too much from the sound card.

[edy]

I also tried output of both oscillofun.flac/wav and youscope.flac/wav versions from my phone output earphone jack to the oscilloscope and some parts of the demo looked bad:

- rotating cubes looked bad - curved lines, not sharp angled edges, didn't meet at corners
- scrolling wavy text was unreadable

These parts looked better:
- Lissajous-type patterns
- Particle cloud effect

Anyone familiar with those demos will know. I am not sure if this is an oscilloscope setting I am doing wrong or my output method, or the effect of the sound DAC in the phone or laptop, my wiring, or even the quality of the original source files (although I thought FLAC and WAV are best as they are non-lossy compared to MP3). I've seen better-looking demos online on Youtube and I just don't see that on my scopes (which otherwise seem to display beautifully).

[AndyC_772]

It's nothing to do with your scope at all. Phones and laptops have audio outputs that are built down to a price, and that price is as close to zero as makes little difference.

If you really want to spend the time and effort on this, you need to find yourself a high quality, low noise DAC which has a good step response. Since the step response contains harmonics which are outside the audio band, a DAC which is "good" in hi-fi terms may or may not actually give a displayed waveform which looks correct.

As a test, generate yourself a square wave and play that through the scope in normal Y-T mode. If it appears square, you have a source which might look OK. If there's ringing and overshoot on the edges, it probably won't - even though there's nothing to say it'll actually sound any worse.

If you're really keen, you could start looking into making an output filter which will give a reasonably flat response with equal group delay across the audio band, and which will attenuate anything above 20kHz or so. That could potentially be a worthwhile project and learning experience, even if the end result in this instance is just a toy.

[AndrejaKo]

Why don't you make some test files and use them to see exactly what's wrong. The wiring most likely isn't a problem. Type of cable isn't important for these frequencies. Expected problems should be coming mainly from noise, if the amplitude is too low, but I doubt that it could be a problem at all.

Try making some test files in say Audacity and play them on the phone and laptop and see how exactly they look. For example, make a chirp that sweeps from say 14 kHz to 20 kHz and see how exactly it looks like on the scope. This way, you'll be able to exactly see what gets attenuated.

Also which scopes do you have exactly?

[edy]

Thanks for the suggestions. I'll use Audacity to make some tones and see how they look on the scopes. The scopes are "JDR Model 2000" which is 20 Mhz, basically a rebrand of the Hung Chang/Tenma 72-720, and the other scope is a Hitachi V-1565 100Mhz.

I've looked at the square wave outputs from the 0.5V 1kHz reference on both scopes, and cross-checked them, and they look sharp. I received 2 pairs of probes with the scopes. One set only works at 1x because the capacitance screw isn't working to allow me to adjust for 10x. However, the other pair of probes works fine at 10x and I have adjusted for capacitance.

I used also a Gabotronics XProtolab as a frequency generator. It will output up to about 31.25kHz. I presume I can also look at a clock signal coming off a crystal or oscillator for higher frequencies, or a digital communication stream from somewhere, just to see if I am still getting a nice signal at higher frequencies, before I run out and buy a pro frequency generator. :-)

[Stonent]

Oscillofun actually was fairly clear coming from my Android phone.  The cube was a tad dirty, but not bad overall.  Absolutely make sure you're using the .FLAC file and not a youtube video or MP3.  I tried both and the output was no good. I could not see anything it appeared to be just static.

The .FLAC was very good.