o scope, sine wave

I'm hoping someone here can help or have maybe seen this before.

I was setting my levels using a scope and a couple meters.

My head unit did no clip at full volume. I had everything on the head unit and processor set flat.

On my front outputs, my sine wave looked a little odd. I had it below the level of clipping. Just past the peak of the positive portion of the wave, the line was not smooth, it had on odd angle to it. The top and bottom peaks were well rounded, and it was just after the positive peak and not the negative peak. Even running the gain on the amp up and down would not change the odd shape. It's not terrible far off from what it should look like, but enough to draw my attention to it.

The other two channels of the same amp looked fine. I swapped the RCA's around on the amp, and the odd signal stayed with the RCAs.

I also noticed that the voltage would be different when I swapped RCA's around. What I mean is that I dialed both the gain for chan 1/2 to right at 20VAC and did the same for chan 3/4. When I swapped the RCAs for the channels, one voltage would be higher and then the other would be lower.

I think am going to go back tonight and substitute different RCAs. Could a pinched or tightly bent RCA cause such a thing? Or is it more likely in the processor.

For what it's worth, the equipment is Alpine INA--W910 head unit, PXA-H701 processor, Alpine PDX-4.150 amp.

Anyone have any ideas or seen this before? I am kind of at a loss.
I wish I had a pic of the odd sine wave. I obviously don't hear it...and now that I have the levels set, the whole system sounds better.

I'd appreciate any of your thoughts/input.

Was your tune frequency very near the crossover frequency setting of either the deck or the amplifier?

i am an idiot wrote:

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Was your tune frequency very near the crossover frequency setting of either the deck or the amplifier?

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I was using 500hz. Both channels of the amplifier are high-passed at 80hz.

Thanks, that *does* spark an idea. While I am substituting RCAs, I will see if there is anything that would make the front channel look different from the rear. While I am sure that I had everything flat and equal, it is a new setup and there is the possibility I overlooked something. It has to be a setting or a buggy processor....

Yea, it sounds like a signal anomaly being introduced by a processor.  If it was due to faulty RCA cables the levels might change (this could be the cause of your voltage issue if one set of RCAs had high resistance) or you might notice a phase shift due to cable capacitance, or it would simply short the output.  But waveform distortion like you're describing is most likely a processor issue.

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DYohn] wrote:

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Yea, it sounds like a signal anomaly being introduced by a processor.  If it was due to faulty RCA cables the levels might change (this could be the cause of your voltage issue if one set of RCAs had high resistance) or you might notice a phase shift due to cable capacitance, or it would simply short the output.  But waveform distortion like you're describing is most likely a processor issue.

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I will mess with it more when I have time. I'll try to get a pic of the waveform. The RCAs that have the odd wave are from the Front 2 output of the PXA-H701. The two RCAs that have the odd wave are in different sets of RCAs. I'm using two sets of AV RCAs. (Three RCAs per set.)

I will substitute the RCA's just to eliminate them and be sure. I think I can switch from using the Front 2 channel to using the Front 1 channel. (But I understand that Front 2 cannot be turned off for whatever reason Alpine thought that to be necessary.) I'll see if that can make it go away.

It's not really a bad distortion and I doubt that it could even be audible. But it was enough for me to notice the difference on the scope when compared to other channels.

Although, the voltage difference does have me puzzled.

I did some more fooling around with it yesterday. Here is what came up with:

I really think it's the processor. I did a little looking, and the Alpine Ai-Net cable only sends left & right information. It does not do front & rear, etc.

The sub shows no sign of the odd wave. I did not swap out for new RCAs, but I did swap existing pairs. No change.

I also changed from the Front 2 output to the Front 1 output from the PZA-H701 and the results were the same. I was also using a different 1KHz sine wave, to eliminate that.

Here is a pic of the wave that I took with my phone:


It is strange, but it doesn't seem to affect the sound or affect music. I did some more tweaking and the system is really starting to sound good. I think about all I have left to do is do some EQ work and maybe play with phasing a little bit.

Ah.  That kind of wave form is most likely due to a digital timing error in your DAC or some oscillator in the system.  It's a sign of an internal issue with the processor, but not anything you can do anything about.  It's a design or component issue inside the processor.

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That's what I had started to think after messing around with it.

When I spent more time with it, I noticed that both peaks had the same sort of thing going on.

I haven't had any issues with noise or any odd sounds. I've got the gains on my 4 channel amp turned all the way down and am getting 25.6VAC output. I also saw the strange wave on the rear channels too.
Maybe something changed, but most likely I just didn't notice it. I was probably just concentrated on looking for clipping at the peaks on the waves.

However, I did not see it on the sub amplifier. The next time I mess with it, I will plug the front channel RCAs into the sub amplifier and take a look at things. (I used a different sine wave signal, so I know it's not the source media making the problem.) So while there's a minute chance it could be the amp, I really suspect the processor.

DYohn, your suspicion of an oscillator or DAC seems to be the most logical. Thanks for your input.