Spectrogram difference between two signals

[garyh]

Just wondering how hard it would be to implement an option for the spectrogram to show the difference between two signals, i.e. between the signal generator and reference mic.

P.S. Thanks for Spectrogram on Qu-Pad app.

[garyh]

If two signals can’t be compared, but since currently there are separate graphs for L & R of stereo signals, how about an option for the difference between L & R of stereo/linked channels?

[Dick Rees]

Gary…

You’re looking at something like SMAART. Studio Six Digital has an affordable software package…but the hardware interface is around $500.00. Really effective comparison is not exactly simple and since the market is relatively small, the unit cost is higher than one would like.

The issue is not so much about looking at two things but having those things carry meaning. Simple displays have simple meaning. More processing yields more meaning…but there will be a cost.

Best example off the cuff would be the utility of RTA : Spectrograph. For me RTA has always been no substitute for my ears. Spectrograph, OTOH, let’s me use what I hear but gives a nice scrolling display with assisted resolution.

Having the ability to use dual FFT is great. But you have to have some added signal processing hardware that (for the moment?) would seem to be external. I doubt it feasible to retro-fit consoles to have the necessary hardware. This means a wait for another console a la SQ, for example.

But the Studio Six stuff is available in a VERY user-friendly package and worth the price of the interface if your need is serious.

[garyh]

Currently the spectrograph shows scrolling levels for both L & R. The software in the console converts the levels across the spectrum into pixel colors. To display the difference between the L & R displays only requires subtraction of one from the other and display these values on a single graph. You could link 2 channels with one channel input being a pink noise generator playing over the PA. The other channel a reference mic in the room. With the difference between these being displayed on the spectrograph you could adjust the PA EQ until the difference is as little as possible (the display would show black all the way across). This would give you a good starting point to then tweak EQ by ear. You could even use this on normal stereo signals to check frequency distribution across your stereo field.

[Dick Rees]

Simple solution as is:

2 iPads

[Dick Rees]

It should work…

[MarkPAman]

Dick

My Studio Six Digital setup has the mic (which I already owned and has a battery, so does not require phantom), apple’s camera connection lead, and Behringer U-Control UCA222. I’ve stuck the Behri in a plastic box with couple of XLRs & a neutrik USB port on the outside and a clamp to fix to a mic stand. Cost under £60 in parts I think.

There’s reference mics pretty cheap too, which if they need phantom, could be run through the desk. So doable at well below the $500.

[Dick Rees]

Mark…

Good to know. Would you care to offer a comparison of the S6D rig to a split/dual screen Qu feature? This would of course factor in screen size options.

[garyh]

I’m thinking something like the attached. The L & R spectrum, representing the difference, would scroll in opposite directions from the 0 second line. As the level difference at each frequency between L & R becomes smaller, the graph would get darker. If the level of a frequency was higher on the left channel, the color representing that difference would start scrolling up. If the level of a frequency was higher on the right channel the color would scroll down.

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[garyh]

Probably better would be to rotate the graph 90 degrees like the attached.

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[garyh]

I think I’ve figured out how to do this with the current board firmware and a mono PA like our church uses. L goes to our house speaker system. Patch R to a channel line in. Connect a reference mic to another channel. Send both to a mono mix and PAFL it. Display spectrogram or RTA on the board screen or iPad. Send signal generator to LR. Invert polarity of channel with main R attached. Set levels for accurate readings and volumes. Adjust delay on channel with R main to oppose phase of reference mic channel. The RTA and spectrogram will now be showing the difference between the generated signal and speaker output picked up with the reference mic. You can also send high fidelity music to LR to show this difference with real world signals. Adjust EQ on LR using RTA and spectrogram for reference. Spectrogram should be as close to solid black as possible.

I haven’t tried this yet, but hope to soon.

[garyh]

Poor mans SMAART.

[Cora Dias]

Hi…i am a new user here. . For me RTA has always been no substitute for my ears. Spectrograph, OTOH, let me use what I hear but gives a nice scrolling display with assisted resolution.Having the ability to use dual FFT is great. But you have to have some added signal processing hardware that would seem to be external. I doubt it feasible to retro-fit consoles to have the necessary hardware.

[nottooloud]

If you’re on this forum, you probably already have the hardware. Qu makes a fine interface for Studio Six apps. Also for full blown Rational Acoustics SMAART. A&H could implement Rational’s API, and display my SMAART trace right on screen in the PEQ frame.

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