Tilt +6 dB / Octave (Step Correction)

Controls: Spectrum Dialog >> Curves >> Tilt+6
Macros: Tilt6LI, RI, LO, RO

This is a built-in Curve that tilts the power spectrum upward at +6 dB per octave. It has an even more specalized use than the above pink noise compensation. Yes, it could be used to compensate for 'brown' noise, which falls at -6 dB per octave. If that's what you need, you are all set.

But the intended use is for quick frequency response measurements which might otherwise require a slow frequency sweep or a long noise average. The principle behind this is that the spectrum of an impulse response gives the frequency response of the system.

If you actually try that, you need a very narrow pulse, only one sample wide. You can experiment using the Generator Pulse Wave option. However, there is not a lot of energy in this narrow pulse, and it may be hard for some types of sources to switch on and back off this quickly. A big voltage step, on the other hand, can have a lot of energy and is very easy to create. The derivative of a step function is an impulse function, so it is perfectly "legal" to apply a step to the system and take the derivative of the response to get the impulse response of the system.

The '+6 dB' Curve simplifies this by making use of the fact that a derivative applies a +6 dB per octave tilt to a spectrum. Here, we simply take the spectrum of the raw waveform, then apply the tilt to the spectrum. If the original spectrum was from a step response, the result will be as though it was from an impulse response. You thus see the frequency response of the whole system on every step.

The Step Response topic covers this method in more detail.

Whether you use an actual impulse, or this Step and Tilt trick, it is important to capture the full response to each event. If the system under test "rings" it must decay to below the noise floor by the end of 1024 samples, or the FFT used to create the spectrum will show the spectrum of the truncated response... not the true spectrum.

Note that sound card inputs and outputs have AC coupling which will pass the ringing, but will greatly reduce or block response frequencies below a few hertz. (But see DC Measurements for ways to get true DC input response.)

One tip: Depending upon particulars of your test setup, you may need to adjust the Trigger Delay to eliminate any time lag before the actual response begins. You can usually tell that you need to do this because the raw waveform trace will show a flat portion before the sharp transient of the response. Just add anough trigger delay to eliminate that flat part.

Caution: Step and impulse tests work on the assumption that the system is linear throughout. This is not the case for many real-world systems, such as electromechanical and biological systems, if you push them hard enough. In addition to amplitude limitations like saturation or clipping, which can be troublesome even with conventional swept frequency responses, some systems are susceptible to slew limit problems. This is where the system has some maximum rate of change that it can't exceed; if you drive it with a step or an impulse, it can't change fast enough to respond to the vertical edge, so it does the best it can and instead gives (typically) a constant slope.

This is not the same as the exponential step response you might get from a linear system, and if you drive the system into slew limiting your resultant frequency response will not be the same as a conventional slow frequency sweep response.

This means that you should avoid or use extreme caution with step or impulse methods for basic research that is exploring the response of an unknown system. These methods are best suited to monitoring known systems, such as in production testing of loudspeakers. When you are testing against a known response, even a certain amount of nonlinearity may be perfectly acceptable if you can still detect changes from the standard response.

As with +3 dB/Octave Pink correction, Step correction does not affect the raw data, only the display. The correction that you see on the display will only be included in .TXT files; if you save a binary file (.DQA, .WAV, or .DAT), you will need to reapply the Tilt when you view it.

Macro Notes:

Tilt6LI=1 applies a 6 dB Tilt to the Left Input, Tilt6LI=0 removes it, and Tilt6LI=x toggles between states.

Use Tilt6RI for the Right Input, Tilt6LO for the Left Output, and Tilt6RO for the Right Output.

See also Spectrum Curves Dialog; Spectrum Control Dialog