# Obtaining an IR from a sine sweep without the dry sweep

I have a test vinyl record that includes an exponential sine sweep as one of the tracks. I have a recording of the sweep through my turntable/mixer, and now I want to calculate the system IR. The problem is I don't have a copy of the clean sweep signal. What's the best way to approach this?

My first thought is to take a best guess at the sweep start and end frequencies and the $$\Delta t$$ between them, then use that to figure out the sweep rate and manually reconstruct the original sweep. But this seems kind of clumsy, is there a better way?

If this is used for channel sounding then you aren't concerned with the actual accuracy of your turntable (causing a frequency shift) in which case I suggest measuring the response in a static condition (non-changing channel) and determining the least square linear fit of the frequency sweep (I would assume but don't know that the test pattern would be a linear sweep) and then from that reconstruct a reference sweep to then use for channel estimation.

Measuring the frequency involves using some for of frequency discriminator process, most of which are also sensitive to amplitude variation, so I recommend hard-limiting the signal prior to the discriminator. Also pay attention to the relationship between instantaneous frequency and the underlying frequency of your ramp as detailed in this "DSP Quiz" question: Simulation of a Frequency ramp

• This approach works very well on very short signal segments. I recommend 2 1/2 cycles. That means the frame size will need to be adjusted in different parts of the signal. At near 2 1/2 cycles the second harmonic is near bin 5 (little leakage) and the third harmonic is between bin 7 and 8 (lots of leakage on a weak component, many bins away). Of course, a little pre-filtering would also improve precision. Three and a half is good too. dsprelated.com/showarticle/1284.php You get a near instantaneous frequency, phase and amplitude reading that is very accurate at every frame. Commented Jul 27, 2020 at 19:32