## Topic: Windowing problem before LPC (Read 3781 times)previous topic - next topic

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• hyeewang
Windowing problem before LPC
##### 16 March, 2010, 09:55:19 PM
Why window the signal before computing LPC coefficients?

What is the whys of windowing?  I know windowing can prevent spectrum leakage,but LPC is none of business of FFT, so it has no spectrum leakage.

• jmvalin
• Developer
Windowing problem before LPC
##### Reply #1 – 19 March, 2010, 10:39:05 PM
Why window the signal before computing LPC coefficients?

What is the whys of windowing?  I know windowing can prevent spectrum leakage,but LPC is none of business of FFT, so it has no spectrum leakage.

You're always windowing -- even when you think you're not. As soon as you're not including an infinite number of samples, it means you've applied some kind of window. What you probably consider as "not windowing" is actually "using a rectangular window". The rectangular window has about the same effect on LPC as it has on FFT (yes, leakage!).

• igor_b
Windowing problem before LPC
##### Reply #2 – 03 April, 2010, 10:13:55 PM
Why window the signal before computing LPC coefficients?

What is the whys of windowing?  I know windowing can prevent spectrum leakage,but LPC is none of business of FFT, so it has no spectrum leakage.

in LPC (linear prediction) you are predicting a value of next sample based on some number of previous samples. if you just take a block of signal "without windowing" (apply a rectangular window on a signal to be correct) then, on a beggining of a block you are actually trying to predict something from a bunch of previous zeros, and at the end of a block you are trying to predict zeros from some signal. that is why the prediction error is very high on the beggining and at the end of a block. by using windowing you make this error much smaller.

• bluepen
Windowing problem before LPC
##### Reply #3 – 18 June, 2010, 01:10:06 PM
Why window the signal before computing LPC coefficients?

What is the whys of windowing?  I know windowing can prevent spectrum leakage,but LPC is none of business of FFT, so it has no spectrum leakage.

in LPC (linear prediction) you are predicting a value of next sample based on some number of previous samples. if you just take a block of signal "without windowing" (apply a rectangular window on a signal to be correct) then, on a beggining of a block you are actually trying to predict something from a bunch of previous zeros, and at the end of a block you are trying to predict zeros from some signal. that is why the prediction error is very high on the beggining and at the end of a block. by using windowing you make this error much smaller.

That sounds reasonable, thx!

Another approach may be utilizing some previous & afterwards points for calculating autocorrelation.

• jmvalin
• Developer
Windowing problem before LPC
##### Reply #4 – 20 June, 2010, 12:07:36 AM
Another approach may be utilizing some previous & afterwards points for calculating autocorrelation.

In theory yes, but in practice there's two problems with this approach:
1) This no longer fits the definition of an auto-correlation and the resulting matrix is not Toeplitz, so you can't use the Levinson-Durbin algorithm
2) The filter you will find is not guaranteed to have its roots within the unit circle. If you're using it only for analysis (FIR), that's fine, but if you're also using it for synthesis (IIR), it means that you're likely to end up with instabilities.