# Preprocessing signal before LPC

I have a signal that contains the speaking of three vowels. The signal is contaminated with dc offset and an interference at 60 Hz. I want to perform LPC for the synthesis of the spoken vowels. Do I have to remove the dc offset and the interference before performing LPC?

My initial signal looks like this:

There is also another question of mine about LPC here https://stackoverflow.com/questions/28909899/weird-output-of-lpc-algorithm

## 2 Answers

Usually before processing the speech signals, they are passed through a high pass filter with cutoff frequency of around 150 Hz to remove such artefacts.

• which will remove DC. Jul 31 '16 at 17:42
• Yes, this will ensure the unnecessary Low frequency and DC components coming from hardware/external source are suppressed/removed. Aug 1 '16 at 4:37
• DC is more than suppressed. it be gone with most HPFs having a gain of $-\infty$ dB at DC. this is more than just an artefact. turns out that because the autocorrelation $R_x[m]$ is the inverse Fourier transform of the power spectrum $S_x[k]$, which is also the square of the magnitude of the spectrum $|X[k]|^2$, removing DC before the autocorrelation insures that $R_x[m]$ is DC free. that means it must go negative for some values (and we know that $R_x[m]>0$ for some values of $m$ (like $m=0$) and gives you a solid threshold for separating peaks in the autocorrelation. Aug 1 '16 at 4:57
• and autocorrelation is definitely used in LPC, but the issue of separating peaks in the autocorrelation is more about pitch detection which might also find some use in LPC. Aug 1 '16 at 5:01
• Yes, LPC coefficient calculation does need auto-correlation. Aug 1 '16 at 5:26

Are you sure that the 60Hz is interference? When looking at the signal, I am not so sure.

Regarding your question about weird output, it looks to me that your LPC may be too small. The LPC size should be at least twice the number of formants.