how to design a lowpass filter for a audio signa

i sent some .wav files as a input to the AM Moudlation scheme in Matlab..i am not getting the clear audio as Output..My Fc=1000,Fs=11025..i attched my input spectrum,the input of the lowpass filter in demodulator spectrum...i know i have some issue in my filter...which filter should i chooose and what is the cutoof frequency i should choose for my filter (you can see the input specturm and get the some knowled3e about the maxmumfrequency of the input )

I got strucked here...so please help me...

Answer 1

[Star Strider]

If you have R21018a or later, use the lowpass function. (Also see the links in and at the end of that documentation page.)

It is also easy to design your own filter:

Fs = 11025;                                                                 % Sampling Frequency
Fn = Fs/2;
Wp = 1000/Fn;                                                               % Passband Frequency (Normalised)
Ws = 1010/Fn;                                                               % Stopband Frequency (Normalised)
Rp =  1;                                                                    % Passband Ripple
Rs = 60;                                                                    % Passband Ripple (Attenuation)
[n,Wp] = ellipord(Wp,Ws,Rp,Rs);                                             % Elliptic Order Calculation
[z,p,k] = ellip(n,Rp,Rs,Wp);                                                % Elliptic Filter Design: Zero-Pole-Gain 
[sos,g] = zp2sos(z,p,k);                                                    % Second-Order Section For Stability

figure
freqz(sos, 2^16, Fs)                                                        % Filter Bode Plot

signal_filt = filtfilt(sos, g, signal);                                     % Filter Signal

[Hari Ijjada]

why you have choosen wp as 1000/Fn becuase i am sending a audio signal...it may have frequencies upto 4000Hz...and iam attaching my input signal spectrum and my output before LPF spectrum...please tell which frequency i should choose...?

[Star Strider]

‘My Fc=1000’

I chose it because you specified the cutoff frequency ‘Fc’ as 1000 Hz.

‘please tell which frequency i should choose...?’

The ‘BeforeLPF’ spectrum appears to have been filtered by a 5 Khz lowpass filter to produce the ‘input’ spectrum. Your signal has some broadband noise but no specific frequency-limited noise, so choose whatever cutoff frequency meets your requirements. The filters will work so long as the cutoff frequency is greater than 0 and less than the Nyquist frequency.

[Hari Ijjada]

sorry ...actually that is my mistake ...that is not Cutoff Frequency...that carrier frequency...

i am getting output now .Sir do you have any filter which process " int " data in matlab ?.

[Star Strider]

That is likely not possible. The documentation for filtfilt indicates that x - Input Signal can only be double. The filter function works with integer inputs, however it introduces phase delays and phase distortion. You might be able to work around that by using a linear-phase FIR filter with it such as those produced by fir1. You will have to experiment with that to see if it works with your signal.