# 2 questions about DSP (beginner)

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#### JCO

##### Newbie level 2 Hello,

I’m starting to learn DSP (Matlab )and I have two questions about it, maybe someone can help me out?
I’m trying to measure a frequency from an Audio Device and I’m using FFT to transform it from the time domain to the frequency domain. But I am doing a downsample before the FFT. But you can also do it after the FFT. Do you know what would be the most efficient (before or after)? And why?
Another question, I am using zero padding to increase my system resolution (up to 50Hz). My frame length is N=256 and sample frequency fs= 44100 Hz. So 44100/50 = 882, so I need to add 626 zeroes (to 256 frame length) to get 50 Hz resolution, I hope this is correct? I believe this is done for FFT. But is this the same for the DFT?

Thank you and regards,

#### KlausST

##### Super Moderator
Staff member Hi,

How do you do the downsampling? Why is it necessary?
Do you use an extra sample rate converter, then look into it´s datasheet about noise, harmonic distortion and other introduced errors.

To calculate the FFT frequency resolution you need the downsampled "sample rate".

If you use an radix-2 FFT algorithm you need to use 2^n samples. (n= integer). So 882 samples are not possible. Maybe you need to expand to 1024 samples. Check on this.

For a DFT you usually don´t need the 2^n samples. But to be sure read on your DFT documentation.

Klaus

#### FvM

##### Super Moderator
Staff member Yeah, beyond other points the article clarifies that zero-padding can't increase the frequency resolution.

#### JCO

##### Newbie level 2 Hi,

How do you do the downsampling? Why is it necessary?
Do you use an extra sample rate converter, then look into it´s datasheet about noise, harmonic distortion and other introduced errors.

To calculate the FFT frequency resolution you need the downsampled "sample rate".

If you use an radix-2 FFT algorithm you need to use 2^n samples. (n= integer). So 882 samples are not possible. Maybe you need to expand to 1024 samples. Check on this.

For a DFT you usually don´t need the 2^n samples. But to be sure read on your DFT documentation.

Klaus

Hi,

Thanks for your advise. I was thinking to decrease the sampling rate with a Downsample block in Simulink and see what would the effect of putting it before and after the FFT (in terms of efficiency).

Regards

#### KlausST

##### Super Moderator
Staff member Hi,

44100 Hz / 50 Hz = 882
So if you use a 1024 FFT you should have a frequency resolution less than 50Hz.

Klaus

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