freq

[MLeo]

Hello,

I'm trying to write a simple hearing aid program and i'm having trouble with reducing the frequency of my input signal to a more audible range.

Any suggestions on what the best way to approach this would be appreciated.

M.Leo

[Havarti]

Hi MLeo,

What hardware are you using?  Also, if you can, posting your code helps people give more suggestions.

Brian

[MLeo]

Hi Brian,

Thanks for your help. I am using a standard microphone plugged into a TerraTec 128iPCI sound card. My output is a simple pair of headphones also through the sound card.

I was going to use a DAC but i thought this way would be more convient (in terms of setting-up).

Ive attached the program bellow.

M.Leo 

[einarsto]

Hi, MLeo!

I'm not sure I understand what it is that you want to do? You have a signal taken from a microphone and you split it into several frequency ranges whose amplitude can be controlled separately. Total range is between 0 and 20 kHz which represents the capacity for a healthy human ear, if I'm not mistaken. Do you want to "move" the power content at certain higher frequencies ranges to lower ones in order to make the sound at these ranges audible for persons with a total loss of hearing at those (higher) ranges? You're sampling at 44.1 kHz, which means that your maximum bandwith lies around 20 kHz according to the Nyquist theorem (with some margin). In order to access frequencies above 20 kHz without aliasing problems you will have to sample at a higher rate, if possible.

regards,

/Einar

Message Edited by einarsto on 04-20-2006 09:45 AM

[MLeo]

Hi Einar,

What Im trying to do is take a range of high frequencies (around 15 kHz) and reduce them down to around the 5 kHz mark.

[johnsold]

To shift the frequencies requires a process called heterodyning, which is used by many radio receivers to shift frequencies. Mathematically it involves multiplying two sine waves of different frequencies and filtering the result. Before filtering the output of the multiplication contains the two original frequencies and the sum and difference frequencies. If all these are far enough apart, filtering out the desired frequency generates the shifted signal.

In your case the frequencies are not too far apart, so the filtering becomes more of a challenge. Do you want to move the 15-20 kHz signals to 0-5 kHz? In principle you would first filter out all signals below 15 kHz, then multiply the remaining signal by a 15 kHz sine wave and lowpass filter everything below 5 kHz.

There can be some practical problems with making this kind of thing work, so tell us more about the details of what you want to do and perhaps someone can help.

Lynn

[MLeo]

Hey Lynn,

I've tried what you said and works great!

Thanks again for your help!

M.Leo