DFT_time2freq
=============
```{function}  DFT_time2freq( t, val, freq, signal_type )
```
Full definition:
```{code-block} matlab
f_val = DFT_time2freq( t, val, freq, signal_type )
```

computes the DFT at the given frequencies

parameter:
- t  : time vector
- val: data vector
- freq: DFT frequency vector
- signal_type: 'pulse' (default), 'periodic'

return values:
- f_val:  single-sided spectrum

example:

    t=linspace(0,1,100);
    t_val=0.9*sin(2*pi*3*t); % sine wave; amplitude 0.9; frequency 3 Hz
    f=linspace(1,5,101);
    f_val=DFT_time2freq( t, t_val, f, 'periodic' );
    interp1(f,abs(f_val),3)
    % ans = 0.8910
    plot( t, t_val )
    plot( f, abs(f_val) )