Re: [Discuss-gnuradio] Resample block for audio signal

[Andy Walls]

Forgot to mention:

7. Restore the correct magnitude of the real audio by multiplying by the
magnitude we picked off of the complex audio signal at the original
frequencies.  (multiply block)

-Andy

On Sun, 2016-03-13 at 16:28 -0400, Andy Walls wrote:
> On Sun, 2016-03-13 at 20:10 +0000, Murray Thomson wrote:
> > Hi Andy,
> > 
> > 
> > Thanks a lot, I wasn't expecting so much help. I will read the
> > flowgraph and I will try to understand it. 
> 
> The magic happens like this:
> 1. convert the (dual sided spectrum) real audio signal into a (single
> sided spectrum) complex audio signal.  (Hilbert block)
> 
> 2. Pick off the audio amplitude. (Complex to magnitude block)
> 
> 3. Get the instantaneous frequency by taking the derivative of the
> instantaneous phase (quadrature demodulator block)
> 
> 4. Make a new complex audio signal by using the instantaneous frequency
> multiplied by the transposition ratio as part of the argument to sin()
> and cos(). (frequency modulator block)
> 
> 5. ***Missing*** Filter the negative side of the complex spectrum to get
> rid of aliases that will fold back in when we convert back to real
> audio.  (***Missing*** IIR filter block)
> 
> 6. Convert the complex audio signal back to real (dual sided spectrum)
> audio signal. (complex to float block) 
> 
> 
> > I've added an extra sine with double the frequency to simulate the
> > first harmonic and when I transpose it I find lots of frequencies. Is
> > this expected?
> 
> Well yes, now that I see the problem.  There needs to be a (pretty
> sharp) filter in between the frequency modulator block and the complex
> to float block, to knock out the negative side of the complex spectrum.
> You picked a case that creates strong aliases (overlapping harmonics),
> so they are noticeable when folding over into the spectrum when going
> from complex to real audio.
> 
> I just tested with voice, and it sounded funny, but fine. :P
> 
> > 
> > I will try to find a solution for it once I understand more how it
> > works. Thank you so much, I hope you've enjoyed with it :)
> 
> Yeah. :)
> 
> Regards,
> Andy
> 
> > 
> > Cheers,
> > 
> > Murray
> > 
> > 
> > On 13 March 2016 at 19:36, Andy Walls <address@hidden>
> > wrote:
> >         On Sun, 2016-03-13 at 12:00 -0400,
> >         address@hidden
> >         wrote:
> >         > Message: 10
> >         > Date: Sun, 13 Mar 2016 12:29:07 +0000
> >         > From: Murray Thomson
> >         
> >         Hi Murray,
> >         
> >         >
> >         > Hi,
> >         >
> >         > This is probably an easy one but I'm stuck and i could do
> >         with some help.
> >         > My goal is to get a musical note from the microphone and
> >         shift its
> >         > frequency to transform the note to a different scale. For
> >         this to happen, I
> >         > need to multiply all the frequencies for e.g. 1.5.
> >         >
> >         > I can achieve an octave of the signal multiplying it by
> >         itself (doubling
> >         > the frequencies). I thought I could do this resampling the
> >         signal but now
> >         > I'm not too sure. Do I need to use an FFT block for this?
> >         >
> >         > I would appreciate if someone can suggest the best way to go
> >         or point me in
> >         > the right direction.
> >         
> >         Since I was recording my daughter's violin audition video
> >         today, I was
> >         in the mood to play around with this one.
> >         
> >         Try the attached *grc file.  Note that you need headphones, or
> >         just keep
> >         the speakers away from the microphone, or the feedback will
> >         ruin
> >         everything.
> >         
> >         Run the flowgraph and select "Up 5th" from the "Transpose" GUI
> >         widget to
> >         multiply by 1.5.
> >         
> >         Double check my variables for A3, B3, C4, D4, etc. and the QT
> >         GUI
> >         chooser widget to make sure I got all the ratios right.
> >         
> >         > Thanks,
> >         > Murray
> >         
> >         Regards,
> >         Andy
> > 
> > 
>