How do I achieve a 90 degree phase shift?

mattybigback's icon

I'm trying to decode some old SQ quadraphonic records. I understand the theory of how they are decoded, but I'm struggling with the Max part of it. I know that I need to shift the phase of each channel by 90 degrees before recombining them (see the attached diagram) but I don't know how to achieve the phase shift in Max. I read the documentation for allpass~ but it didn't solve anything.

Max seems like it would be perfect for this - I'm just unsure how to proceed. Any ideas?

Fig-3.jpg
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jninek's icon

try hilbert~. It falls apart at low frequencies though.

Peter McCulloch's icon

If for some reason Hilbert~ doesn't get the job done (that would be the first thing I'd try, too), you could try using the FFT and add a fixed phase offset to one channel.

mattybigback's icon
Max Patch
Copy patch and select New From Clipboard in Max.

Hilbert~ did the trick! I got it working in the end. For anyone who is interested here's my patch