Quadrature Filters

I’m working on extracting color information from an
NTSC signal. The color information is encoded with
quadrature multiplexing using dsb-sc. I want to
demodulate the signal but it requires a carrier that
is in phase with the transmitter. The signal does
include a color burst which should be used to sync the
local oscillator.

My approach is this:

Make a high pass filter to eliminate the luma
information, leaving me with mostly chroma
information. Pass this through a very high Q IIR
filter tuned to the carrier frequency. The output of
the high Q filter will resonate long enough to be used
to demodulate the chroma information.

My problem is: how to I design a high pass filter that
will only output positive frequency?

Another approach I thought of was to use the fast
convolution method (FFT filtering) but instead of
multiplying against a filter just setting the bins of
the frequencies I want to take out to 0. Not sure if
that would actually work.

I’ve seen lost of abstracts of papers that talk about
quadrature filters but not sure if they are worth the


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