I have a real time series sampled at 32 MHz. So, when I channelized it and plot it against time, I get a Frequency vs. Time image plot where the frequency domain spans 16 MHz. To elaborate, this basically entails taking, say, 2048-sample chunks of data and doing a Fourier Transform on each to get a 1024-channel '16 MHz'-wide Fourier Transform over time (because we only care about positive frequencies). It looks something like this:
Every vertical line in that plot is a Fourier Transform of 1024 channels. Which means that the time step between adjacent vertical lines is 2048 samples wide or 64 microseconds wide.
Now, what I want to do is convert this time series so that it looks like it was sampled at 48000 Hz, but not actually. Basically, I want the 16 Mhz frequency channel to correspond to a 24 kHz channel. So, I still want to preserve the structure of the data. So, a naive low-pass filter doesn't work, because I don't actually want to remove the higher frequencies, I just want to "redshift" it all. I want to go from a
0 - 16 MHz band to a
0 - 48 kHz band. But those pulses that pulse at 4 Hz, I still want them to pulse at 4 Hz.
The way I would try to do this, is by binning the channels. So, if I have 1024 channels (as from above) - I'd try to bin them into, say, 32 channels (and compress the spectrum by 32 times) - which basically means that I would take blocks of 32 channels and sum them or average them somehow to get one channel. Now, I really want to compress this spectrum 666 times but it doesn't matter. I really just need to understand how to meaningfully combine multiple Fourier Transforms into one.
Unfortunately, the only way I know how to do this is by taking the amplitude of the Fourier Transforms and then summing up adjacent channels to bin the channels. But this LOSES phase information - and I lose the pulses that I want to survive. So, how do I those these channels into a bin meaningful without losing both phase or amplitude information?
More concisely, my question is the following:
Given N Fourier Transforms of N real functions, is there a meaningful way to get a sum of them without losing phase and amplitude information?
Of course, this doesn't have to be the question if someone knows a better way to handle my overall problem without binning channels.