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Re: Recovering one irregular signal in the presence of another stronger one--*in utero* pulse ox

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Phil Hobbs wrote:
> Hi, all,
> 
> Hoping there are still some DSP folks round here despite the evil Google 
> ban. (But I repeat myself.)
> 
> I'm working on a completely noninvasive sensor for fetal blood oxygen, 
> using optical sensing through the mom's abdomen.  It's a very low SNR 
> measurement on account of all the attenuation.
> 
> The mom's heartbeat modulates her pulse-ox signal, which is much 
> stronger than the fetus's on account of the scattering and absorption in 
> maternal tissue.
> 
> The data are several time series.  The main issue is the variability of 
> both pulses, which smear out the spectra and therefore knock the peak 
> heights way down towards the noise.  There are weak multiplicative 
> effects between maternal and fetal signals, as you'd expect.
> 
> What I'm looking to do is something like:
> 
> 1. Use a digital PLL to find the time-dependent maternal pulse rate.
> 
> 2. Resample the data accordingly, and notch out the first 5 or so mom 
> harmonics.
> 
> 3. Do the PLL thing on the fetal pulse, and signal average to pull out 
> the average fetal pulse ox signal.
> 
> Extra credit: sometimes the baby's pulse can cross the first or second 
> harmonic of the mom's, and it would be good to preserve both pulse 
> shapes accurately.
> 
> Resampling a noisy signal isn't necessarily the most well-conditioned 
> operation, so I'd welcome suggestions for just how to do this.
> 
> Thanks
> 
> Phil Hobbs


I'd be tempted to take a cuff-style/smartwatch heartrate reading from Ma 
and use that as the top of a search tree for the other frequencies. "Top 
of a search tree" isn't really all that much help but it's a start.

The thing I'd wonder is whether the fetal ox cycle isn't a harmonic of 
Ma's respiratory cycle. Actually, if it's not that's kind of an even 
more interesting question.

 > There are weak multiplicative
 > effects between maternal and fetal signals, as you'd expect.

This makes me think of an old Stroboconn/Peterson tuner. Obviously, 
those are solving a much simpler problem. I'd be tempted to play
with the aliasing/sidebands.

The peterson/Stroboconn had a base model of the frequency under 
measurement and provided a visual , intentionally aliased display
of the difference. It literally wagonwheeled.

This probably isn't much help, but it's clearly a profound problem. 
Seems  like a thing more for metaphor than for mechanism. Find the story 
and the mechanism reveals itself. I'd almost bet that there's a model in
the brain of every neonatalist.

-- 
Les Cargill

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Recovering one irregular signal in the presence of another stronger one--*in utero* pulse ox Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2021-05-18 18:31 -0400
  Re: Recovering one irregular signal in the presence of another stronger one--*in utero* pulse ox theman@ericjacobsen.org (Eric Jacobsen) - 2021-05-18 22:50 +0000
    Re: Recovering one irregular signal in the presence of another stronger one--*in utero* pulse ox Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2021-05-18 19:16 -0400
  Re: Recovering one irregular signal in the presence of another stronger one--*in utero* pulse ox Christian Gollwitzer <auriocus@gmx.de> - 2021-05-19 07:37 +0200
    Re: Recovering one irregular signal in the presence of another stronger one--*in utero* pulse ox Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2021-05-19 11:31 -0400
  Re: Recovering one irregular signal in the presence of another stronger one--*in utero* pulse ox Les Cargill <lcargil99@gmail.com> - 2021-05-30 21:14 -0500
    Re: Recovering one irregular signal in the presence of another stronger one--*in utero* pulse ox Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2021-06-02 12:39 -0400

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