Well, I tried capturing the signal entirely in the upper lobe in GQRX and then downmixing it in GNU Radio. I confirmed that it’s centered and that the DC spike is shifted off the signal. Still no good, unfortunately. I know it’s detecting it because the file writer is writing bytes, but when I play the video in the GNU Radio Video viewer, it’s just white noise, which is the same as before when I just ran the DC blocker. So it seems this might be unrelated.
This stuff is hard 