Hi @roadfun, thanks for those results, very comprehensive. I should point out I’m no expert in interpreting the heatmap results but I’ll give it a go, and that’s one reason it’s interesting to see results from others, especially when there are other tests to compare with, as you’ve provided.
I can see that a higher gain will introduce more noise, like cranking up the ISO on a photograph. I can see that the heatmap will also show increased noise in the form of an in-band glow and that visually one can compare how the signal lines look against that glow. The goal is to get the most detail/signal without needlessly ramping up the noise.
The problem is that while I can look at the heatmaps and say to myself “ah, that looks like the one with the best contrast” it’s not clear that the decoder ‘sees’ the same result. It may well be that it can see signal despite what we would consider visually to be a lot of noise. If I set the gain to the best scan visually, perhaps I am needlessly dragging it down.
The good news is that if the best gain is found then we can look at those heatmaps and say “ah, so that’s what it looks like visually” and now we know for next time and it should be much easier. As a methodology this visual approach does appear to give a decent enough starting point from which to iterate.
I’ve been doing some more testing with my own setup which has the Uputronics filtered preamp. This does a great job of killing nearby signals and introduces a lot of glow on the heatmap at high gains. Logically it seems like I don’t want to be running it up there. Until recently I’d been using AGC but it’s not clear to me how that actually works moment to moment.
I found a really interesting post from @obj who is staff who said that AGC works best with steady signals. In the context of ADS-B decoding this implies that tuner AGC ultimately gives you a result which gives you a decent result in preference to max performance. Even setting a fixed gain is a compromise between increasing performance much of the time, perhaps at the expense of performance at certain times of the day. But it’s certainly true that setting tuner gain can be used to avoid needless in-band noise from one’s own setup, and it’s not clear how AGC copes with that – does it overall do a good job of mitigating it or is it confused by it.
I set my gain to 20.7 based on that heatmap image looking like the one with the best contrast. My positions and aircraft dropped from normal AGC. I then tried it on 49.6 but reduced to 48 based on obj’s post. My positions and aircraft have gone to nearly a high yesterday. I know I’m well into the glow on the heatmap at this level. My assumption is that if I drop it a touch things could get better still as I reduce the noise, but clearly dropping it to 20.7 is too much. So for today I’m upping it to 38.6, which on the heatmap has a fair bit of glow but is a step down from what’s at 48.
Looking at your scans you’ve been running at 42 and I see little difference between the noise at 42 and the noise at 49.6. The glow you have at 49.6 is less than the glow I upped my gain into yesterday and dump1090 got more info from it. So I would be inclined to change your gain to 48 (avoiding 49.6 as per obj’s comments) and see if dump1090 can extract a bit more info from the tuner at that level.
Those signals you have down at 880MHz and 930MHz, could they impinge on your tuner’s performance? I don’t know but I don’t think upping your gain from 42 to 48 will dispoportionally increase them such that they could somehow become any more or less of a problem.
Perhaps once you’ve played with the gain you can try buying the Uputronics 1090MHz filtered preamp. You can see how it kills out of band signals nearby in my initial post. Perhaps it will have a much greater benefit for you with those signals nearby, as well as boost your in-band signals. Then you’ll be in the same position as me, seeing how much I need to reduce the gain to eliminate extra in-band noise but not needlessly going lower. The good news is that merely adding the preamp increased everything including range and aircraft, so even on AGC it ends up in a pretty good configuration.
Your numerical gain tests appear to give the best aircraft decoding at that highest gain, with higher numbers for the components at lower gains. Perhaps this too suggests that a gain around 48 will give you a slight boost over what you have now.
I’ll continue tweaking my gain here over the next week or two and once I’ve found what seems to be the optimum result I’ll post more info with the relevant heatmap scan to see what that visually looks like for future reference. It should give an idea of what kind of noise we can see on the heatmap knowing that the decoder can cut through it.
Not an exact science but I remember from tuning in to skip from other countries in the 1980s or finding the right balance of gain over tuning there’s an element of chasing a moving target to this.