The Community Team spent some time looking at the Signal Precessing Private Beta today, and we saw a lot of good things: You have a good amount of meta activity (and you are very civil!), you are asking good, on-topic questions, and giving good answers. The DSP SE community seems to have a strong feel for what does and doesn't belong here.
However, being a niche site, Signal Processing doesn't yet have the volume of content we like to see when we move a site into public beta. The more great content a site has when it enters public beta, the easier it is to tackle content quality problems that may be introduced by an influx of new users.
This is not a warning or a problem. We re-evaluate private beta sites on an ongoing basis, and have confidence that, given a little more time to increase the quantity of content (while maintaining your current quality), Signal Processing will find its way into public beta.