Hybrid algorithm changed to prevent subsonic frequency content from causing irregularities in background noise. All UM400 receivers with 1.x firmware should be considered radioactive and upgraded immediately to v2.8. Likewise, UCR411 (not UCR411A) receivers containing v1.x firmware must be upgraded. UM400 and UCR411 (not UCR411A) with v1.x firmware are not fully compatible with other Digital Hybrid products. This algorithm change was made 3 months after production release of the 400 Series, and only UCR411 (not UCR411A) receivers running version 1.x firmware and UM400 transmitters running v1.x firmware are affected.