50dB noise floor is quite high. On my DVB-T USB dongles using an outside discone, I see approx 60dB noise without any shielding or filtering on the dongle or USB power source. Judging from what I hear on my antique Arvin floor model AM radio, these computers and printers put out quite a bit of RF garbage.
For an experiment, I wrapped the DVB-T USB in foil and dropped the noise by about 3-5 dB. Will be permanently mounting the DVB-T and the NooElec HamItUp upconvertor PCB in a single shielded case with one connecting USB cable w/ferrite. See if that mod provides a better noise floor. Here are a few images captured on FM and Public Safety frequencies for comparison.
This first image shows a typical noise level centered near 88Mhz with RF gain at 0dB.
Second image is an average RF gain of 16 - 17dB to provide 30 - 35 FM carrier levels for good FM broadcast reception.
This image shows average RF gain of 25 - 30dB for Public Safety 159Mhz