No Useable Signal

[jr_tech]

QAM is the digital modulation method for cable TV, while ATSC is used used for over the air digital TV.

QAM stands for quadrature amplitude modulation, the format by which digital cable channels are encoded and transmitted via cable television providers. QAM tuners can be likened to the cable equivalent of an ATSC tuner which is required to receive over-the-air (OTA) digital channels broadcast by local television stations; many new cable-ready digital televisions support both of these standards. Although QAM uses the same 6 MHz bandwidth as ATSC, it carries about twice the data (38.47 Mbit/s @256QAM per 6 MHz channel) due to the lack of error correction; however, this requires a significantly cleaner signal path, such as distribution through hybrid fiber-coax digital cable.

http://en.wikipedia.org/wiki/QAM_(television)
jr