Final answer:
Set-top boxes can reliably receive signals below 20 MHz through specialized modulation techniques, synchronizations protocols, and receivers designed to tune to specific low frequencies, as well as avoiding interference from higher frequency VHF TV channels.
Step-by-step explanation:
The question, "What enables the STBSet-top boxes upstream carrier to be located below 20 MHz and still be reliably received?", is primarily concerned with the frequency modulation and reliable signal reception of STB (Set-Top Boxes). For upstream communication in set-top boxes, frequencies below 20 MHz can be used effectively despite them being relatively low. This is possible through the careful design of the signal modulation technique and robust synchronization protocols. Just like how a cell phone's weak transmitter and receiver are tuned to specific frequencies within the microwave range, set-top boxes can tune to frequencies below 20 MHz for the upstream carrier. Additionally, the receiver's capability to detect weak signals, similar to how satellite dishes are tuned to resonate at specific frequencies to catch TV signals, plays a critical role in receiving the upstream signal reliably. Furthermore, since the VHF (Very High Frequency) TV channels operate between 54 to 88 MHz and 174 to 222 MHz, set-top boxes using a frequency below 20 MHz would avoid interference from these channels and utilize a less cluttered spectrum for the upstream path.