The portion of a bi-directional passageway that carries information from a subscriber’s home, back to a signal collection and processing center. In cable broadband systems, the return path is synonymous with “reverse path” and “upstream path.” The signal collection center is the headend; “bi-directional” is synonymous with “two-way.”
The U.S. cable return path spectrum is located between 5 MHz and 42 MHz — although often it stops as low as 30 MHz — depending on equipment and plant conditions. Spectrally speaking, the 37 MHz or so of bandwidth that embodies the return path is an inhospitable zone, highly susceptible to two types of noise: Ingress (signals leaking in) and impulse (electrical spikes.)
What makes it even worse is this crazy little fact: Most noise — upwards of 70%, by some estimates — originates in the home. That noise mixes with the intended signal as it moves upstream, and gets amplified along with the intended signal as it makes its way to the headend. So the desired signal and noise from eight homes goes to the tap, which gets joined to other pockets of eight homes, before it gets to the amplifier; at the point of the optical node, the noise and intended signals from as many homes are connected to the node are coming in. This effect is called “noise funneling.”
The harsh conditions in the return path usually require a sturdy modulation type, such as QPSK (quadrature phase shift key), which is slower than downstream modulation formats (like QAM), but withstands noise better. This is not unlike driving on a road pocked with deep potholes: You have to go slower, so as not to damage your car (or your head, on the ceiling). On a smooth road, without potholes, you can drive faster without apparent road risk to your vehicle. Oversimplified, the return path is the bumpy road; the downstream path is the smoother road.
Nonetheless, the return path is critical to two-way applications, which require a way for subscribers to interact, instead of just passively receiving TV pictures or broadband services.
For the first few decades of cable television, the return path wasn’t needed. Television signals were broadcast downstream, through the plant, to homes. Subscribers turned on their TV, and watched. They didn’t “click” to initiate or pause or rewind a TV show, or to invoke a bound application. They didn’t use PCs to connect to the Internet — and so on.
In the late ’70s, some operators experimented with television and data services that encouraged consumers to interact. At that point, attention started to focus on building a two-way path to augment the existing one-way, downstream plant. Mostly, it involved the installation of modules into existing amplifiers that fed a signal upstream, to the headend, then balancing the two-way signal path.
These days (2005), cable’s return path is viewed as a critical competitive differentiator against satellite TV providers, because it’s kind of hard to create a return path from millions of homes, to a satellite, to destination. It’s what’s propelling VOD to be a time shifting mechanism for more and more types of television, rather than being contained to “digital PPV” only. It’s what gives passage to just about any form of interactivity, save what can be done with a built-in DVR. In simple, competitive terms, cables return path is what will likely go farthest to attract and retain customers, at least relative to satellite. (Meaning, telco video also comes with a return path, so that fight will be fought on different terms.)