# How Cable Modems Fix Signal Distortions Before Sending Data > A method for cable modems to pre-filter data so it arrives clearly at the central office, preventing signal errors caused by the messy physical wires between them. - **Patent:** US 6665308 - **Original title:** Apparatus and method for equalization in distributed digital data transmission systems - **Owner:** Terayon Communication Systems Inc - **Granted:** 2003 - **Status:** Public domain (expired) - **Times cited:** 188 - **Field:** telecommunications, semiconductors, mechanical ## What it does This patent describes a way to clean up digital signals sent from a home modem to a central office. Instead of the central office doing all the work to fix signal distortion, it calculates the necessary corrections and sends those instructions back to the home modem. The home modem then uses a precode filter to adjust its own signal before it even leaves the house. This ensures that by the time the data travels through the noisy cable lines, it is already optimized to be read correctly by the central receiver. ## What it does NOT cover - Does not cover signal equalization that happens entirely within the central office without sending instructions back to the remote unit. - Does not cover analog signal modulation techniques that do not involve digital precode filtering. - Does not cover systems where the remote unit calculates its own equalization coefficients without receiving them from the central unit. - Does not cover general data transmission that lacks a training phase for channel characterization. ## The clever bit Instead of just trying to fix a distorted signal after it arrives, the system calculates the 'inverse' of the distortion at the destination and tells the sender to apply that inverse before transmitting. ## Real-world examples 1. DOCSIS cable modem systems 2. Upstream broadband data transmission 3. Digital subscriber line (DSL) signal processing ## Why it matters This technology was essential for the early deployment of high-speed cable internet (DOCSIS standards). By offloading the complexity of signal correction to the home modem, it allowed cable companies to maintain stable, high-bandwidth connections over existing, interference-prone copper cable infrastructure. ## Frequently asked questions ### What does How Cable Modems Fix Signal Distortions Before Sending Data cover? A method for cable modems to pre-filter data so it arrives clearly at the central office, preventing signal errors caused by the messy physical wires between them. ### Who owns patent US 6665308? Terayon Communication Systems Inc owns this patent, granted in 2003. ### When does this patent expire? This patent has expired and is now in the public domain — anyone can use the invention freely. ### What is patent US 6665308 cited by? This patent has been cited by 188 later patents that build on its ideas. ### What problem does this patent solve? This technology was essential for the early deployment of high-speed cable internet (DOCSIS standards). By offloading the complexity of signal correction to the home modem, it allowed cable companies to maintain stable, high-bandwidth connections over existing, interference-prone copper cable infrastructure. ### What does this patent NOT cover? Does not cover signal equalization that happens entirely within the central office without sending instructions back to the remote unit. **Full plain-English explainer:** https://patentbrief.org/patent/us/6665308/apparatus-and-method-for-equalization-in-distributed-digital-data-transmission-systems **Original patent:** https://patents.google.com/patent/US6665308 --- _Source: PatentBrief — https://patentbrief.org. Patent facts are from public records; the plain-English explanation is PatentBrief's._ ## Related patents Semantically similar inventions in the PatentBrief corpus: - [How Self-Healing Data Loops Automatically Elect a New Master Controller](https://patentbrief.org/patent/us/4677614/data-communication-system-and-method-and-communication-controller-and-method-therefor-having-a-dataclock-synchronizer-and-method) — A communication system where multiple nodes in a loop can automatically take over as the master controller if the current one fails, ensuring the network stays synchronized. - [How Multiple Computers Share a Network Cable Without Crashing](https://patentbrief.org/patent/us/4063220/ethernet-packet-network) — This patent describes how multiple computers can share a single communication cable by listening for other transmissions and stopping their own if a collision occurs, then trying again later. - [How Modems Use Guard Time to Switch Between Data and Commands](https://patentbrief.org/patent/us/4549302/hayes-modem-escape-sequence) — This patent describes a method for modems to safely switch from sending data to accepting commands without accidentally triggering that switch while transmitting normal files. - [How Cable Boxes Download Software Updates Remotely](https://patentbrief.org/patent/us/5440632/reprogrammable-subscriber-terminal) — A method for cable television boxes to automatically download and install new software updates sent over the air from the cable provider's main office. - [How Pulse Code Modulation Digitizes Analog Signals](https://patentbrief.org/patent/us/2266401/pcm-pulse-code-modulation-reeves) — A foundational 1938 patent describing how to convert continuous sound waves into a stream of digital numbers for transmission.