Tag – You’re It!  Signal Tags and Their Use in Cable Signal Leakage Detection

Cable signal leakage technologies have been around for more than three decades, and while the underlying concept of detecting CATV signals leaking out of the network has not changed, the technologies to accomplish this have changed considerably.  Shortly after the first commercial systems were released, vendors began adding “tags” or unique RF signals into their systems to enable better detection and later discrimination of signals coming from a specific network operators system.  These tags have evolved over time, and now greatly increase overall system sensitivity and accuracy in addition to simply discriminating between systems.

History of Cable Signal Leakage Tags

Through the years much has changed about how tags have been used to enhance signal leakage capabilities.  Listed below are some of the categories of changes:

Types of Tags:

  • The earliest systems didn’t use tags at all, a warble tone was modulated onto existing analog signals to make them recognizable to tech’s in the field
  • In some cases an FM tone was AM modulated by a step attenuator moving through a 25dB range providing an audible indication of signal strength based on the number of steps that could be heard.
  • Some of the earliest tags were simple AM or FM-modulated carriers combined into cable downstreams
  • Eventually multiple CW carriers were used with a known offset between them. This was the status quo for over a decade until recently.
  • The latest breakthrough is the introduction of digital chirp tags. Traditional dual CW tags would occasionally generate false positives when over the course of millions of samples a random off-air interferer would replicate the tag signature.  Based on RADAR concepts, the digital chirp tag is virtually immune to false detects as it is impossible to occur “in the wild”.  The transmission characteristics of the chirp tag also enables significant gains in sensitivity – smaller leaks can be detected at greater distances than ever before.

Number of Tags:

  • A single tag was the standard until the early 2010’s
  • Second monitoring frequency and tag was added by most vendors to accommodate monitoring the UHF band in addition to the existing VHF (primarily aeronautical) band.
  • As cable downstreams were expanded to 500, 750, 860, 1000, and eventually 1220 MHz additional tags were added up to the current 4 tags offered by the most advanced systems.

Tag Frequency Agility

  • Early systems were hardware-locked to a single frequency
  • Later systems enabled adjustment within narrow (<5MHz) range
  • Leading systems today offer complete frequency agility – tags can be placed anywhere between 130-1220MHz enabling complete flexibility in defining optimal channel lineups

“Tagless” Options Emerge

The introduction of up-to 192MHz wide carriers with DOCSIS 3.1 created large blind spots where signal tags were used.  To combat this, vendors introduced methods to detect the OFDM carriers themselves leaking out of the plant minimizing these blind spots.  While there are compromises in sensitivity and discrimination when using tagless methods, they are good options to use in conjunction with tags for complete spectrum coverage.

Other leakage detection approaches involving sampling RF feeds before transmission to the field and looking for a signature match with field detectors.   Adoption of this approach has been limited due to the complexities of setting up and operating a system using this method.  Links between headends, trucks, and field receivers must always be in constant sync or risk bringing the CLI program to a complete stop.


Cable operators around the world are increasingly realizing the overall plant hardening value of signal leakage systems, and the introduction of breakthroughs like the revolutionary tag methodology above, increased tag counts, and tagless fallback methods will only serve to accelerate this trend.  If you want to learn more about other breakthroughs in modern signal leakage technology or about signal leakage fundamentals in general please visit this site.


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