What was WB900 ?

The WB900, also known as the 1+1 Carrier System or Analogue Pair Gain System, was introduced by British Telecom (BT) in the early 1970s to address the shortage of physical telephone lines. This system enabled two separate telephone subscribers to share a single line while maintaining complete privacy and allowing simultaneous use. 

Key Features of the WB900 System:

  • Dual Subscriber Support: The system designated one subscriber as the ‘Audio Sub,’ who used the line at standard audio frequencies, and the other as the ‘Carrier Sub,’ who operated at higher frequencies (between 19 and 96 kHz). This frequency separation allowed both subscribers to use the line concurrently without interference.
  • Privacy and Simultaneous Usage: Unlike earlier shared service lines, the WB900 provided complete privacy between the two parties, enabling both to make or receive calls simultaneously. 
  • **Equipment Requirements:**
    • Audio Subscriber: A filter unit was installed at the distribution point (e.g., on a pole) to prevent interference between the audio and carrier signals.
    • Carrier Subscriber: An electronic conversion unit was installed at the subscriber’s premises, with a corresponding unit at the exchange. These units facilitated the modulation and demodulation of signals at the higher carrier frequencies. 
  • Power Supply: The carrier equipment at the subscriber’s premises was powered by a nickel-cadmium battery, which was charged via the exchange battery when the line was not in use. In cases of heavy usage, an external mains-powered 9-volt supply was sometimes provided to ensure consistent operation. 

Evolution and Replacement:

Over time, the WB900 system faced challenges with the advent of new technologies, such as modems and touch-tone signalling, which interfered with its operation. To address these issues, updated versions were developed, with the final iteration being the WB900/9. In the 1980s, BT introduced the Digital Access Carrier System (DACS), a digital technology successor to the WB900, which further improved line-sharing capabilities and compatibility with modern telecommunications equipment. 

In summary, the WB900 was a significant technological advancement by BT that optimized the use of existing telephone infrastructure, allowing for efficient line sharing between two subscribers with enhanced privacy and functionality.