I got started in the two-way radio and mobile phone business back in the mid-1970s.  In those pre-cellular days, phones cost thousands of 1970s dollars, and there were only a small handful of channels for making phone calls.  Two-way radios were also suffering from spectrum overload.  Various technologies were being invented to alleviate the problem.

For mobile telephones, the answer was cellular technology, which allowed the reuse of available channels based on geographic distance.  For two-way radios, trunking systems, which share channels dynamically between users, helped immensely.  Both services benefitted from new spectrum allocations at 800 MHz as well.

The Emergence of ACSB: Amplitude Compandored Sideband

But it was clear that more was needed, especially for those users in the VHF frequency range where additional spectrum was unavailable.  Conventional FM had been whittled down to the narrowest possible bandwidth, so there was a great deal of research on alternative forms of modulation.  The performance of standard amplitude modulation, or AM, was terrible compared to the FM systems of the day.  Something that would perform as well as FM but with a reduced bandwidth was needed.  The solution was found in a specialized form of single sideband, known as ACSB, or ACSSB, amplitude compandored sideband.

ACSB (which was a trademarked term by Sideband Technology, later Aerotron) used a conventional single-sideband radio, but added audio compression and expansion (similar to that used in wireless microphones to reduce noise) as well as a pilot tone to lock the receiver to the transmitter, and help synchronize the compandors.  The result was a system that sounded and behaved very much like FM but with a bandwidth of one-fourth that of a similar FM signal.

ACSB had some success, especially in the special industrial radio service, which relied heavily on VHF spectrum.  I installed several systems for oil companies and various construction firms during the 1980’s.  Eventually, ACSB moved to spectrum in the 220 MHz range.  Segregated from other two way bands, it eventually was phased out, replaced by digital technologies which had matured by then.

The Revival of ACSB: Meeting Modern Communication Needs

In the early 2000s, the pending FCC reallocation of the upper UHF television band to cellular use rekindled interest in ACSB, this time for a new application.  Up until the reallocation, wideband FM was used for wireless intercoms in the entertainment industry.  Using frequencies originally designed for wireless microphones, these intercom systems provided full duplex operation (simultaneous talk and listen, just like a landline telephone), making them invaluable in the fast paced world of the entertainment industry.

However, the loss of spectrum created a huge issue:  Those intercom systems could no longer be used.  Without the extra spectrum, there was not enough spectrum to support full duplex operation.  A company contacted Radio Design Group looking for a solution.  After a lengthy investigation, we determined that ACSB would be a good fit.  The client agreed, and a new intercom was designed that met all of the requirements of the old system but could fit into the new, smaller frequency allocation that the FCC mandated.

Overcoming Communication Challenges in Demanding Environments

The product was met with immediate success in the entertainment industry.  But that was not the only market for this new-old technology.  As it turned out, there was another user of these systems, and their need was, if anything, more urgent than the entertainment industry.  Nuclear power plants.  Their systems were also becoming obsolete, and their environment was even more demanding than stage or studio, primarily due to severe multipath issues inside the containment, where these units are used.

Containments are made out of steel and concrete and are very reflective of radio waves, which bounce off the walls in numerous and unpredictable ways.  Because of this problem, narrowband FM systems and digital systems work poorly, if at all.  The old wideband FM systems were fairly tolerant of multipath, and, as it turned out, the ACSB systems were even better.

The biggest problem for digital systems is that the multiple random signal reflections create serious timing ambiguity making it hard to recover the data.  Analog systems suffer from the flip side of the coin:  The multiple reflections create frequency-selective notches in the signal.  In narrowband FM systems, this results in intolerable audio distortion and noise.  But in ACSB systems, the notches are barely, if at all, noticeable.

The result is an intercom system that has proven to be very reliable, even in the most difficult of areas.  ACSB, an old technology by today’s standards, is still going strong, having found an important niche in the communications industry.

To learn how RDG can help you implement ACSB into your RF communications systems, contact us for a consultation.

Jim Hendershot will be part of the panel discussion on Tuesday, March 26 at the IWCE Conference, Which Spectrum Options Make the Most Sense for Your Utility.