QSIndustries offered the first successful electronic reverse unit (E-unit) around 1983, followed by plug-in circuit board sound systems.
MTH adapted QSI systems in 1994. I purchased a Premier Southern Ps-4 from a Forumite (20-3006-1; 1401) made that year. The last two lines of Features on the box end flap state, "Protosound (TM) Digital Sound Package Designed by QSI [QSI herald with a small yellow lightning bolt on a black background]." This system has basic sounds.
As posted above, MTH and QSI added features quickly, including Passenger Station Announcements (PSA) and Freight Station Announcements (FSA), remote control of couplers, and a host of others.
QSI products offered the best operation and sound systems available. As PhillyReading posted, lots of them were installed in Weaver and Williams locomotives. They were designed to fit Lionel postwar locomotives. The reverse board was wide enough to keep the plastic sides of a Lionel postwar Geep from moving inward. Art Boynton of QSI had a table at York for years.
As I suspected, the system in my Ps-4 worked with my ZW but not with newer transformers. I contacted MTH as well as J and W Electronics. MTH Parts brought a circuit board from a RailKing Southern Ps-4 to York, and Wayne Renga (J and W) installed it. Now I can use newer transformers that produce "pure" sine wave AC. Other transformers produce "chopped" sine wave AC. QSI products are designed for Lionel Electrical Operating Standards [postwar transformers]. "Chopped" sine wave AC will damage or destroy them, though they may run at first.
PS-2, designed by MTH for DCS [command control], superseded PS-1.
The term "PS-1" was coined [here on the Forum, I think] to identify these earlier systems after PS-2 was introduced. "PS-1" was not printed on MTH boxes. "ProtoSound by QSI" was.
Protosound features were accessed by turning track voltage up and down [but not off]. That seems tedious now. But back then it was state of the art. As GGG posted, locomotives could be assigned their own ID, then another ID for multiple-unit consists. They could run independently of other locomotives that would not start until they were "called." MTH added a PROGRAM button to Z-4000 transformers to make that process easier and faster.
QSI and PS-1 systems are designed for conventional operation. But they incorporate many features of command control.