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In the mid-1970s, Harold Hallikainen modified a Moseley TRC-15 remote control with what was named the TEL171. It proved very popular and was eventually manufactured and sold with TRC-15s by another manufacturer. Hallikainen's story:
Back when I was the chief engineer for an AM/FM station, we were inspected by the FCC. The DJ on duty had a lot of trouble reading the meters. Besides the zero and calibrate controls, and their interaction, DJs just could not read meters. Also, the TRC-15 had several scales, some of which were two times another scale. So, I'm with the FCC inspector at the transmitter site where we're running a Bauer 707. The inspector drops it to night power (250W), then calls the DJ and asks for the meter readings. The DJ gives him day (1kW) readings by reading the wrong scale (since all readings were about twice the night readings during the day). The inspector was not impressed.
My first modification to the TRC-15 was to replace the calibrate control in the circuit with a fixed resistor that made the meter read high (excess gain). I then put the calibrate pot across the meter, making it a shunt gain control. The interaction between the controls was gone. You adjust the zero, then the calibrate, and the the zero was still good.
But, DJs still could not read the meter. Since the TRC-15A uses a voltage to frequency converter to send the voltage samples to the studio, I tried using a frequency counter at the studio. This was complicated since I had to subtract the zero frequency and adjust the timebase for scaling. It was really too complicated (microprocessors were new at the time).
So, I then discovered the MC14433 dual slope A/D. This output a multiplexed BCD reading from -1999 to +1999 for voltage inputs of -1.999V to +1.999V. I ran through 3 CMOS chips to a UART. The logic had a digit counter and a multiplexer. When the ADC finished a conversion, it reset the counter and routed one of the ADC digit select lines to the strobe input of the UART, causing the UART to send the BCD for that digit along with a two bit digit identifier (the counter value). The output of the UART went through an optical coupler so the ADC section could float, then to an XR2206 FSK modulator.
The studio had an LM3900 based band pass filter to get rid of the TRC-15A control tone, then an XR2211 FSK demodulator, then into a UART and some more discrete logic that sorted out the digits and sent them to the front panel display board that had CD4511 BCD to seven-segment LED latch/driver chips. The display board also had a bipolar PROM that we'd program for each station that would place the decimal point in the proper location for that particular metering channel.
That was the original system. We showed it at NAB. I think it was KCBS that said they'd buy one if there was a display at the transmitter site to allow one person calibration. So, I designed a board similar to the studio display, but using an LCD, since there was not enough power available on the floating supply to drive an LED display.
The TEL171 proved pretty popular. David Green Broadcast Consultants (a large distributor of the day) started buying new TRC-15A systems from Moseley, shipping them to us, where we'd put in the TEL171, then ship them on to him for sale to stations.
The TEL171 had a 25-pin D connector on the front panel of the studio display. I figured that would be valuable some day. I prototyped a plug-in MC6800 system to give automatic control and logging, but we never went into production on that system. Bill Bordeaux, of Interstellar Communications, though, DID make an adapter for the Commodore 64 computer that plugged in to that jack. We sold it as the ITO177 Intelligent Transmitter Operator. It added words to Commodore Basic to allow control and metering of the TRC-15. I eventually took another approach to automatic control and logging. That was the PCC180 system. It was an MC6800 based system that included XR2206/XR2211-based modems to emulate the control end of different transmitter control systems. It could emulate the Moseley TRC-15A with our adapter, the Moseley DRS-1A, and the TFT 7610. It had loop through relays that would take over the phone lines from the studio unit. It ran a version of Microsoft 6800 Basic that I licensed from Bill Gates ($3,000 plus $50 per copy). I wrote the code to interface with the Basic and emulate the transmitter control systems in 6800 assembly. I did not have a computer at the time, so I built a modem and dialed into "The Source," a time share computer system. They had a 6800 assembler, so I used that to develop the system. Several of those systems were sold. One went to WVON/WGCI in Chicago. I went back there to install it. That was my only time in Chicago. The PCC180 then grew into the DRC190, which was our complete transmitter control system.
Everything was written in assembly. Debug consisted of burning an eprom, staring at the results and the code, and guessing what was wrong. I could put in a breakpoint by hand programming a 0x3f (software interrupt) in the eprom programmer, then running it again. I could then look at registers, memory, the stack, etc. Today, I write stuff mostly in C for PIC microcontrollers for the cinema industry (test equipment, sound processors, digital cinema, closed captioning, etc.). With the debugger, I can single step through my code, look at any variables, or whatever. That, and a logic analyzer, makes it wonderful to debug code. While designing remote control equipment, I never had a logic analyzer or debugger. I did have a Tek 535 scope that served double duty as an oscilloscope and a heater.
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