A Stereo Generator That Worked
I think there are too many technical things that the general public take for granted. The development of FM stereo was one.
WHEN STEREO WAS NEW
During the infancy of FM stereo, I worked for WCRB, a “Concert Music ” Station in Waltham, Massachusetts. Mr. Richard L. Kaye was the Chief Engineer, Program Director and Station Manager (although I was appointed the Chief Engineer at WCRQ in Providence, Rhode Island his “sister” station).
At the time, when there were very few FM stereo stations on the air, the “H H Scott” company of Maynard, Massachusetts was attempting to design a stereo receiver. Unfortunately, There were no stations on the air that transmitted a stereo signal that Daniel Von Reklinghausen, Scott’s Chief Engineer, could use to test off-air performance of his new design.
Therefore, Reklinghausen made his own low-power transmitter using the recently approved “General Electric” specification that used a 19 kHz pilot tone and 38 kHz switching between channels.
AN ON-AIR TEST
Mr. Von Reklinghausen asked WCRB’s Chief Engineer, Program Director and Manager, if he could install his prototype stereo generator in WCRB’s new FM transmitter which had a wide-band input.
Mr. Kaye was enthralled at the opportunity and started to rebuild the studio and program path for stereo operation. Although it was wide-band, the broadband input to the new Collins was designed for SCA. Changing a capacitor helped extend its low-frequency response, and we were ready to go.
My main assignment was to build the stereo studio in rented space at Boston’s Sheraton Plaza Hotel. Since stereo consoles did not exist at that time, I made my own using high quality Daven attenuators and conventional circuitry with “canned” solid-state amplifiers that used Octal plug-ins, designed and produced by Amherst Electronics. Audio transformers were “Linear-Standard” types.
BETTER THAN THE POOR IMITATIONS
Once WCRB was on the air with a stereo signal, other so-called stereo stations came on-the-air.
I say, “so-called” because they really did not transmit stereo audio. Some stations like our competitor WCOP cheated, using a Heathkit audio generator to insert a fake 19 kHz “pilot” signal. In fact, quite a fewt FM stations started doing this to fool radio receivers and their owners into thinking they were tuned to a genuine FM stereo station.
When I discovered this and mentioned it to the FCC Engineer-in-Charge for the Northeast District, EIC Nathan Hellenstein, WCOP tried to cover over their game by accusing me of gaining my knowledge by trespassing and seeking to have me arrested.
Suffice it to say, those who truly appreciated the WCRB broadcasts were usually owners of H H Scott stereo receivers.
DESIGNING THE EXCITER
Fast forward about five years. I was working for CCA, trying to make a Direct-FM exciter somebody else had designed actually work.
The problem was that the frequency would drift all over the place as the temperature changed. Without a complete re-design, the only way to stabilize was to put the critical circuitry in a temperature-controlled box. This is what we did, and it actually got into production – the “Works in a Drawer” was touted as an advertising advantage for the exciter.
In the meantime, while learning to fly at the Westchester Airport, I met Bill Johnson who owned an airplane based there. Mr. Johnson was the Chief Engineer of Wilkinson Electronics. Once he knew I was working for Bernie Wise, he asked me to drop by his company which I did.
The owner, Guffy Wilkinson, asked me to leave CCA and Mr. Wise (who had a bad reputation in the industry) and come to work for his company to design a new Direct-FM exciter and stereo generator. So it was at Wilkinson Electronics that I designed a stereo generator that actually worked according to FCC Specifications.
A DIFFICULT TASK
It was not easy.
We had to overcome several issues. First of all, the specifications were written by lawyers, not engineers. Early developers took the specifications literally and subtracted the L and R channels to produce the stereo sub-channel, modulated a 38 kHz subcarrier to produce a DSB signal, and then added it to the sum of the L and R channels.
At the time, no manufacturer had been able to do this in a manner that met the FCC specifications. In fact, Arno Meyer of Belair Electronics, maker of FM stereo monitors, claimed that it could not be done and he had the mathematics to prove it.
MAKING A REALLY GOOD STEREO GENERATOR
Unfortunately, there were time-delays between the main channel and the subchannel that could not be resolved.
My solution to produce the required AF spectrum was by chopping up (sampling) the left and right channels, producing square waves, then filtering the result, all in-line with no differential delays. I designed a phase-linear elliptical filter to do the job. This was critical for RCA had copied this system, but filtered the result with a “store-bought” Torotell Filter that had a sharp cutoff. (Their engineers ignored the fact that square waves do not have second harmonic components, only odd-order products so you do not need a sharp-cutoff filter.)
The result was the Wilkinson SG-1E a stereo generator that featured at least 60 dB separation up to 7500 Hz (more than 50 dB separation up to 15 kHz). FM Signal to Noise was 75 dB or better.
Wilkinson SG-1E
Stereo Generator
Bill Johnson and I brought our new stereo generator up to Arno’s lab. He connected it to his new Modulation Monitor and it worked as advertised. He was so happy he bought our stereo generator on the spot – and wired the FCC’s Standards office in Laurel, Maryland that they needed to purchase one as well so they could properly test monitors which needed “Type Approval.”
A VERY POPULAR PRODUCT
Guffy Wilkinson sold hundreds of these stereo generators to broadcasters all over the world who wanted to upgrade their facilities by replacing the Heathkits and, finally, transmit true stereo.
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Richard Johnson has been designing electronics for broadcast for over 55 years, including the first solid state kilowatt AM transmitter, as well as a complete 50 kW AM transmitter, one of which is still reported to be on the air after 50 years.
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