I could swear that they had been but couldn't remember exactly when it had come into effect, so asked google for the answer ...
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When and why did the FCC mandate that all AM radio stations must use a crystal for broadcasting?
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The FCC never mandated the use of crystals specifically, but their increasingly strict frequency stability requirements in the 1920s and 1930s effectively made crystal-controlled transmitters the only viable option. Before the invention of crystal oscillators, AM radio stations struggled to maintain a stable frequency, which led to significant broadcast interference. ... ....
Why crystal oscillators became the standard
The push for crystal control was driven by the need to solve the growing problem of adjacent-channel and co-channel interference.
Etc etc etc .... ....
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I don't think that's correct, I distinctly recall old magazine and newspaper reports that it was mandated that all broadcast stations were required to use only crystals for operation, seems like it was sometime in the 1930s
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You are correct that it was in the 1930s that the FCC's regulations effectively led to the universal use of crystals for broadcast radio. However, there was never a specific rule stating "all stations must use crystals." Instead, the FCC imposed increasingly strict frequency stability requirements that could not be reliably met by older LC (inductor-capacitor) oscillator technology. This regulatory pressure was a more subtle but equally effective mandate.
Your memory of magazine and newspaper reports is likely accurate, as the trade press of the time would have extensively covered these new, demanding regulations and the technological shift they caused.
Why FCC regulations made crystals necessary
Solving interference problems: In the early days of radio, LC circuits were susceptible to frequency drift caused by changes in temperature, humidity, and vibration. With broadcasting frequencies only 10 kHz apart, this drift led to significant interference between stations, creating a "beat note" that was a "disconcerting annoyance to listeners".
Preventing adjacent-channel interference: The FRC (Federal Radio Commission), and later the FCC, worked to organize the chaotic airwaves by assigning specific frequencies. However, these assignments were meaningless if stations could not stay on their assigned frequency. By the early 1930s, the required stability was an extremely precise 50 Hz deviation from the assigned frequency, a tolerance that was practically impossible for older LC circuits to meet.
The crystal solution: The quartz crystal oscillator provided the stability and accuracy needed to meet these new standards. The natural mechanical resonance of a quartz crystal is highly resistant to environmental changes. This allowed stations to reliably broadcast on their assigned frequency, solving the interference problem for both broadcasters and listeners.
Market adoption: This regulatory push, combined with the superior performance of crystals, drove the industry to adopt the new technology. Early adopters like New York's WEAF began using crystal control in the mid-1920s, and by the mid-1930s, it had become the industry standard. Amateur radio operators also embraced the technology, which further drove down the cost of crystals and made them more accessible.
In short, you are correct in remembering that crystals became mandatory for broadcast radio in the 1930s, but it was the effect of regulation, not a single, explicit rule, that brought about this change
I asked AI (Google) Why are AM broadcast band crystals not manufactured anymore? And it replied to me a bunch of information about crystal radios, to which I replied: "No, not crystal radios, why aren't crystals manufactured anymore for AM Broadcast Band transmitters?
Here's it's reply:
AM broadcast band transmitter crystals are no longer mass-manufactured for commercial use because modern broadcast equipment uses more advanced and flexible frequency control methods, making fixed-frequency crystals an obsolete technology for the industry.
The reasons for this transition include:
Technological Advancements: The development of modern frequency synthesis techniques, such as phase-locked loops (PLLs) and temperature-compensated crystal oscillators (TCXOs), allows transmitters to generate highly stable and precise frequencies digitally. This provides greater flexibility and stability than using individual, fixed-frequency quartz crystals.
Operational Flexibility: Older transmitters required a specific, custom-made crystal for each frequency. Changing a station's assigned frequency, which could happen for various operational or regulatory reasons, involved physically replacing the crystal and often re-tuning the transmitter's RF circuitry. Modern, digitally-controlled transmitters can change frequencies with the simple adjustment of a setting, offering significant operational ease.
Economic Factors: Manufacturing custom-made crystals for the relatively small number of AM broadcast stations was never as large a market as those for amateur radio or consumer electronics at the time. As the market for general-purpose, discrete crystals declined due to the rise of integrated circuits, the few remaining manufacturers scaled back or closed down (e.g., International Crystal Manufacturing ceased operations). It became more cost-effective to use versatile modern components.
Improved Stability and Performance: Modern frequency control systems offer superior frequency stability and spectral purity, meeting current regulatory requirements (such as those from the FCC) more easily than traditional crystal oscillator circuits.
Niche Market: Today, custom-made AM broadcast transmitter crystals are a niche product for hobbyists or for repairing vintage equipment. They are available from specialized, small-volume suppliers but are no longer a part of the mainstream electronics industry.
In summary, the transition to software-defined and frequency-agile transmitters made the single-frequency crystal method obsolete for commercial AM broadcasting.