01-04-2014, 11:03 PM
Ah.. I may have an explanation for this behavior. When these radios were made, broadcasting standards were far different than they are now. Mostly, modulation peaks were limited to 100% on both positive and negative excursions. Also, program material was not processed at either the recording studio nor the radio studio in order to maintain a high average level (volume wars). Radios could be designed with a wider range of detectors and get relatively low distortion.
On the other hand, modern broadcasting is different. Regulations now allow positive excursions of 125% while keeping negative excursions to 100%. Compression/expansion allows stations to keep the average level close to 100% for the majority of the time (some stations process so much that their dynamic range is reduced to around 6dB.)
With these combined differences, the radio may process the signal outside of the linear part of the curve of the detector. This would cause distortion which on a scope would look like flat topping of the detected waveform. This will show up more on music than on speech, with some kinds of music (higher bass or sharp peaks) causing worse distortion than others.
On the other hand, modern broadcasting is different. Regulations now allow positive excursions of 125% while keeping negative excursions to 100%. Compression/expansion allows stations to keep the average level close to 100% for the majority of the time (some stations process so much that their dynamic range is reduced to around 6dB.)
With these combined differences, the radio may process the signal outside of the linear part of the curve of the detector. This would cause distortion which on a scope would look like flat topping of the detected waveform. This will show up more on music than on speech, with some kinds of music (higher bass or sharp peaks) causing worse distortion than others.
