DavidEduardo said:
rbrucecarter5 said:
AM music sounds really good on all recently designed radios. By recently designed, I mean within the past 15 to 20 years. Another piece of iBiquity misinformation / outright lies was their contention that AM radios are limited to 3 to 4 kHz response.
Actually, the data came from an NRSC workgroup headed by Bob Orban. The group purchased a variety of current production model radios and measured the response; it was determined that the best AM bandwidth was around 6 kHz (the bandwidth many HD AM stations have adjusted for today).
The study also concluded that the receivers, as a group, rolled off severely over 4 kHz.
I don't see how you could even buy AM radios with the old "all American 6" transistor design with 3 IF coils. Unless you go to thrift shops and garage sales or something. I needed a replacement IF transformer a year ago, and tried in vain to find one. After buying three or four bargain radios - all, as in ALL were Sanyo or some other IC based, one ceramic filter AM, very wide audio response. I am sorry to have to say this, but I think Bob Orban purposely skewed the results by carefully researching which few of the existing production radios used the 45 year old reference design - probably to support a result greatly desired by iBiquity. It would not be first time a respected engineer followed the money or was intimidated into submission. I would say a 35 year old reference design, but by 35 years ago, ceramic filters were already making inroads and few radios had more than two IF cans. Most had two IF cans, making wider response than the old three can approach at the expense of a few images on the radio, forcing even more radios to include the IF can with the ceramic element inside. Radio Shack was always a bit ahead of the curve, they downsized the IF can count in the late 60's, two was standard in their gear into the mid 80's when they, too, started using ceramic filters. GE and Sony were the last holdouts on the traditional IF can designs, but even they succumbed to the pressure - with Sony using their proprietary CX1129 chip and GE getting out of the market.
Current production Sony based on their IC with 50 kHz AM IF are about the only narrow IF response AM radios on the market. Everything else has Sanyo or some derivation, one ceramic filter (if you are lucky), tiny ferrite bar or ridiculously small loop. Silicon Labs has chips that digitally sample the AM band, and have reasonable audio response. They are a little expensive, though, and not that common. Even digitally tuned radios use a two IC design, one to generate tuning voltage for varactors, the other the same old radio on a chip as everybody else. iHome docking stations are a good example. If by some miracle they made the IF narrower, the radios would be impossible to align because a 3 to 4 kHz mistake in the tuning somewhere on the band would mean they couldn't tune all stations. So they HAVE to be wide band to accommodate the sloppy tuning voltage generator IC. Very high end AM radio sections may add one IF can in addition to the ceramic filter, and it makes a huge difference to the noise floor of the receiver. But they are still way wider than 6 kHz audio response. If the radios are narrow audio response, it is done with high and low pass filters in the audio section to equalize the radio for talk and sports - not the IF where it would help clean up IBOC artifacts. So - yes - where did Bob find the antiques???? Sure was in iBiquity's best interest. I'd sure like to see his list of radios, because they would be a gallery of antiques in all likelihood.
Don't believe me? Go buy an "all American 6" transistor based design with three IF cans, or even an IC based radio with three IF cans and tell me the manufacturer and model. Good luck with that!