Announcing 'hifi-am', to discuss High Fidelity AM tuners and hobbyist transmitters

Hi-Fi AM....what the hell is that????!!!....I thought it died out 10 minutes after it was announced about 5 years ago. Kim

AM has always been relatively 'hi-fi' (15 to 20kHz), but the bandwidth of most receivers is such that you receive crap. A half-way decent receiver will give really good sound, making all those red-necks on talk-back sound like they were in the same room, dammit.

Ken

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Huh?! At least in the US the AM channel is 5kHz wide, hardly enough to broadcast 15-20kHz audio. Even the FM audio channel is only 10Khz (with two for stereo). TV is the same. Where are you getting 15-20kHz????

I dunno, but I'm real happy to know the BW tain'tb so wide as to be able to smell that redneck cheap beer 'n' chile fart.

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Channel spacing is irrelevant - it's why local stations aren't allocated adjacent channels. Ditto the FM band. It's the difference between 'channel spacing' and 'bandwidth'.

Ken

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If your local AM station violates the FCC specs for bandwidth, transmitter power, and modulation, then you must live in or very near to Mexico.

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Okay, I'll put it somewhat clearer - you can have a bandwidth greater than the channel spacing if the licencing authority is smart enough to not put someone else locally on an adjacent channel. And, in fact, they are that smart and have done so since day one (maybe two, they were celebrating with a few drinks on day one).

Ken

Please don't be spreading misinformation. You can "have a bandwidth greater than the channel spacing" if the licensing authority is dumb enough to let you radiate outside your allocated channel.

And somebody said earlier that FM broadcast has a bandwidth of 10 KHz. Come on, people, some of these newbies might believe that crap!

The channel allocation for US FM broadcast is 100 KHz. I'm not sure what the audio bandwidth is, but it's a lot better than 10 KHz. It's "high-fidelity," remember those terms?

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AM channels are allocated at 10 kHz intervals. The FCC allocates locally on alternate channels, so during the day, you can usually get at least 10 kHz of usable audio bandwidth. With all the de-facto pre-emphasis that broadcasters used to overcome lousy AM receivers, the FCC standardized with NRSC. Since the NRSC rules went into effect, broadcasters are required to apply pre-emphasis and band-limit the audio to 10 kHz. Prior to that, you could go out to 15 kHz. I used to maintain an old 250 watt RCA transmitter that could modulate to 20 kHz.

FM Stereo with a pilot at 19 kHz, can have audio bandwidth of nearly 19 kHz. Anti-aliasing and reconstruction filters limit the usable bandwidth to about

TV Stereo is similar to FM, except that the pilot is tied to the horizontal sweep frequency, so the absolute maximum bandwidth is about 14 kHz.

Broadcasters use so much audio processing, you can hardly call any of them high fidelity anymore.

Unless you cross the Border.

Drive along the Canadian Border and listen to a lot of beautiful audio with noticable better clairity then US stations, I used to love to listen to Canadian jazz and classical and techno stations in the Buffalo area, it made long drives to service calls much more pleasent. Also less a lot less background hiss. Evidently they do not preprocess the audio as much to create fake "loudness" like US stations do to increase ratings and revenue. Or does the CBC just have higher standards?

Steve Roberts

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Non-commercial stations use much less processing, as a rule, because they're usually not competing for the same listeners or revenue. The NPR stations (US) often sound quite good. CBC is non-commercial, if I'm not mistaken. I'm not sure what regulatory body controls the commercial stations in Canada, and I'm not sure if they specify how much compression they can use.

The FCC used to have something in the regulations about broadcasters not "substantially" altering the dynamic range of the program material, but no one seemed to pay any attention to it, and it wasn't a testable (i.e., enforceable) constraint anyway. I haven't read the rules in more than 20 years, so I don't know if it still exists.

Indeed true!

Back in the late 1950s and early 1960s, I was chief engineer in a Trenton, NJ radio station that employed a Gates BC-5B 5,000 watt transmitter, and the facts were just as this gentleman posted them. At that time you could create a great deal of problems for yourself if you exceeded the 10-Khz channel assignment.

This is certainly true of any AM station of which I am aware. This began around 1960, when someone came up with the 'bright idea' of placing a GE 'Sta-Level' amplifier in front of an RCA limit clipper. This increased the average level of modulation to something like 98% without risking over-modulation, thus increasing the station's coverage range and advertising revenues.

The downside was that the audio quality went down the toilet, since the FCC 'Proof of Performance' requirements didn't take this factor into account. When 'Proof of Performance' testing was performed for the FCC, we would simply input an audio sinewave at a sufficiently low level that no significant compression or clipping took place! Of course, this was not true of the day to day broadcasting, where you could get a call from the station manager if he/she saw the modulation monitor dip below 98% for any significant period of time.

Naturally, the audio broadcast sounded like, and in fact was, total audio crap!

Harry C.