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Old 29th May 2006, 05:16 PM   #1
2litre is offline 2litre  United States
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Default Frequency response

Hi All,

Not sure if this is the right forum but,...

I was looking at car radios at my local parts store to see if anyone still made the old style 2 shaft radio cassette decks to fit my '72 bug (want to upgrade my old high zoot Kraco) and I noticed something about the AM and FM band frequency responses. (I guess that's the term)

I forget the absolute stated values but, I noticed that the FM band covered almost the full audio spectrum, something like 30Hz to 18KHz but the AM band only covered something like 100Hz to 12KHz.

Regardless of the brand, (the super cheapies had even worse numbers in both bands) is this narrowed frequency response normal for the AM band? If so, what would one consider good span numbers for the AM band?

R/

Jim
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Old 29th May 2006, 10:13 PM   #2
2litre is offline 2litre  United States
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Answered my own question I guess.

en.wikipedia.org -
Because of its susceptibility to atmospheric interference and generally lower-fidelity sound, AM broadcasting is better suited to talk radio and news programming, while music radio and public radio mostly shifted to FM broadcasting in the late 1960s and 1970s. Frequency response is typically 40 Hz–7 kHz with a 50 dB S/N ratio

No wonder all the music on AM sounds like it comes from an old console radio.

R/

Jim
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Old 30th May 2006, 01:23 AM   #3
lndm is offline lndm  Australia
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The low radio frequencies used for AM broadcast mean there is not much room when you slice it up, and you trade off bandwidth versus available station allocations. The AM broadcast band wasn't designed to be inferior however, at the time, 9kHz or so was considered plenty.
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Old 30th May 2006, 07:21 PM   #4
2litre is offline 2litre  United States
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Thanks for the reply,

Just have to consider AM's age. Kind of explains how come most of the radios of the era had full range drivers and small cone 'tweeters'. Full range at that time could have a 10K top end and be great. Now those are good wide band drivers we pay dearly for.

Unfortunate that this station I always listen to doesn't have an FM side. Much of the old stuff would sound real nice on FM, another 9K on top and 20Hz down low.

R/

Jim
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Old 30th May 2006, 08:18 PM   #5
EC8010 is offline EC8010  United Kingdom
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It's a long time since I looked at this, but broadcasting frequencies are allocated by worldwide agreement. As I recall, the last WARC (World Association of Radio Communications?) deemed that MW frequencies had to be multiples of 9kHz. The upshot of this (and this is why I remember about the WARC) is that our local radio station (BBC Radio Solent) had to change its frequency from an engineeringly useful 1MHz (use it as a frequency standard) to 999kHz. Now to the point; 9kHz between AM stations means 4.5kHz deviation. In other words, your upper audio frequency is 4.5kHz. To maintain balance, divide that number into 500,000 to give a low frequency limit of 111Hz. MW is slightly better than a telephone line (300Hz-3.4kHz).
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Old 31st May 2006, 07:00 PM   #6
2litre is offline 2litre  United States
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Cool, thanks.

Wrote the radio station and asked if they currently did, or had they ever thought of simulcasting on the FM band.

I got a rather sloppy answer back. Anyway, they do an HD broadcast off the sideband of an FM station. Oh well. I'm not rushing right out to get an HD tuner anytime soon.

R/

Jim
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