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

Discussion in 'General Electronics' started by Jon Noring, Jul 8, 2004.

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  1. Jon Noring

    Jon Noring Guest


    I've created a discussion group (mailing list) to discuss high
    fidelity AM tuners and hobbyist transmitters. Appended below is the
    more detailed group description.

    To subscribe to this group, either go to the group home page (at
    Yahoo) and subscribe there (you will need a YahooID):

    Or, if you don't have a YahooID, send a blank email to:

    Looking forward to seeing you there!

    Jon Noring


    Group Description for hifi-am:

    The purpose of hifi-am is to explore the possibilities of high
    fidelity AM tuners and hobbyist transmitters for medium-wave (or
    better known as the broadcast band, appr. 520 to 1720 khz.)

    Although most commercial AM broadcasts are definitely not high
    fidelity (due to various factors including, in many countries, audio
    bandwidth restrictions), there are nevertheless a few commercial
    stations which broadcast with audio quality approaching high fidelity.
    For example, in Australia, because the number of stations is fewer and
    the country quite large, many stations broadcast with quite wide audio
    bandwidth, approaching 15 khz.

    There is steady growth in interest (in the U.S.) for non-licensed
    hobbyist broadcasting in the BCB (per FCC "Part 15"), which is
    restricted to 100 milliwatts and a three meter antenna (among a few
    other minor restrictions). With a high efficiency antenna, it is
    possible for such neighborhood broadcasting to be heard about 1/2 to 1
    mile from the transmitter. There are quite a few progressive and
    alternative stations broadcasting this way. For a wonderful example,
    see Radio KMTZ.

    More importantly, it appears there is no specific restriction on audio
    bandwidth for Part 15 transmissions. Thus, if this is true, the
    broadcasts can approach high fidelity (15 to 20 khz audio bandwidth.)

    Although discussion can focus on the general topic, it is hoped this
    group will catalyze the development of hobbyist kits for high fidelity
    AM tuners (digital, solid state, and tube) and for high fidelity
    low-power (Part 15) AM transmitters. It should be fun to consider the
    many possibilities.
  2. Neil

    Neil Guest

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

    Ken Taylor Guest

    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.

  4. Keith

    Keith Guest

    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????
  5. Activ8

    Activ8 Guest

    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.
  6. Ken Taylor

    Ken Taylor Guest

    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'.

  7. 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.
  8. Ken Taylor

    Ken Taylor Guest

    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).

  9. Rich Grise

    Rich Grise Guest

    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?
  10. Karl Uppiano

    Karl Uppiano Guest

    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
    16 kHz.

    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.
  11. FM Stereo with a pilot at 19 kHz, can have audio bandwidth of nearly 19 kHz.
    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

    Steve Roberts
  12. Karl Uppiano

    Karl Uppiano Guest

    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.
  13. 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.
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