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FM hiss, vintage 1973 receiver

Discussion in 'Electronic Repair' started by mc, Jan 3, 2006.

  1. gb

    gb Guest

    Karl -

    You beat me to that answer, which is correct.
    I installed a set of 2 horizontal staked loops for one college broadcast
    station (they had no desire for mobile users) and the antenna was free from
    a commercial station that upgraded to CP as pointed out by Karl).

    gb
     
  2. Smitty Two

    Smitty Two Guest

    Hey, I've got a Nikko 5055 out in the shed. Are these things considered
    respectable? I've thought about giving it away several times, but can't
    quite make myself do it.
     
  3. That's not what's happening. Stereo noise cannot be "out of phase"
    because it occurs at entirely different frequencies from mono noise.
    When the receiver detects a stereo signal (by noting the presence of the
    19 KHz pilot), the detected bandwidth must be at least 53 KHz, in order
    to be able to receive the (L-R) subcarrier. When the receiver does not
    detect the pilot, the detected bandwidth is reduced to only 15 KHz, plus
    there is de-emphasis which does not apply to the composite stereo
    signal. It's the nearly four-to-one bandwidth ratio and the lack of high
    cut (de-emphasis), that accounts for the difference in noise. With a
    "perfect" receiver, a stereo signal needs to be nearly 30 dB stronger
    (29.7, AFAIR), to produce the same signal-to-noise ratio as a mono
    signal.

    Isaac
     
  4. Peter Larsen

    Peter Larsen Guest

    FM stereo is transmitted as sum and difference, and the difference
    channel has some 10 dB less dynamic range than the sum channel. So much
    for your implied theoretical impossibilty.
    To the differnece signal rather, just a hunch, it makes sense because it
    explains the differnce in sn-ratio for sum and difference channels.
    You certainly seem to know more sbout the technicalities of this than I
    do, I will just add that FM emphasis/deemphasis standars are slightly
    different, my general understanding of these matters is however correct.

    The propoerty that the noise is identical and out pf phase between the
    channels is generally used as a means of automated noise suppression in
    case of weak signals, on some tuners it is switchable whether it occurs.

    Kind regards

    Peter Larsen
     
  5. Asimov

    Asimov Guest

    "Isaac Wingfield" bravely wrote to "All" (03 Jan 06 22:10:23)
    --- on the heady topic of "Re: FM hiss, vintage 1973 receiver"

    IW> From: Isaac Wingfield <>
    IW> Xref: core-easynews rec.audio.tech:186070
    IW> sci.electronics.repair:353881


    IW> In article <>,
    IW> That's not what's happening. Stereo noise cannot be "out of phase"
    IW> because it occurs at entirely different frequencies from mono noise.
    IW> When the receiver detects a stereo signal (by noting the presence of
    IW> the 19 KHz pilot), the detected bandwidth must be at least 53 KHz, in
    IW> order to be able to receive the (L-R) subcarrier. When the receiver
    IW> does not detect the pilot, the detected bandwidth is reduced to only
    IW> 15 KHz, plus there is de-emphasis which does not apply to the
    IW> composite stereo signal. It's the nearly four-to-one bandwidth ratio
    IW> and the lack of high cut (de-emphasis), that accounts for the
    IW> difference in noise. With a "perfect" receiver, a stereo signal needs
    IW> to be nearly 30 dB stronger (29.7, AFAIR), to produce the same
    IW> signal-to-noise ratio as a mono signal.


    The extra noise arises because the stereo difference signal (L-R) is
    on an amplitude modulated subcarrier and thus more prone to
    atmospheric noise same as with an AM radio.

    A*s*i*m*o*v

    .... Children come from God. He can't stand the noise either.
     
  6. Karl Uppiano

    Karl Uppiano Guest

    Stereo FM is transmitted as L+R (baseband) and L-R (subcarrier) but nothing
    is intrinsically "out of phase". The dynamic range isn't terribly relevant
    either, except that we tend to undermodultate the L-R channel when
    monophonic or highly correlated stereo, so noise is more noticeable. The
    encoding is a way to symmetrically encode stereo over a single broadcast
    channel.

    Pre-emphasis does indeed apply to the composite signal. The left and right
    channels are pre-emphasized, then encoded. After you decode the stereo
    channels, they are de-emphasized. The L-R audio in the subcarrier is
    pre-emphasized.
    The reason stereo is noisier is because the L-R information is shifted up
    (effectively more than doubling the audio spectrum) and then shifted back
    down, bringing the noise that is up there back down with it. It's very
    simple, really.
     
  7. Yes. I was involved in the design of the very first broadcast quality
    stereo generators that actually met all the FCC specs. They were
    designed and manufactured for RCA in the mid-to-late 1960s.

    Your comment about the 10 dB reduction in dynamic range is not correct.
    The difference channel is exactly that: the analog sum of the right
    channel and the inverted left channel. No other processing is done to
    limit the dynamics.

    I see you are posting from Denmark. To be fair, I do not know the
    technical details of stereo broadcasting in Europe; it may indeed be
    different from what is done here in the states.

    Isaac
     
  8. Bill Taylor

    Bill Taylor Guest

    Peter Larsens original statement about stereo noise being out of phase
    between L & R channels is quite correct. Noise in the S (L-R) signal
    will appear as anti-phase signals in the L and R channels after
    decoding, and will disappear if the channels are summed to mono. After
    all, that's what the S signal is.
    The noise in the S channel is also made worse by the triangular shape
    of noise in FM signals, And the S signal occupies twice the bandwidth
    as the M signal - 38KHz rather than 19KHz.

    Bill Taylor
     
  9. Peter Larsen

    Peter Larsen Guest

    I make no claim of knowing the facts of this, and I am very glad that
    you take the time to explain it.
    I am also puzzled, because my understanding was that the difference
    channel only was broadcast without preemphasis. By the rationale of
    compatibility with mono receivers your claim that M as well as S are
    broadcast without preemphasis is an impossibility.

    "S" does not mean "stereo", it means "side" as is this about Mid Side
    Stereo. I may be wrong, but I do not from your explanation understand
    myself to so be and I think your wording "there is de-emphasis which
    does not apply to the composite stereo signal" should have been "there
    is a deemphasis which does not apply to the difference signal".
    And *because* it is the difference channel it signal appears in opposite
    polarity in the left and right stereo channels after matrixing. Summing
    L and R mathematically eliminates it.
    There is a difference in time constant of emphasis/deemphasis, but I
    will leave specs to those that know them. I can not really bridge your
    detailed explanation to a simple "I am wrong because so and so" that
    fits my points and voids them, but it may be because I am listening to
    BBC televison while reading news ...

    Our cable FM is btw. atrocious, I think they recevive the RF via a
    tuner, AD converts it, bitreduces it again, DA converts and retransmit
    as FM on the local cable net - are they equally insane over your way?

    Kind regards

    Peter Larsen
     
  10. mc

    mc Guest

    Nikko STA-5010. FM stereo demodulator is a UPC554C chip.
    Yes, I think so. They weren't as widely sold as Pioneer or Marantz, but
    they had a following. Someone sold a mint-in-box Nikko 5055 on eBay
    recently.
     
  11. Barry Mann

    Barry Mann Guest

    If the gain of the RF stage or the IF strip is low, there will be some
    noise.

    A fairly common failure in older units is the trimmer capacitor(s) on
    the tuning capacitor -- especially the plastic cased trimmers.

    Other than as a hobby, I don't recommend spending much time messing
    with the unit. So many parts are at the end of their expected life
    (especially the capacitors), you could be chasing a cascade of failures
    -- or not, no one really can predict what the next failure might be.

    I can imagine an oscillation somewhere is upsetting the AGC or stereo
    decoder.

    -----------------------------------------------------------
    spam:
    wordgame:123(abc):<14 9 20 5 2 9 18 4 at 22 15 9 3 5 14 5 20 dot 3 15
    13> (Barry Mann)
    [sorry about the puzzle, spammers are ruining my mailbox]
    -----------------------------------------------------------
     
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