FM hiss, vintage 1973 receiver

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

"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> > mc posted a question compliant with usenet standard: 3416554677697809809 > section B, page 27, paragraph 8: Any initial question must omit at least > one piece of vital information, otherwise it can not be considered for > followups. >

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.

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.

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

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

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

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.

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.

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