FM hiss, vintage 1973 receiver

[gb]

Almost all commercial FM stations in the US are circularly polarized (1/2
power vertical; 1/2 power horizontal, 90 degrees out of phase). That
covers both types of antennas, and if you have a CP receiving antenna, you
can dramatically reduce multipath if it is oriented toward the
transmitter.

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

 

[Smitty Two]

Nikko STA-5010. FM stereo demodulator is a UPC554C chip.

Circuit diagrams somewhere on www.covingtoninnovations.com/audio.

Thanks!

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.

 

[Isaac Wingfield]

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.


Noise in Stereo FM is out of phase between the channels, and thus
disappears when the signalchannels are added to mono.

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

 

[Peter Larsen]

That's not what's happening. Stereo noise cannot be "out of phase"

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.

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.

To the differnece signal rather, just a hunch, it makes sense because it
explains the differnce in sn-ratio for sum and difference channels.

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.

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.

Isaac

Kind regards

Peter Larsen

 

[Asimov]

"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 <[email protected]>
IW> Xref: core-easynews rec.audio.tech:186070
IW> sci.electronics.repair:353881


IW> In article <[email protected]>,

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.


Noise in Stereo FM is out of phase between the channels, and thus
disappears when the signalchannels are added to mono.

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.

 

[Karl Uppiano]

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.

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.

To the differnece signal rather, just a hunch, it makes sense because it
explains the differnce in sn-ratio for sum and difference channels.

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.

 

[Isaac Wingfield]

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,

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

 

[Bill Taylor]

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.

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.

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.

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

 

[Peter Larsen]

Your comment about the 10 dB reduction in dynamic range is
not correct.

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".

The difference channel is exactly that: the analog sum
of the right channel and the inverted left channel.

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.

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.

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?

Isaac

Kind regards

Peter Larsen

 

[mc]

Nikko STA-5010. FM stereo demodulator is a UPC554C chip.

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.

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.

 

[Barry Mann]

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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