Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ukma!uflorida!haven!uvaarpa!mcnc!unccvax!dya
From: dya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC)
Newsgroups: sci.electronics
Subject: Re: Stereo vs. Mono: Q about AM Stereo
Keywords: AM,radio,stereo,mono,FCC
Message-ID: <1480@unccvax.UUCP>
Date: 3 May 89 13:02:48 GMT
References: <4046@bgsuvax.UUCP> <1968@hp-sdd.hp.com>
Organization: Univ. of NC at Charlotte, Charlotte, NC
Lines: 94

In article <1968@hp-sdd.hp.com>, robert@hp-sdd.hp.com (Robert Navarro) writes:
>    I'm not laughing at the person who asked for proof that stereo is better
> than mono because in one important way it's NOT. The signal to noise
> ratio for Stereo is lower than mono. This means the signal sounds more
> noisy for stereo. To see this just tune your FM radio to a station that
> barely comes in in stereo and then turn the stereo off. (This is why

         This is only true for multiplexed stereo FM.  However, if you listen
to stations which serve your community of license, the FM noise penalty is
approximately 3 dB for stereo (because the total permissible deviation is
reduced by half to accomodate the subcarrier).  Multipath can exacerbate
the problem somewhat; but within the primary service area (60 dBuV) a properly
functioning stereo receiver should have a 3 dB noise impairment over mono.
> 
>    Furthermore, AM is badly bandlimited. The baseband signal for AM is only
> 5KHZ! Most music requires at least twice this for decent sound 
> reproduction. To include stereo into AM would require more bandwidth.
> And, unless you use single side band (which usually adds some noise 
> in practice) the 5Khz for the baseband signal now must be divided
> up into two channels for stereo. Can you imagine what a 2.5Khz bandlimited
> signal would sound like!!!!

	No, no, no, no, no, no, no.

	The baseband signal is **not** limited to 5 kHz.  Where do people
get this stuff (and we AM broadcasters wonder why the band is deteriorating).
There are AM **receivers** which are bandlimited to 5 kHz; but the fidelity
limit (twice the bandwidth, by your own emissions) generally is unlimited
within the service area.

	Nice AM broadcasters have installed NRSC-1, the national voluntary
standard which mandates a 10 kHz baseband signal with 75 uS preempasis.

	BTW, even crummy old C-QUAM doesn't cut the baseband audio in 
half...the FDMA analogy isn't true.  The Kahn ISB system **does not**
suffer from the problems of multipath distortion; the mono and stereo
service areas are **identical**.

> So, while stereo AM may sound more like an original stereo music 
> recording, it will also sound noticably noisier than mono 
> no matter what AM stereo standard is picked. 

	Do you own an AM stereo receiver? Do you live in a market 
which has radio stations using the Kahn ISB system? Do you own an
NRSC-1 compliant receiver (with 10 kc bandwidth and deemphasis)?

	If you had owned appropriate AM receivers, you would know
why the Kahn ISB system is not noisier in stereo.  The recovered
signal to noise ratio of each sideband is exactly the same as 
both sidebands summed together...(think about it).  If that
doesn't work, consider the fact that under zero signal conditions,
the output of the "phase" discriminator is zero, and the
output of an "envelope" detector is whatever it was in mono.
> 
> By the way, FM is better not because it is in the higher frequency
> ranges, but because FM is inherently a less noisy modulation
> scheme for radio frequencies than AM. FM does take up more bandwidth
> than AM but at the higher frequencies that bandwidth is 
> availiable. So FM trades off greater bandwidth for less noise. 
> Because of this inherent superiority, AM really doesn't stand a chance
> in terms of high quality (noiseless) reception.
> 
	Say **what**?
        FM is not inherently a less noisy modulation scheme than
AM. What do you think the FM stereo subcarrier is...suppressed
carrier, double sideband AM!

	I'll concede that AM has all kinds of garbage in it whenever
there is a thunderstorm, or your next door neighbour cranks up their
Electrolux.  Sometimes.  It's not AM's fault, though it is a uniquely
AM problem.  An AM signal at 108 mHz with the appropriate carrier
to noise ratio would be just as clean as a wasteful :-) FM channel
occupying nominally 200 kHz.

	FM radio stations in many major markets have **severe** problems
which are created by receivers and by gross short spacing.  When FM
was envisioned, no one ever believed that every class "C" FM would
put up 100 kw and 2000 feet facilties.  There are problems with
three-signal intermodulation (ever been to Boston? Norfolk? Minneapolis?
Charlotte?) which render 70 dBuV signals (city grade) every bit as
trashy as AM stations.  Where there is gross short spacing, such
as exists in central North Carolina, service areas are dramatically
cut, and program interruption from distant station capture is a
common problem.  While thunderstorms go away, a grossly short spaced
FM is usually forever.

	AM has taken the (well deserved) rap entirely too long
because of the available receivers.  It is not, however, as bad
as suggested.  FM stereo coverage problems, short spacing, intermod,
and so on are problems unique to FM, they do interrupt service and
spoil reception more or less permanently.

York David Anthony
DataSpan, Inc