Re: [ARSCLIST] FM reception way back when

["Charles A. Richardson" <charlesarichardson@xxxxxxxxxxx>]

Hi George;  A little history.  There were two competing stereo  
systems, Crosby and GE Zenith.  Crosby was much better than the GE  
Zenith with stereo specs almost as good as mono FM.  But Crosby was  
not as good in mono compatibility and also it may not have allowed for  
SCA service, then a money revenue for the stations.  Lots of mono, and  
no stereo receivers at the time.  GE Zenith also had big money and  
political connections so it won out over the technically better Crosby  
stereo system.  GE Zenith did have the pilot tone issues you mention,  
along with more multipath distortions   There had to be an audio  
bandwidth filter used to cut the audio off above 16K HZ to protect the  
19 K Hz pilot.  Mono FM had wider bandwidth up to 20 KHZ, and a  
simpler signal with no audio phase summing issues either acoustic or  
the L + R and L - R stuff to mess it up.  Also the 75 micro second  
audio emphasis curve in broadcast and 75 de-emphasis curve in  
receivers helped  signal to noise a lot too.  But when there were hot  
high level high frequencies, over modulation was a concern.  So we now  
have a FM stereo degraded signal to noise situation due to the GE  
Zenith compromise with the old FM mono receivers calling the technical  
tune. Likewise when TV transitioned  from Black and White to NTSC  
Color.   Black and White, when done with a live broadcast of a wide  
bandwidth single tube camera, seen on a wide bandwidth B & W monitor  
or off air monitor was stunning in its detail.  When compromised NTSC  
color came in we got dot crawl,  scan line crosstalk, reduced  
bandwidth, gray scale and color problems in dealing with three color  
signals when seen on both black and white as well as standard color  
TVs.   When TV audio went from mono to stereo, it too was degraded in  
signal to noise, but saved somewhat by the CBS CX stereo audio noise  
reduction system, that was also used on 12 inch Laserdiscs.

Back to FM.   Stereo FM signal to noise was not very good because of  
the issues mentioned.  In the early 1970s Dolby came on the scene with  
their noise reduction systems. These were used in studio audio,  
consumer B formats, and Dolby film improvements.  Dolby also wanted to  
improve FM stereo and came out with their model 334 Dolby B FM encoder  
to clean up FM stereo noise issues.  I was involved in getting the  
first NPR FM station converted over to Dolby B FM.  When everything  
was done right it worked very well.  We measured about 94 DB SN in  
Stereo and 104 DB SN in mono FM.  Rather impressive improved  
performance, which could be made even better if one later switched  
over to the newer and better Dolby C and Dolby S systems, that would  
have made the SN about 10 DB better.  Dolby FM failed in the  
marketplace and the Dolby  FM 334 system had to be taken out.  Why the  
failure?  On the transmission side, there was more audible noise found  
in the Modulator, STL Link, Audio Console, and music program sources  
that compromised Dolby FM performance as the Dolby would show up their  
noise shortcomings.   Many stations would not change their other  
equipment or level practices, but had to engage in the loudness wars  
to keep listeners.  The Dolby FM Encoder also changed the transmitter  
pre-emphasis from 75 microseconds down to 25  microseconds which  
technically was a good improvement to allow more high level high  
frequency signals without over modulation violations.  But  FM  
receivers also needed a switch to change their de-emphasis back and  
forth from 75 microseconds for non Dolby stations to 25 microseconds  
for Dolby Stations.  One also had to line up signal levels to Dolby  
standards and have a Dolby B decoder at the audio line output of the  
tuner or the receiver.  All this was all just too much for the general  
public.  Listeners did not usually hear programs of master tapes of  
wide range classical music. Their taste was for other music types with  
little dynamic range.  Most did not have wide range hi-fi stereo  
systems either, but instead listened on car radios and table top  
radios.  Dolby FM sounded worse to them with its lower modulation  
levels and changed pre-emphasis.  It sounded duller to them as a  
result. There were no radio or tuners that would automatically and  
simultaneously do three things.  1. Change the De-emphasis between 75  
and 25 microseconds.  2. Turn the Dolby B decoder on and off,  and 3.  
align the signal levels for proper tracking of the tuner's Dolby  
decoder to the Dolby FM station.  When tuning to a non dolby station,  
the above had to revert to the old standard FM settings.    Many  
listeners also thought loud was good and powerful.  Some did have  
reception problems that chronic high modulation levels and various  
processing helped solve.  Thus Dolby B FM stations lost listeners with  
Dolby B FM, so they pulled the plug on it. Accordingly, it is hard to  
actually improve FM on air broadcasting unless one can address and fix  
all issues at the same time in a systematic way. The marketplace, bad  
technical compromises, government, and consumers were just not willing  
to do what it took to make things better for the long term good of FM  
Broadcasting in a comprehensive rational orderly way.    A familiar  
story.    Anyone want to go back to Crosby stereo or Dolby B or Dolby  
S FM today?   With new low noise  quiet analog and digital radio  
station program sources it would sound fantastic, but no one will  
fight all the battles to do the job right as it must be done.  Adding  
Surround Sound would cause other headaches and problems.  So over the  
air FM is, by its
market place, standards, technicalities, and FCC rules, limited and  
stuck where it is as to its sonic performance.  Satellite, cable,  
fiber, internet, etc get around FM broadcast's old obsolete standards  
and limitations, so they largely take the market.

Hope this helps you.    Charlie Richardson


On Oct 7, 2008, at 2:03 AM, George Brock-Nannestad wrote:

> From: Patent Tactics, George Brock-Nannestad
>
> Hi Folks,  Bob Cham wrote:
>
> ............
> > The problem lies with the multiplex system that we use.  When FM went
> > STEREO, the FCC didn't want to obsolete all the mono receivers, as
> > they had when the band changed frequency.  Thus we broadcast L+R
> > (mono), and L-R, which are combined to make stereo.  It doesn't
> > exactly make for a robust stereo signal, but it is compatible.
>
> .......................
>
> Isn't the problem that in stereo, the pilot tone (multiplied by two)  
> is AM
> modulated by the L-R and not FM at all? This was the system that won  
> out.
>
> Kind regards,
>
>
> George