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Re: [ARSCLIST] FM reception way back when

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,


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