[BC] Mono Expands on the FM Band

[Robert Orban]

At 07:58 AM 1/17/2006, you wrote:
>From: Mark Humphrey <mark3xy at gmail.com>
>Subject: Re: [BC] Mono Expands on the FM Band
>To: "Broadcasters' Mailing List" <broadcast at radiolists.net>
>Message-ID:
>         <74b029b80601170731n7375f9caj35f72890085a3d98 at mail.gmail.com>
>Content-Type: text/plain; charset=ISO-8859-1
>
>There was another drawback to Crosby's proposal, though.  He proposed
>relatively high injection of a wide FM subcarrier centered at 45 kHz to
>carry the L-R signal.  This would have required the L+R main channel to be
>backed off about 6 dB, so the FCC considered the system somewhat
>incompatible with existing mono receivers.    The approved GE/Zenith system
>only requires about 1 dB main channel penalty to accomodate the pilot tone.
>
>In his patent application, Crosby mentions that the L-R signal might only
>acheive a 40 dB signal/noise ratio, but it probably wouldn't have degraded
>as much as GE/Zenith's in fringe coverage areas.
>
>Check the uspto.gov database for his patent, it's number 2,851,532.

Mono loudness loss was one problem for the Crosby system. This was the 
direct result of a subcarrier that sucked up a constant amount of baseband 
modulation regardless of the L-R level. It was at its least efficient with 
center-channel (pure L+R) audio because the subcarrier just sat there, 
doing nothing but taking up modulation.

The other problem was that there was an intrinsic amount of nonlinear 
distortion built into the system because FM subcarriers have 
(theoretically) infinite bandwidth, and some of that has to be truncated to 
fit the subcarrier in the allotted baseband spectrum.

Bob Orban