[IRCA] Synching-up--would attempting to revive this 70+ year-old technology make any sense?

["Dan Strassberg" <dan.strassberg@xxxxxxx>]

Seventy-odd years ago, when crystal oscillators were not nearly as stable as
they are today, when AM DAs were in their infancy, and when NARBA had not
yet come along to place two or more 50-kW AMs with protected skywave service
on approximately half of the US Class I channels, an interesting AM
broadcast technology--synchronized carriers--was born. It survived for
nearly a decade and apparently died on 3/31/1941 with the advent of NARBA,
which mandated the use of approximately half of the US Class I channels by
two (and occasionally three) powerful stations per channel. These stations,
designated Class IB, were granted exclusive use of their channels within
their 0.5 mV/m 50% skywave contours and used directional antennas to prevent
interference within the co-channel IB station's protected nighttime-skywave
service area.

Prior to NARBA, a large number of what became Class IB stations used--or
attempted to use--carrier synchronization to minimize co-channel
interference. I presume (but don't know) that the implementation was somehow
different from that of synchronized operation of stations in the same
general area that simulcast the same program. Examples of synchronized
simulcasts on nearby transmitters were WBZ Boston/WBZA Springfield MA (on
990 pre-NARBA) and WLLH Lowell and Lawrence MA (on 1370 pre-NARBA). The WLLH
synchronized simulcast continues to this day and, of several dozen such
simulcasts currently on the air in the US, is to my knowlege the only one
that the FCC does not classify as "experimental."

Examples of non-simulcast synchronized operations during the 1930s included
WBAL Baltimore/WTIC Hartford (probably on 1040 or maybe 1050 pre-NARBA) and
KEX Portland OR/KOB Albuquerque (maybe on 1140 or 1150 pre-NARBA). I know
that the synchronization of the nearby transmitters was sometimes
accomplished by sending the "master" carrier by wire to the "slave" station.
That is, at one point, WBZ used coax to send its carrier the roughly 100
miles to Springfield. This arrangement might later have been replaced by the
use of frequency dividers to produce an audio-frequency signal that could be
sent over commercial voice-grade lines to Springfield, where the the signal
was used to drive chain of frequency multipliers. From what I can tell, the
frequency divider/multiplier scheme must have been problematic and very
likely never worked satisfactorily but for stations widely separated
geographically (KEX and KOB, for example) may have been the only scheme that
could be implemented because long runs of coax must have presented many
technical and legal challenges.

Today, however, such a scheme, if it had any value, might be implemented
with the aid of geosynchronous satellites instead of any sort of wired
connection between stations. As I see it, the potential value,
if any, would be limited to a few AM stations, on whose channels nearly all
co-channel interference comes from one dominant station. Here in the Boston
area, I can think of two examples, WBIX 1060, whose only significant
co-channel interference comes from KYW and WAMG 890, whose only significant
co-channel interference comes from WLS. WBNW 1120 has in the past received
more interference from WPRX Bristol CT than from KMOX. The satellite-based
scheme should not be extraordinarily expensive to implement. My question is
whether it would have any value. Would elimination of the sub-audio
heterodyne between the carriers improve the listenability of the signal from
stations subject to co-channel interference from a single dominant station?
Your thoughts please.

--
Dan Strassberg, dan.strassberg@xxxxxxx
eFax 707-215-6367







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