[BC] AM HD power levels

[Phil Alexander]

Bob,

There was a time when the only audio "processor" was  
the hand of the control room operator on a pot. <g>

In those days there were far less high energy, high 
frequency modulation instances. In fact, I suspect 
any peak hold audio spectrograph would have been 
classically triangular. 

I started in radio at an FM that had **NO** audio 
processing equipment except for a 100 W yellow bug lamp 
on top of the console that was slaved to the modulation 
monitor's overmod relay.

Later, when I worked in master control at WBAA (long 
before the invention of NPR) the operator on duty was 
the only audio processor although we did have a peak 
limiter at the Tx for preventing negative overmodulation. 
The program was primarily serious/classical music in the 
evenings when I worked, and we were required to ride gain 
to maintain approximately 600 peaks per hour over 90%. 
All monitoring was done off air. The station did transmit 
30 Hz to 15 kHz on AM. Switching from OTA to program 
monitor was audibly imperceptible to most although I could
detect a slight difference. (When I was a kid, I could 
hear the difference because my ears had good response out 
to about 19.7 kHz and I could not stay in a room that had 
a TV with a noisy flyback transformer.) BTW, at WBAA, 
failure to monitor OTA and failure to average 600 peaks 
per hour were both cause for dismissal. This should give 
you an idea of the radio I was taught during my formative 
years.

Many of the pre-1960 radios in serious high fidelity 
systems had excellent frequency response, and perhaps 
a noise limiter that could be switched in and out for
AM. The practical effect was a system that corresponded
with the FCC audio proof of performance requirements.

I suppose it is fair to say that technically AM radio was 
not a system because the FCC did not **control** the 
receiver technical specification, but, practically speaking, 
anyone who wanted to receive the full audio spectrum of 
a station could buy a good tuner and plug it into their 
hi-fi. The point is that competent receivers were readily 
available. A couple of the names I recall were Sherwood and 
Fisher. As you mention, the H.H. Scott stuff was pretty 
good too. 

The early stereo exciters lacked the mono audio quality
of earlier mono only exciters. As examples, the switch
from Phasitron to direct FM in the Collins left something
to be desired, and the addition of stereo with what was
essentially an SCA generator in the Gates was a joke. My
record for getting one of the tube type factory refurbished 
exciters to hold separation (IIRC either 27 or 29 dB) was 
7 minutes after complete tune up on the bench in Quincy. Of 
course it did hold main channel frequency somewhat better 
than its successor, the infamous TE-1.

---------------------------------------------
Phil Alexander, CSRE, AMD
Broadcast Engineering Services and Technology 
(a Div. of Advanced Parts Corporation) 
Ph. (317) 335-2065   FAX (317) 335-9037




On 30 Aug 2008 at 14:27, Robert Orban wrote:

> If I recall correctly, most pre-1960 radios did not have significant 
> audio response up to 15 kHz, which would have been the case if the 
> radios and transmissions were indeed a "system" at that time. 
> Meanwhile, the 10-15 kHz audio sidebands splattered into 
> second-adjacents at night.
> 
> I'm not sure what you're talking about regarding early FM stereo's 
> being technically very poor. By 1963, Collins offered a competent 
> switching-mode stereo generator and direct-FM exciter (with which I 
> worked at my college radio station) and H.H. Scott had already 
> developed and marketed a good switching stereo decoder (I owned a 
> Scott tuner). Moreover, the FCC's separation specs (which exceeded 
> the performance of all stereo phono pickups at the time) were more 
> than adequate to preserve a technically perfect stereo image, which 
> only requires about 20 dB of separation according to peer-reviewed 
> papers from the AES Journal.