[BC] Wow, I wonder if you feel the same way about AM?

[Robert Orban]

At 05:35 PM 2/2/2010, Broadcast List USER wrote:
>I forgot to mention, yes I know.
>
>However, it was brought on by the audio processing at the stations.
>
>The stations kept turning up the high end, causing more interference,
>and sibilance.  The receiver manufacturers, especially car radio
>manufactures kept rolling off the high end in response.

That's not the way I remember it. The Delco radio in an early 1970's 
Buick Skyhawk that I owned was one of the narrowest AM OEM adios I 
can recall. But in 1972, there was very little in the way on 
pumped-up high end on AM transmissions. Most stations were running 
Audimax/Volumax or DAP 310s. However, no one in the US was running an 
audio lowpass filter, so stations transmitted to 15 kHz or beyond and 
were stepping on 2nd adjacents for no good reason. Meanwhile, these 
GM radios, with poorer than toll-grade telephone response, rolled off 
highs so much that they compromised speech intelligibility if 
broadcasts were "flat." (I recall Ford and Chrysler radios of the 
time as having considerably wider audio bandwidth than the GM radios.)

When I worked on the Optimod 9000 (which was introduced in 1978), I 
added HF EQ in response to radios like this. In addition, I put in an 
11 kHz lowpass filter to help mitigate second-adjacent interference. 
CRL, which also offered processing much more aggressive than the Max 
Bros. or the DAP, first came into national prominence about the same 
time. But the narrowband receiver cart definitely came before the 
boosted-highs horse.

>Auto manufacturers don't want cars coming back for radio complaints.
>So, they kept rolling off the highs to ensure that wouldn't happen.
>That made AM stations pump up the high end even more.
>
>No offense intended to Bob, who I greatly respect, but do you recall
>the "car radio emulating filter" that came with the 9100?  WOW!

This was the "monitor rolloff filter," which I believe we still sell 
as an optional accessory to Optimod-AMs. It was based on the average 
rolloff of about 20 radios, which I measured in the early 1980s. 
Audio response was -3 dB at 2 kHz and the curve shape approximated a 
third-order Bessel filter. The NRSC measurements of AM radios done 
around 2005 indicated that nothing much changed in 25 years.

Bob Orban