Tuesday, September 29, 2009

Audio Frequency Response, From Dave M.

Hello gents,

You had me scratching my head on AM transmitter frequency response questions, and I finally found some corroboration of my 30++ year old memory about KAAY.

But first, some more trivia - KAAY's audio chain had a 75Hz high pass audio filter that intentionally limited low frequency response (60Hz power supply hum). Felix didn't like hum and he said it just ran up the power bill. During audio proof of performance, we disabled the filter to test frequency response at 50Hz, which I seem to recall was the lower A.F. limit.

On the upper end we tested to 7500Hz for the formal report, and we also made readings at 10kHz as I recall but I don't remember if 10kHz was part of the formal report [it may have been and I just forgot this detail]. Bottom line is/was KAAY always used the full 10kHz bandwidth allowed at the time. The old RCA-BTA50F would do it without complaining. 10kHz @ 100% modulation, that is. A 50-kW dog whistle!! Heh.

Measurements were made at 25%, 50%, and 100% modulation at several frequencies (I don't remember for certain, but seem to remember 50Hz, 1kHz, 5kHz, 7.5kHz, and 10kHz). We used a precision attenuator at the microphone input of the main studio, and measured the frequency response and THD distortion at the audio output of the station's modulation monitor at the transmitter site. We could resolve frequency response to within 1/10 dB. We also measured total system S/N end to end.

Since then the FCC rules relating to AM frequency have been modified. This is not surprising since early measurements were intended to monitor the performance of "tube" amplifiers, whereas now almost everything in the audio chain now is solid state of some sort, except for, perhaps the transmitter final P.A.

Here's the snippet of information I found.

"The limitation on AM fidelity comes from current receiver design. Moreover, to fit more transmitters on the AM broadcast band, in the United States maximum transmitted audio bandwidth is limited to 10.2 kHz by an NRSC standard adopted by the FCC in June of 1989, resulting in a channel occupied bandwidth of 20.4 kHz. The former audio limitation was 15 kHz resulting in a channel occupied bandwidth of 30 kHz. Source: http://en.wikipedia.org/wiki/AM_broadcasting "

Here's a link of a fellow HAM who might be able to shed some light on this mysterious subject:


Regards all,


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