Let's keep in mind that having audio frequency components above 3 kHz
doesn't necessarily require us to have a transmitted bandwidth in excess
of 3 KHz. That's why we have passband tuning, VBT, and the ability to
set the low frequency and high frequency corners on our rigs.
Much of the discussion thus far about intelligibility vs. upper
frequency limit *in the HF ham bands* has been anecdotal. Two rigs with
identical transmitted passbands may sound quite different if one is used
with the stock OEM microphone and the other with a specially tailored
mic response, such as that provided by the Heil elements.
I used to work with an engineer who had spent much time in the audio
products department of a major consumer products company. He claimed
their design practice for audio stages for inexpensive clock radios,
portables, and the like was to maximize the apparent fidelity of cheap
units by making the mathematical product of the high frequency and low
frequency corners of the designed-in audio response curve equal to
400,000 (units of Hz squared, I guess). This, by the way, is what you
get when you multiply the traditional hi-fi limits of 20 Hz and 20 kHz.
Thus, if your receiver audio was limited to a maximum of 3300 Hz, for
instance, they felt the low frequency roll-off should be around 120 Hz
for best clarity.
The psychoacoustics of intelligibility on the ham bands seems to be a
fairly complex issue to me. I know, for instance, that I used to tune a
very crowded 40M or 75M phone band in a contest by using the 800 Hz
mechanical filter in my Collins 75A-4; judicious use of the Passband
Tuning knob allowed me to position the filter passband exactly where
*my* ears needed it for maximum intelligibility. Was it hi-fi? No.
Was it toll-circuit audio? No. Was it the right bandwidth (and the
right *positioning* of that bandwidth) for maximizing my ability to copy
individual stations on an unchannelized crowded band? Yes.
Bud, W2RU
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