Jim wrote: So when you use a communications mic, like a Shure 444 or the wildly overpriced Heil mics, you're simply compensating for that standard that was wrong in the first place! All of these mics have some variation of that peaked response in the 2-4 kHz region.
---------------------- As far as the audio bandpass range is concerned, Ham practice followed commercial practice. The use of an upper frequency of about 2.5 kHz for communications goes way back to WWII at least when AM transmitters were designed with a bandwidth of 5 KHz (2.5 KHz per sideband). In the 1960's we carefully rolled off telephone bandwidths for conventional lines at 2.5 KHz. I haven't worked in the telephone industry since, but I believe that's still the upper limit due to the use of frequencies right above 2.5 KHz for signaling. (Wideband lines were used for broadcasters and other services of course, and AM broadcasters were allowed exactly twice the bandwidth - 10 KHz - for music.) Yes, I'm familiar with the standard "communications" mics. A classic example is the D-104. It has a characteristic that puts the output at 500 Hz a full 12 dB below the output at 2.5 kHz. It continues to slope to more than 20 dB below the peak at 100 Hz. I have never heard the claim that response was intended to overcome the upper frequency limit of the transmitter, but that extensive research had shown that a rising characteristic within the normal communications bandwidth of about 300 to 2500 Hz produced the highest level of intelligibility. That rising characteristic (high-frequency emphasis) is usually provided by the mic, as you pointed out, while the low frequency cut was traditionally provided by the choice of capacitors in the audio amplifiers. In the K2, the low frequencies are suppressed very effectively by the OPT1 filter if it's properly aligned. Are you suggesting that those low frequencies are somehow being distorted or causing other problems in the audio I.C. in the K2 before they get to the filter? Ron AC7AC _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
