From: "Jeff Edmonson" <[EMAIL PROTECTED]>
I'm hoping that Don can come in here, and remind me how it is/was
he was peaking the audio at around 3kc, and then rolling the whole thing off at 3.5kc (or slightly higher). And, what's the bottom end?
I do it by using a rising pre-emphasis between curve in the mic preamp, starting at 800 cps and peaking 8-10 dB at 3 kc/s and above. Also, I use a D-104 mic (for highs) and a dynamic mic (for lows) mixed together in phase. The low-pass cutoff is accomplished using passive L-C filters. With the flip of a switch I can choose between a gradual hf cutoff filter at 5-6 kc/s, a very sharp cutoff filtor at 3400 cps, or no lowpass filter at all. The bottom end goes down to 30 cps or so, but the scope shows visible distortion on a sinewave below about 40 cps.
With the 3400 cps filter in the circuit, the response curve is flat from 40 cps to 800 cps, then rises steadily up to about 2 kc/s and begins to flatten off beyond 3 kc/s, but hits a brick wall at 3400 cps. I bought the filter at a military surplus store in Washington DC back in 1973 ( I wish I had purchased half a dozen of them). There is virtually no attenuation at 3300 cps; at 3400 it is over 25 dB down, and at 3500 cps you can't detect the signal on the oscilloscope. The only other filter I have ever seen that has that sharp a cutoff is one designed by W2WLR that he sent me to evaluate.
One thing that is often overlooked is that the audio system capability needs to extend one octave above and below the intended frequency response of the program material, to minimise phase distortion effects. This is per information in the United Transformer Corp. catalogue. For example, if you intend to transmit high quality voice audio from 100 to 3500 cps, your audio chain from the mic through the modulator stage needs to be as flat and distortion free as possible from 50 to 7000 cps. Good "communications" quality audio 300-3000 cps would require the transmitter to be capable of 150-6000 cps (that is almost exactly the audio specification of the BC-610). Any limiting of frequency response should be done using shaping circuitry at low levels, not by limiting response at high levels by using transformers with poor frequency response, or putting capacitors across interstage or modulation transformers, or similar gimmickry often seen in ham rigs.
My system seems to work well with my own voice. However, when N3DRB came by for a visit, he operated my station, and everyone told him his voice didn't sound nearly as good as it does on his rig at home. Each operator should make sure the rig is taylored individually to his or her voice.
Don K4KYV _________________________________________________________________ Send and receive Hotmail on your mobile device: http://mobile.msn.com
