Audio processor for SSB transceiver
With the heat up of our H.F. weekly contact net, I tend to have more operation on SSB radio recently. Unfortunately I have a low pitch voice. Information in human speech has a large portion hiding in the higher pitch audio spectrum. For a low pitch voice, most of the sound power does not create readability. Since SSB transmission involved linear power amplification, keeping the low pitch voice in a transmission is actually a waste of power. Following is the circuit of a high pass filter I have tried out to improve my signal's readability. This circuit consists of microphone amplifier and a high pass filter (frequency cut point selectable at 400Hz, 600Hz and 1050Hz).
Gain of the amplifier is adjusted by changing value of the 10K resistor (first LM741 pin 4-6). Such gain is higher when value of this resistor is increased. In brief, max. gain of 30db can be achieved by changing the resistor to over 200K. Yet a higher gain in the first stage will peak up the frequency response and alter the overall flatness of the whole unit. It is better to keep gain of the first stage to be under 15db so that the second stage (a unity gain filter section) can perform full function.
Below shows the frequency response simulation of such a circuit. Three resistors are switched alternatively to generate different profile :
Input node 8, output node 2, switch at node 3 to 50k, 18k and 6.8K.
Frequency response with R=50K
Peak gain 10db between 500Hz to 7KHz, natural tone profile.
Frequency response with R=18K
Flat gain 14db 400Hz to 3Khz, telephone quality profile
Frequency response with R=6.8K
Low cut 500Hz, peak gain 16db 450Hz to 1050Hz, QRM penetrating profile.
Follow up work will be to make a stand alone unit with the following modules :
Stay tune for the update portion on this project.