Many transceivers, like my old ICOM IC725, have no microphone audio compressor. As a consequence, the transmitter power is not fully used. I found with normal speech on average only about 10% of the maximum transmit power is used, and as a consequence the signal sounds thin and is hard to hear.
Looking at a speech signal, voiced speech consists of regular narrow and high pulses and much smaller waves in between. The goal of speech processors is to reduce the amplitude of those pulses and to increase the in between waves. In this way, the speech signal sounds louder but still natural, and is not overloading the transmitter. In effect, it reduces the Crest factor or the Peak to Average Power Ratio (PAPR). This effect can be obtained by a logarithmic transfer function of the amplifier, which amplifies small signals, but not strong signals. It can be implemented with an operational amplifier with 2 anti-parallel diodes in its feedback path. Diodes have an almost perfect logarithmic current/voltage function, which is used to give the amplifier this logarithmic characteristic.
The design also includes an audio output for headphones. Since most Mic plugs have an audio out pin (like my IC725), this makes it possible to hear the received audio, and also to monitor ones own voice after the compression. The compressed voice has a monitor volume knob on the case. The received audio from the transceiver has potentiometer on the board to set the general loudness, and it can be fine controlled by the volume knob of the transceiver.
The amplifier uses very little power, such that it doesn’t need external power, but simply uses the current that the microphone socket supplies for an electret microphone (8V, less than 10 mA total, including the LED signalling its operation).
Test of the compression amplifier: Without amplifier my IC725 had about 10% maximum power, measured at the internal power meter and with an external power meter. Using the compression amplifier, I found the output is less dependent on the distance of the mouth to the microphone, and the average output was increased to about 40% of the maximum output. This is about 6dB more, without an HF amplifier!
I also received very positive remarks on my audio quality.
Gerald Schuller, DL5BBN
- Schematic OpAmpProcFed16.sch
- Qucs Schematic OpAmpProc.pdf
- Qucs Plots OpAmpProcPlots.pdf