Tech_corner|Psk31|Overview:
PSK31 Overview
PSK31 rivals radio teletype as an entry mode into digital communications. In fact, if you start with the software described here, you can operate both modes and many more including CW and Slow Scan TV. However, few modes give you the 'punch' you get from PSK31. You simply interconnect your transceiver with your home computer, bring up the free software and you are ready to go. You type on your keyboard and watch your QSO on the screen. Slow on the keyboard? No problem. Computer aided macros can automate most of your transmissions.
PSK31 uses "Phase Shift Keying" as the method of modulating your signal. It sends the signal at a bit rate of 31.25 Hz. (Hence PSK31.) The important thing to remember here is that the output signal is only about 32 hertz wide. Yes, 32 hertz. Compare that with a teletype signal bandwidth of about 175 hertz and a phone bandwidth of 2,500 to 3,000 hertz. PSK31 is actually narrower than the average CW Signal. This concentrated bandwidth does wonders for reception!
The concentrated bandwidth of a PSK31 signal also allows for many transmissions in the bandwidth of a single USB phone signal. Note in the sidebar that hams generally use just one or two standard frequencies per band. Microphones and speakers do a fine job of converting phone signals to something we can understand with our ears. Computer sound cards can scan the same audio frequencies and digitize them into something a computer can understand. In fact, the computer sound cards can do our ears one better trick. They can focus on any little section of the audio range -- like the 31.5 hertz PSK signal. Thus, a PSK31 signal based on an audio tone of 600 hz would be detected as a different signal from one at 700 hz. Software programs like Ham Radio Deluxe/Digital Master 780 do this for us. However, you still need to tell the software package which signal to decode. You do this by viewing and clicking the 'waterfall' display on your computer screen.
The computer generates a waterfall display by monitoring the output of the sound card as it scans the audio portion a received USB signal and by showing the signal strength on the screen. Each scan is shifted downward as a new scan is made generating a waterfall affect.
The screenshot to the left shows a portion of a waterfall display showing a signal at 14,070,630 Mhz, 14,071,120 Mhz and 14,071,700 Mhz. To begin decoding a particular signal, you would simply click on the signal with your mouse. Text is automatically displayed on the screen above.