Sampling rate tolerance: Because sound cards are very simple, they cannot always give exactly the right sample-rate. This is because the sample-rate crystal may not be accurate, or because the electronics can only program in steps and will choose the nearest step to the rate you want. Since PSK31 needs to generate accurate frequencies for the tones and the 31.25 baud data clock, this error must be corrected. To allow for this, you can make a fine adjustment of the sample-rate value used by the PSK31SBW program to calculate the frequencies it generates. This done by "fine-tuning" the value in the sample-rate box in the setup screen, moving it slightly above or below the starting value (11025 or 8000 or a multiple of 500Hz). For PSK31, the data clock must be set to within 1000ppm (0.1%). To get this accuracy you need a frequency counter with at least a 10 sec timebase, or an accurate frequency standard with a 1 kHz or 10kHz output and an oscilloscope. If you are going to use a counter, first use the Tx Freq box to enter a frequency that you will measure. Choose a frequency that is a multiple of 500Hz, as high as possible up to 3000Hz, but lower than 1/3 of the sample-rate. Then key F8 to make the soundcard give a pure tone at the chosen frequency. Measure the audio output frequency from the soundcard. If the measured frequency is too high, use the setup box to increase the sample-rate setting. For example, if you chose a frequency of 2000Hz and you measure 2002Hz and your sample-rate was set to 8000, increase the sample-rate to 8008. Check the frequency again and it should now be correct. If the measured frequency is low you decrease the sample-rate setting to correct it. The sample-rate value does not need to be a whole number: Use the same format for decimal numbers that your WINDOWS system uses. If you have a frequency standard with a low-frequency output such as 1000Hz or 10 kHz, you can use it synchronise the oscilloscope so that you can check the frequency accurately, or use a Lissajou figure. The sample-rate setting will be saved to the PSK31.INI file when you terminate the PSK31SBW program.