Sound card clock calibration

This concept goes above and beyond the Digital Net but is important when operating some digital modes.

All sound cards are not 100% accurate in their sampling rates. In normal audio sound applications, this tends not to matter. Digital modes like MFSK, SSTV, and Olivia require an accurate sound card clock. Other modes are more forgiving like MT63 and PSK. A difference can be calculated between the stated sample rate and actual sample rate of the card. Experiencing one or more un-calibrated sound cards results in decoding errors or little to no decoding of a strong signal.

Some programs can ‘auto correct’ on receive. The results are often ‘best guess’ and not always correct. “Auto Slant” in MMSSTV is an example. Those who commonly operate those modes will want transmitting stations to calibrate their sound card.

Each program has its own method for doing this. Here are common examples:

SSTV: http://www.wb9kmw.com/WB9KMW/sstv_files/tutorial/That_Pesky_Slant.pdf. I prefer this method: http://www.wb9kmw.com/WB9KMW/sstv_files/tutorial/That_Pesky_Slant_WWV_Alternative.pdf.

Fldigi: http://www.k8jtk.org/2015/10/19/nbemsfldigi-sound-card-calibration/