Processing an analog signal to square wave using a SN74LS14 Schmitt trigger?
The SN74LS14 is intended to be used with TTL logic levels, not with random analog voltages.  The specs you quote say that a voltage above 1.6 volts will be seen as a High, and below 0.8 volts will be seen as a Low.  The actual switching threshold will be somewhere between 0.8 and 1.6 volts, but there is no guarantee exactly where.

You should use an analog voltage comparator such as an LM339 to "square up" your analog signal.  With a comparator, you can set the switching thresholds and hysterisis to suit your application.