What happens to a Tau2 which is operating at a frame rate less than 15 Hz as driven by the EXT_SYNC signal?

Below about 15 Hz the Tau2 will stop generating analog video (most analog monitors will not work correctly much below 30 Hz). Digital video will still be output. While a usable image can be obtained at rates below 15 Hz after the flat-field correction (FFC) is done at the desired rate, dynamic range will be limited and there may be other compromises.