Here is where the major soundscaping effects occur. The phase filter is in fact six filters and FET/bipolar transistor switches that alter the signal path through the filters of audio fed from the left hand effects. The six different ‘Treatments’ (selected by front panel rotary switch) are Vibrato, Phasing 1, Phasing 2, Waa, Waw and Meow all of which are are the result of different paths that audio is fed through via filter sections. For any given phase filter configuration above, there is in addition, six different modulations you can apply to vary the frequency of the phase filter. Varying the frequency alters the amount of phase shift that occurs in the audio signal. These modulations take the form of ‘transients’ i.e. waveforms that are produced only between the attack and decay sections of say a plucked guitar string. Outside of this ‘attack/decay’ envelope the modulation waveform goes back to a simple sine wave. The six transients are slow sine, fast sine, increasing amplitude sine, decreasing amplitude sine, ramp up and ramp down. Modulation speed is controlled via slider (or pedals) and modulation amount by another slider. Here is a great feature. The time taken for transients to appear modulating the phase filter is controllable. So there is a slow option controllable via the modulation ramp time slider/pedal. Finally there is a slider/pedal that directly shifts the frequency of the phase filter. This allows you to ‘sweep’ the audible range of the filter if you use pedal control.The ‘Growl’ switch option drops the fundamental by two or three octaves depending on the switch position. As mentioned above, these sub-harmonics are used to modulate the phase filter and are not just simply mixed with the original sound. This gives amazing/bizarre new modulations! The amount of growl is controllable via the depth slider and also if the increasing/decreasing sine modulation is selected, by the decaying envelope extracted from the attack/decay section of the input audio.   My impression of the phase filter is that it's the ‘guts’ of the Hi-Fli and produces an astonishing range of phasing effects from subtle vibrato to screeching wah sounds all with transient modulations on top! The Envelope and Fuzz bypass switch allows you to bypass completely the left hand effects section of the Hi-Fli and send audio directly into the Phase Filter. This was something (along with Growl) that EMS seem to have added in the Mk2 version of the Hi-Fli... so you wont see it on earlier Mk1 versions. It's a simple idea but allows for yet more different sounds... ’clean phasing’ if you wish. Finally the bypass mix slider is a simple dry/wet mix control of the audio output.