Martin (1992: 510):
In effect, mode is the semiotic construction of communication technology. As far as interpersonal space is concerned, what is critical is the way in which various channels of communication affect the kind of interaction that is possible between speaker and listener. This is conditioned by the kind of feedback that is possible, depending on whether or not the speaker and listener can see each other and at the same time whether or not they can hear each other.
 To be clear, communication technology provides the material means of expanding the semiotic system of mode.
 The unacknowledged source of these ideas is Hasan (1985/9: 58):
The second factor to be considered under mode is that of PROCESS SHARING. Is the addressee able to share the process of text creation as it unfolds, or does the addressee come to the text when it is a finished product? Here again, there are degrees of process sharing from the most active — as in dialogue — to the most passive — as in a formal lecture. The degree to which process sharing can occur is closely related to CHANNEL. … The first channel I will call PHONIC, the second GRAPHIC.
At the memorial to honour the life and work of Ruqaiya Hasan, Martin accused Hasan — falsely — of not having acknowledged her sources. Evidence here.