1. A method for producing from a given series of digital input words, an output in the form of an analog waveform approximating, in analog amplitude with respect to time, values of a notional digital signal that would be formed by stepping through each of said digital input words at a stepping frequency selectable over a range of frequencies between a lower stepping frequency limit and an upper stepping frequency limit, said analog waveform having frequency components not exceeding a limiting frequency, comprising the steps of: deriving a plurality of digital samples from said digital input words, said samples being equivalent to a sampling of said notional digital signal at a sampling frequency which exceeds said stepping frequency; applying said samples to a programmable digital filter arranged to filter said samples to produce a sequence of digital output words, at a predetermined conversion frequency, said conversion frequency being at least twice said limiting frequency and independent of said selected stepping frequency; and converting said output words from digital form to analog form at said conversion frequency to thereby yield said analog waveform as said output.
1. A method for producing from a given series of digital input words, an output in the form of an analog waveform approximating, in analog amplitude with respect to time, values of a notional digital signal that would be formed by stepping through each of said digital input words at a stepping frequency selectable over a range of frequencies between a lower stepping frequency limit and an upper stepping frequency limit, said analog waveform having frequency components not exceeding a limiting frequency, comprising the steps of: deriving a plurality of digital samples from said digital input words, said samples being equivalent to a sampling of said notional digital signal at a sampling frequency which exceeds said stepping frequency; applying said samples to a programmable digital filter arranged to filter said samples to produce a sequence of digital output words, at a predetermined conversion frequency, said conversion frequency being at least twice said limiting frequency and independent of said selected stepping frequency; and converting said output words from digital form to analog form at said conversion frequency to thereby yield said analog waveform as said output. 4. A method according to claim 1 including the step of arranging said digital filter as a low pass filter.