A Simple Phasing Effect Circuit

'Phasing' is an effect (heard on popular records in the late 60s and 70s) that selectively filters audio frequencies. Delaying the sound waveform by a few milliseconds and adding it back to an undelayed version of itself results in some frequencies being cancelled while others are reinforced - what happens to a given frequency depends on the relative phase of the two components - if there is a 180 degree shift then there will be cancellation (or partial cancellation if the amplitudes are different) or (if there is no relative phase difference or amplitude difference) a doubling of the amplitude. The delay is simply achieved using a loudspeaker, air and a microphone. The delayed and direct waveforms are added using a mixer (figure 1).

diagram of the system
Figure 1: The Phasing System

Assume that the speed of sound in air is 333 m/s (335 is closer but 333 is easier to remember and the speed varies with temperature and pressure anyway) then a delay corresponding to 1 metre of air will be approximately 3 milliseconds. A frequency of 333 Hz has a period of 3 ms so there will be a reinforcement at this frequency. At twice this frequency (666 Hz) the period is 1.5 ms so there will be reinforcement here too (the waveform will have been delayed exactly two cycles) and also reinforcement at 3 times, 4 times 333 Hz and so on. However, at 167.5 Hz (half of 333 Hz) the relative delay will be half a cycle so that (partial) cancellation will result; similarly at 1.5 times, 2.5 times, 3.5 times 333 Hz (and so on) cancellation occurs.

If this was all there was to it then the effect would sound like listening to a sound through a drainpipe. However, changing the delay over short periods of time will sweep the peaks and troughs in the spectrum up and down in frequency - the human ear is particularly sensitive to picking out these dynamic changes (since the movement of resonances in the vocal tract (formants) is what speech is all about).

By way of example here's a recording I have processed using the arrangement described above. First the original and then the result of processing. (The music is a short excerpt from 'Snoopy vs the Red Baron' by the Royal Guardsmen). The effect works better with some sounds than with others - those sounds with a wide spectrum work best - percussion is one (hence the choice in my example) or highly polyphonic music. A solo flute is not a good choice.

The spectra of the sound 'before' and 'after' are shown in figures 2 and 3 below. They were prepared using Audacity sotfware (see below); time is plotted horizontally (in seconds) with increasing frequency vertically. The power at each frequency is indicated by the colour - grey the lowest through blue to pink and red where the power is greatest. The movement of the filter in frequency can be seen in fig. 3 with the multiple peaks and troughs showing clearly.


spectrum of unprocessed signal
Figure 2: Spectrum of source


spectrum after processing
Figure 3: Spectrum of Phased signal


Last updated: 22 October 2003;   © Lawrence Mayes, 2003