Frequency modulated, (not amplitude modulated), with a 37Hz square wave,
followed by a double dose of leveller on the maximum setting, then boost the 500-1500Hz band by about 10db.
Daleks and Davros on Youtube for comparison … http://www.youtube.com/watch?v=TXrUyOurnOI
[those upset by bad acting and cheapo special effects shouldn’t watch 
]
The vocoder effect is more Cylon than Dalek, (I’m ashamed I can tell the difference 
).