[sdiy] clock frequency shifter?

[tom wisdom]

hi all,

i'm building a lo-fi sampler box for an audio electronics workshop at 
school, using the ISD voice recorder chips that have been mentioned 
previously on this list in connection with a Radio Shack kit. i want to 
make them behave a bit more interestingly. :)

this chip has an internal clock that sets the sample rate at 8kHz. 
there's also an XCLK input on the chip, that, according to the datasheet 
(which is at 
http://www.winbond-usa.com/products/isd_products/chipcorder/datasheets/1400/), 
can be connected to an external clock signal. for the chip's default 8 
kHz sampling rate (yep, it is going to sound like pants), the clock rate 
is 1.024 MHz.

there's an anti-aliasing filter on the audio input, so i don't think 
it's possible to increase the frequency range of the thing by 
overclocking it. but i would like to be able to vary the speed of sample 
record and playback by varying the clock rate, say with a pot.

so: is there a way to take the output of a 5MHz crystal and vary it 
semi-smoothly down to somewhere near 1 MHz? or alternately, a 
very-simple VCO that does this range? could i just use an LFO circuit 
with component values adjusted accordingly, or are there weird effects 
that come into play? linearity and stability are obviously not very 
large concerns. any suggestions are welcome (it's due in a couple weeks).

tom