It's also possible to use a regular optocoupler
jon, do you mean led - transistor/darlington, or led - ldr?
It's *possible* to use LED+LDR. I don't think it sounds as good, mainly due to the light memory and the time it takes to fire up, though the VTL5C1 has similar turn on time to the chip. The H11F1 is also cheap ... part of the reason I was originally excited about this design, actually.
Incidentally, I talked to Ray a bit over e-mail about some other aspects of this particular design if anyone's wondering about a few parts:
1. R4 (the 470K base > ground) basically sets the maximum compression swing, linearizing the response some.
2. In my version, I built it with a 22K on the input, instead of 100K. Ray did the math, and the difference in volume attenuation (assuming the LED in the chip turns on fully) is negligible, so you can in some ways control the overall brightness of the effect by changing that resistor. I like my compressors to be slightly brighter.