I think what Dave is talking about is one of Yamaha's plug in boards. Yamaha makes several plug ins that can be installed into the PSR9000Pro. These boards include more polyphony, more effects and in most cases, more midi channels. There is one board which is the vocal processor board which adds a vocal harmonizer and additional effects, and this is probably what he is talking about. However, there is already the same vocal processor in the PSR9000 series and I don't know if you will be able to install this second unit. I've wondered about this and I will have to check this out.
George Kaye