//This somewhat empirical value of 4, in combination with a kMinSamplesToProcess
//= 16, gives the best performance in the sense of minimizing the likelihood of
//DAC buffer underflow and ADC buffer overflow during the period just after
//sampling starting when the filters are initialising.
const int kMultirateInitNumInnerSamples = 2;

...

out = new RingBufferEnum::Enum( in.get(), (9*5) + 5, 9 * 5 * kMultirateInitNumInnerSamples);