Topic: digi9652 sample rate

Hi, I imported some ADAT tape tracks into Cubase 4 via the Digi9652. After disconnecting the Alesis ADAT recorder, I found Cubase was playing the file back at a faster sample rate. I measured the word clock output on the Digi9652 and it was running at 51.6 Khz, internal sync. When I reconnected the fibre optic line to the adat, it went back to 48 Khz. Then I supplied word clock in to the digi9652 supplied from an Alesis BRC and it was running at 48 KHz again. Any idea why the internal clock is running so fast without and external sync source? Is there any way to adjust this?