26
the DISPLAY key to cancel the read from tape operation. To
disable the automatic power up read from tape prompt, set read
tape to off in the commands menu.
On initialization the default AI-2 control mode is set to auto. The
AI-2 will check the control ports for communication and switch
accordingly, if no communication is detected then the AI-2 will
switch to reader chase mode.
The AI-2 system reference, and code type and rate are then set
depending on the controller type detected. The AI-2 always de-
faults to a sample rate of 48.000 KHz. In normal operation the
sample rate, frame rate and code type are shown in the top line of
the display. If required, use the AI-2 reference menu to change the
default selections.
4.2A Time Code and Sample Rates
It is extremely important to understand that the ADAT transport is
unlike an analog transport where time code recorded on an audio
track has a fixed relationship with the program material. The ADAT
is unique in that any time code type and any time code number can
be selected for a piece of audio on tape. This provides great
operational flexibility, but if not clearly understood can potentially
lead to synchronization problems.
To determine the ADAT time code position, the AI-2 reads the
current digital audio sample number, or number of samples from
ADAT tape zero (digital audio sample zero) from tape and calcu-
lates a time code number based on the number of samples per
frame and the AI-2 time code offset.
The number of samples per frame is a ratio of sample rate to frame
rate. For example, at a sample rate of 48.000 KHz and a frame
rate of 30 fps, the sample rate ratio is 1600, so for every 1600
digital audio samples the tape moves forward from zero, the ADAT
time code position will advance by a frame. At 25 fps the sample
rate ratio is 1920, so for every 1920 samples the ADAT time code
position will advance a frame.
