Alesis ADAT-XT manual Polarity Differences, Connections, Here are some possible solutions

Chapter 5: Multiple ADAT-XT Operation

POLARITY DIFFERENCES

The original ADAT was designed to invert its analog input signal internally before the audio was sent through the A/D (analog-to-digital) converter and recorded onto tape. The digital representation of the data (as stored on tape) was therefore inverted relative to the analog input (i.e., a positive voltage at the analog input is stored as a negative number on the tape). When this digital signal was played back from tape, the data was sent to a D/A (digital-to-analog) converter, and the analog signal output of the D/A was inverted once again before going to the output jacks. This resulted in the input to output phase being correct, and no phase problems were experienced when using a single ADAT or a multiple ADAT system.

However, neither the digital input nor the digital output was inverted. Therefore, if one transfers the ADAT’s digital audio data to a DAT machine or hard disk recording system (using the AI-1 or a similar digital interface), the audio output from the other device would likely be out of phase with the ADAT, since it probably does not invert the analog signal after its D/A converters. Although this in and of itself will not usually be a problem, it may cause some confusion if multiple correlated audio signals are combined from multiple sources since there is a possibility that signals could become out of phase.

To prevent this from occurring, the XT design has been improved so that the polarity remains constant from the analog to digital, and back to analog, domain. Like the original ADAT, the result from one machine, or multiples of the same machine, is the same: The input to output phase is correct. However, if two cloned tapes (copied digitally either with ADATs, ADAT XTs, or both) are played back in sync on both an ADAT and an XT, the outputs of the two machines will be out of phase relative to each other. Since it does not usually serve any practical purpose to play the same signal from two different tapes with two different model machines simultaneously, this should not present a problem for you. This design improvement is only mentioned here to explain this possible situation should you happen to experience it.

Possible problem: Let’s say you had been using a multiple ADAT system and you recorded a stereo signal across two machines (a stereo drum recording on tracks 8 and 9), where there is a common element to each track (i.e. center signal). If you replace one of these machines with an XT, the result would be that the track being played back on the XT will be out of phase from the track played on the ADAT, canceling the center signal.

Here are some possible solutions:

•Use a phase switch on your mixer (if your mixing board provides this) to invert the phase of the problem track to compensate.

•Wire your patchbay or ELCO cable to reverse the input and output polarity of the XTs in the system, but use standard cables on the ADATs in the system. This is recommended only for studios where master tapes will be exchanged between units or sent out of house.

•Digitally bounce one of the stereo tracks to the other ADAT so that both sides play back from the same machine.

•Most importantly, it is better to plan recording sessions so that stereo tracks reside on the same physical tape in a multiple ADAT system.

CONNECTIONS

64 ADAT XT Reference Manual