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Chapter 5: Multiple ADAT-XT Operation
64 ADAT XT Reference Manual
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.
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