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ADC1 Instruction Manual Page 16
Using ADAT S/MUX
Proper S/MUX Identification is a
Must
S/MUX
2
allows recording 4 channels at 88.2
or 96 kHz using a standard 8-channel 44.1 or
48 kHz ADAT recorder. S/MUX
4
allows
recording 2 channels at 176.4 or 192 kHz
using a standard 8-channel 44.1 or 48 kHz
ADAT recorder. In either case it is important
to identify S/MUX recordings so that they can
be properly decoded upon playback. Failure to
properly decode an S/MUX recording will add
unwanted artifacts to the audio. The severity
of these artifacts is a function of the high-
frequency content of the original digital audio
signal, and may range from inaudible to very
objectionable. This variation in severity can
make it difficult to accurately spot a problem
just by listening to a portion of the recording.
An ADAT S/MUX
2
recording will have pairs of
nearly identical tracks (1≈2, 3≈4, 5≈6, and
7≈8). Unfortunately this can be mistaken for
4 stereo pairs at half of the original sample
rate. There is no substitute for proper
labeling. This labeling should include the
sample rate of the recording.
An ADAT S/MUX
4
recording is somewhat
easier to identify because it will have groups
of 4 channels that are nearly identical
(1≈2≈3≈4, and 5≈6≈7≈8). In error, S/MUX
4
could be played at ¼ of its original sample
rate, and sound almost normal. S/MUX
4
could
also be mistaken for S/MUX
2
and could be
played at ½ of its original sample rate. Please
note that these changes in sample rate will
not alter the pitch of the audio but will
introduce errors. These errors may not be
discovered until it is too late.
S/MUX Must be Decoded Before
Digital Processing
No DSP process should be applied to an
S/MUX signal before it is decoded. S/MUX
must be decoded before it reaches the
internal processing in a DAW or a digital
console. Many such devices include S/MUX
decoders at their digital interfaces. These
decoders must be properly enabled for S/MUX
and must be disabled for standard ADAT
inputs.
Sample Rate is the Key that
Controls S/MUX
Most devices (including the ADC1)
automatically enable and disable S/MUX in
response to changes in sample rate.
Therefore it is essential that all S/MUX
equipped A/D converters, D/A converters,
digital consoles, digital audio workstations,
and digital processing devices be set to
identical sample rates. There is one exception
to this rule: A non-S/MUX ADAT recorder can
be connected to an S/MUX interface, but the
recorder must be set at ½ (S/MUX
2
) or ¼
(S/MUX
4
) of the actual sample rate.
S/MUX should not be used for
Sample Rate Conversion
If two devices are connected with an ADAT
S/MUX interface and the devices are set to
different sample rates, a crude form of
sample rate conversion will occur. For
example, if an A/D converter is set to 96 kHz,
and it feeds a digital console that is set to 48
kHz, the system will appear to down convert
from 96 kHz to 48 kHz. This would be a useful
feature if the digital filtering was correct. The
problem is that this ad-hoc sample rate
converter is lacking the low-pass filter that
prevents aliasing.