Adat S/Mux - Benchmark ADC1 USB Instruction Manual

ADAT S/MUX

Proper S/MUX Identification
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S/MUX allows the recording of 4 channels at
88.2 or 96 kHz using a standard 8-channel
44.1 or 48 kHz ADAT recorder. S/MUX
the recording of 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.
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An ADAT S/MUX 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.
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An ADAT S/MUX 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
could be played at ¼ of its original sample
rate, and sound almost normal. S/MUX
also be mistaken for S/MUX
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, and these errors may not be
discovered until it's too late to correct them.
ADC1 USB Instruction Manual
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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 and these
decoders must be properly enabled for S/MUX
and disabled for standard ADAT inputs.
Sample Rate Controls S/MUX
Most devices (including the ADC1 USB)
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
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(S/MUX ) of the actual sample rate.
S/MUX is not 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.
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) or ¼
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