ADC16 Instruction Manual
Page 26
ADAT S/MUX Tutorial
Proper S/MUX Identification
S/MUX
2
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
4
allows 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.
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, and these errors may not be discovered
until it's too late to correct them.
ADAT S/MUX
2
Flag
The ADAT specification was amended in February of 2001 to include an S/MUX
2
flag. The ADAT
Interface carries 4 'user bits' per frame, and user bit U2 is now designated as an S/MUX
2
flag. U2
should be set high when the interface is carrying S/MUX
2
. The ADC16 follows this standard.
Unfortunately, many devices ignore the user bits, and therefore, these devices will not respond to
the S/MUX
2
flag. Again, the user must use caution when using an S/MUX connection.
No S/MUX
4
Flag
The February 2001 addendum to the ADAT specification made no provisions for an S/MUX
4
flag.
User bit U2 should not be set in S/MUX
4
applications. Unfortunately, this means that the S/MUX
4
user bit configuration is identical to normal ADAT usage. Consequently, at the current time, user
bits cannot be used to indicate S/MUX
4
. This could change in the future. Again, the user must use
caution when using an S/MUX connection.
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.
