OPERATION
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PreSonus 2008
2.3 DIGITAL CONNECTIONS AND SYNCHRONIZATION
The FireStudio LightPipe works with any digital device equipped with ADAT lightpipe I/O. This includes digital
mixers and standalone A/D devices like the DigiMax series of microphone preamps by PreSonus.
2.3.1 What is ADAT optical?
“Alesis Digital Audio Tape” or ADAT was pioneered by Alesis to simultaneously record eight tracks of digital
audio at one time. The ADAT lightpipe protocol was developed to stream eight channels of 20 or 24 bit audio
over an optical cable at 44.1kHz or 48kHz to allow for digital transfers between ADAT digital recorders.
Today “ADAT” is used as the standard abbreviation for the ADAT lightpipe protocol and is still used to
transfer eight channels of digital audio over a single fiber optic cable. The ADAT digital interface has been
adopted by many audio manufacturers as a compact way to transfer digital audio data between devices. Today
many audio devices ranging from consoles to synthesizers are equipped with ADAT I/O.
ADAT optical uses the same type of optical cables as the TosLink two channel protocol. These cables can be
purchased at your favorite local recording store.
2.3.2 What is S/MUX?
“Sample Multiplexing” or S/MUX is used to transmit high bandwidth digital audio using lower bandwidth
technology, i.e. ADAT lightpipe. By using S/MUX, a 96 kHz digital audio stream can be transmitted through
a protocol based on lower sample rates. S/MUX works by using demultiplexing to join two or more digital
audio channels to represent a single higher bandwidth channel. As discussed earlier, the ADAT lightpipe
protocol allows for eight channels of digital audio streaming at 44.1 kHz or 48 kHz. By using S/MUX
technology, two channels are joined together to represent one channel at 88.2 kHz or 96 kHz. In this way,
you can stream four channels of digital audio at 88.2 kHz or 96 kHz over the same lightpipe connection
originally designed to stream eight channels of audio at 44.1 kHz or 48 kHz. The FireStudio LightPipe is
equipped with dual S/MUX technology so that, should you choose, you can record 16 channels of audio at
88.2 kHz or 96 kHz.
2.3.3 BNC Sync and Word Clock
When using multiple devices connected through digital audio formats like S/PDIF, AES/EBU, ADAT or
TDIF, it is necessary to synchronize them to a single word clock generator. Word clock is used to keep a
perfectly timed and constant bit rate between all synced devices to avoid data errors. A word clock generator
creates digital pulses that contain no other data (i.e. audio). These pulses clock the internal oscillators of
each device and are essential to avoid frequency drift.
A word clock signal is bundled with the audio data in the ADAT Lightpipe protocol; however, many engineers
prefer to keep word clock sync and audio separate from each other. This is where BNC word clock
connections come into play and the FireStudio LightPipe has BNC word clock input and output for just this
purpose. You will find BNC cables used to deliver dedicated word clock in many quality studios and
broadcast facilities worldwide. BNC cables are rugged, lock into position, and can carry clock signals much
farther than the standard optical cable. A BNC word clock cable is a 75
Ω
, shielded coaxial cable with
standard ‘twist-lock’ BNC-type connections on each end.
Please note: BNC cables are made in several
impedances. The FireStudio LightPipe requires an impedance of 75
Ω
to achieve consistent sync.
