The ADAT Lightpipe, officially known as the ADAT Optical Interface, is a standard for transferring digital audio between equipment. This interface was originally developed by Alesis.
Cables and interface
ADAT Lightpipe gets its name from the fiber optic cables it uses to carry data. Toslink connectors are found at either end of the cables, which means they are identical to S/PDIF optical cables. Even so, the data streams of these two protocols are completely incompatible. The ADAT optical interface can support up to eight channels at 48 kHz, 24 bit. The S/PDIF interface, on the other hand, is primarily used to transfer stereo or multi-channel surround sound audio. Lightpipe devices have recently been interfaced via FireWire.
ADAT Lightpipe is capable of carrying eight channels of uncompressed digital audio at 24 bit resolution at 48,000 samples per second or four channels at 96,000 samples per second. The protocol was first used to transfer digital audio between ADATs, but it was always intended to be improved over time.
No matter the audio depth, all Lightpipe signals are transmitted at 24 bit resolution. The most significant bits hold the audio information, and the remaining bits are comprised of a string of zeros. If a 16 bit signal, for example, is sent through Lightpipe, the audio information will be contained within the first 16 bits, and the other eight bits will be made up of zeros. The device receiving the signal will ignore any information it can’t process. For example, if a 20 bit signal is transferred from a Type II ADAT to a Type 1 (which operates only at 16 bits), it will ignore all the bits below the 16 MSBs.
Higher sample rates can be used with a proportionately reduced number of channels; however, this was not supported by the original ADAT machines.
The primary advantage of ADAT Lightpipe is its ability to transfer digital audio and facilitate a perfect transfer of information. Another advantage is that it is “hot-swappable,” which means that it’s not necessary to turn off devices when plugging in or unplugging.
Use in ADAT systems
Lightpipe was originally designed to be used with Alesis ADATs, but it is a very versatile interface. However, there are certain limitations. When it comes to a straightforward digital audio transfer, the receiving device can be synchronized to Lightpipe’s embedded clock signal to achieve a 1:1 digital copy. Additional synchronization will be required between devices for transport control. For example, if you use two ADAT machines simultaneously to achieve a 16-channel throughput, you will need better transport control to ensure that the ADAT machines play in sync. Nine pin D connectors are utilized to transfer transport information. The Alesis ADAT HD24 also features a MIDI Time Code to help synchronize with MIDI-enabled devices.
There is an abundance of digital audio transfer protocols available in the market. The most popular professional interface is the AES3, which transmits two channels of digital audio over a balanced XLR cable. This protocol was created by the European Broadcasting Union and the Audio Engineering Society. The consumer version of the AES3 is the S/PDIF (Sony/Philips Digital Interface). It uses either RCA leads or optical cables that are exactly the same as Lightpipe cables. Another protocol is the MADI-X, which is capable of carrying 64 channels of audio at 48 kHz or 32 channels at 96 kHz. However, it does not have an embedded clock signal, so an outside one will be required.
More recently, USB and FireWire interfaces have become the most popular ways to transfer data, particularly in home and semi-professional studios. They offer several advantages over Lightpipe, including the fact that all kinds of information can be transferred and compatibility is nearly universal. Also, a single cable can be used to both send and receive data, while Lightpipe uses two separate leads to achieve this. Yamaha’s mLAN protocol uses the FireWire interface exclusively.