The ADAT Lightpipe, officially the ADAT Optical Interface, is a standard for the transfer of digital audio between equipment. It was originally developed by Alesis but has since become widely accepted,[1] with many third party hardware manufacturers including Lightpipe interfaces on their equipment. The protocol has become so popular that the term "ADAT" is now often used to refer to the transfer standard rather than the Alesis Digital Audio Tape itself.
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Lightpipe uses fiber optic cables (hence its name) to carry data, with Toslink connectors at either end, making them identical to S/PDIF optical cables. However, the data streams of the two protocols are totally incompatible. S/PDIF is mostly used for transferring stereo or multi-channel surround sound audio, whereas the ADAT optical interface supports up to 8 channels at 48 kHz, 24 bit. Recently, Lightpipe devices have been successfully interfaced via FireWire.[2]
Lightpipe can carry eight channels of uncompressed digital audio at 24 bit resolution at 48,000 samples per second. Initially used for the transfer of digital audio between ADATs, the protocol was designed with future improvements in mind. All Lightpipe signals are transmitted at 24 bit resolution, no matter what the depth of the audio; information is contained within the Most Significant Bits and the rest of the bits remain a string of zeros. For example, if a 16 bit signal is sent via Lightpipe, the first sixteen bits contain the audio information while the other eight are simply occupied by zeros. The receiving device ignores information it cannot process. For example, a 20 bit signal going from a Type II ADAT to a Type I (which only operates at 16 bits) will simply ignore the bits below the sixteen MSBs.[3]
Higher sample rates can be used with a proportionately reduced number of channels, although the original ADAT machines did not support this. The Alesis HD24 recorder demultiplexes high sample rates across multiple ADAT channels. A signal with a sample rate of 96 kHz is thus effectively split up into two signals of 48 kHz each, maintaining audio compatibility with the original ADATs.
Lightpipe's main advantage is bit-transparent transfer of audio information. The lightpipe is "hot-pluggable", which means devices do not need to be turned off for plugging in or unplugging (although it is advisable to mute the receiving equipment, since there will be a large signal spike when the connection is made). The optical connect avoids ground-loops, which can be troublesome in larger installations, and will not transfer any harmful electrical spikes from one device to the next.
Lightpipe was designed for use with the Alesis ADATs, and although extremely versatile, there are a few limitations. For straightforward digital audio transfer, the receiving device can synchronize to the lightpipe's embedded clock signal, achieving a 1:1 digital copy. For transport control, additional synchronization is needed between devices. (For example, using two ADAT machines at the same time to achieve 16-channel throughput would require better transport control; otherwise, the two ADAT machines would be very unlikely to play in sync.) Nine pin D connectors are used to transfer transport information. The Alesis ADAT HD24 also offers MIDI Time Code for synchronization with MIDI-enabled devices.
From the ADAT Lightpipe patent (US 5297181), the bitstream is transported in a frame of 256 bits. This frame is repeated at the desired sample rate (e.g. 48 kHz). Each of the eight channels contain 24 bits (MSB first), so 24 x 8 = 192 bits are allocated per frame. For synchronization, 6 sync (one) bits are interleaved per channel, adding 6 x 8 = 48 bits. The remaining 16 bits in the frame are allocated as: 4 user data bits, 2 sync bits and a frame sync pattern of 10 zero bits. The 10 zero bit pattern is unique in the frame and aids in synchronizing the bitstream at the ADAT receiver.
User data bit allocations[4]
The transmission speed of the user bits is equal to the sampling rate (e.g. 48000 bits per second)
There are numerous digital audio transfer protocols. The most commonly used professional interface is AES3, developed by the Audio Engineering Society and the European Broadcasting Union, which transmits two channels of digital audio over a balanced XLR cable. S/PDIF (Sony/Philips Digital Interface) is the consumer version of this protocol, which uses either RCA leads or optical cables identical to lightpipe cables. MADI-X can carry 64 channels of audio at 48 kHz and 28 channels at 96 kHz, although it always requires a separate clock signal as it does not have one embedded.
However, recently, certainly in home and semi professional studios, USB and FireWire interfaces are the most popular means of transferring data. Their advantages over Lightpipe are large: compatibility is almost universal, all kinds of information can be transferred and a single cable can both send and receive data, whereas Lightpipe requires two separate leads for this. Yamaha's mLAN protocol exclusively uses the FireWire interface.