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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, 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 to the Alesis Digital Audio Tape itself.
Cables and interface
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.
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.
Higher sample rates can be accommodated with a reduced number of channels. While The original ADAT machines did not support this, the Lightpipe format was modified using bit-splitting techniques by the company Sonorus. Known as S/MUX (short for 'sample multiplexing'), this connection allows 4 channels at up to 96 kHz, or two channels at up to 192 kHz, on one optical cable. Most manufacturers implementing ADAT Lightpipe now support this S/MUX interface extension.
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.
Use in ADAT systems
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.
In order to fit 8 channels within the bandwidth limits of the standard TOSLINK transceiver modules, the bitstream is not biphase mark coded like S/PDIF. Instead, NRZI coding is used, where a 0 bit indicates no transition and a 1 bit is a transition. 8 audio samples at 24 bits per sample plus 4 user bits (196 bits total) are sent in groups of 4 data bits followed by a 1 bit to force a transition. This totals 196×5/4 = 245 bits. 10 consecutive 0 bits followed by a 1 bit provide frame synchronization.
One frame is sent at the desired sample rate, for a bit rate of 256×48 kHz = 12.288 Mbit/s, which is within the specified 15 Mbit/s capacity of the popular TOTX147 /TORX147  TOSLINK transceivers.
User data bit allocations
- User bit 0 is designated for Timecode transport
- User bit 1 is designated for MIDI data transport
- User bit 2 is designated for S/Mux indication (96 kHz sample rate mode)
- User bit 3 is reserved and set to 0
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 up to 24-bits 192 kHz 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 can carry 64 channels of audio at 48 kHz, 32 channels at 96 kHz or 16 channels at 192 kHz.
- Robjohns, H. 2007. Digital Interfacing. Sound on Sound. Volume 22, Issue 4, p.105.
- M-Audio ProFire Lightbridge, an example of a Lightpipe wireless interfacing implementation
- Alesis. 199?. ADAT LX20 Reference Manual. Chapter 8, p.52. Available online: . Accessed 24 August 2007
- Digital Interfacing Sound on Sound.
- ADAT project, ACKspace wiki
- Wavefront Semiconductor. 2005. AL1401AG Datasheet. Page 2 - pin description table. Available online: . Accessed 15 January 2010
- Sonorus S/MUX  Accessed 15 January 2010
- Wavefront Semiconductor application note AN3101-10 S/Mux Receiver. 2005.  Accessed 15 January 2010