ADAT Lightpipe
Encyclopedia
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 the Alesis Digital Audio Tape itself.
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 channel
s at 48 kHz, 24 bit. Recently, Lightpipe devices have been successfully interfaced via FireWire.
at 48,000 sample
s 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 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.
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.
are used to transfer transport information. The Alesis ADAT HD24
also offers MIDI Time Code for synchronization with MIDI-enabled devices.
User data bit allocations
The transmission speed of the user bits is equal to the sampling rate (e.g. 48000 bits per second)
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.
Digital audio
Digital audio is sound reproduction using pulse-code modulation and digital signals. Digital audio systems include analog-to-digital conversion , digital-to-analog conversion , digital storage, processing and transmission components...
between equipment. It was originally developed by Alesis
Alesis
Alesis is a company based in Cumberland, Rhode Island, that designs and markets electronic musical instruments, digital audio processors, audio mixers, digital audio interfaces, recording equipment, drum machines, professional audio and electronic percussion products...
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
ADAT
Alesis Digital Audio Tape or ADAT is a magnetic tape format used for the simultaneous digital recording of eight analog audio or digital audio tracks at once, onto a Super VHS tape that is used by consumer VCRs.- History :...
" is now often used to refer to the transfer standard rather than the Alesis Digital Audio Tape itself.
Cables and interface
Lightpipe uses fiber optic cables (hence its name) to carry data, with ToslinkTOSLINK
TOSLINK is a standardized optical fiber connection system. Also known generically as an "optical audio cable," its most common use is in consumer audio equipment , where it carries a digital audio stream from components such as MiniDisc, CD and DVD players, DAT recorders, computers, and modern...
connectors at either end, making them identical to S/PDIF
S/PDIF
S/PDIF is a digital audio interconnect used in consumer audio equipment over relatively short distances. The signal is transmitted over either a coaxial cable with RCA connectors or a fiber optic cable with TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high...
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 channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
s at 48 kHz, 24 bit. Recently, Lightpipe devices have been successfully interfaced via FireWire.
Data transfer
Lightpipe can carry eight channels of uncompressed digital audio at 24 bit resolutionAudio bit depth
In digital audio, bit depth describes the number of bits of information recorded for each sample. Bit depth directly corresponds to the resolution of each sample in a set of digital audio data...
at 48,000 sample
Sampling (music)
In music, sampling is the act of taking a portion, or sample, of one sound recording and reusing it as an instrument or a different sound recording of a song or piece. Sampling was originally developed by experimental musicians working with musique concrète and electroacoustic music, who physically...
s 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 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.
Advantages
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.
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 connectorsD-subminiature
The D-subminiature or D-sub is a common type of electrical connector. They are named for their characteristic D-shaped metal shield. When they were introduced, D-subs were among the smaller connectors used on computer systems....
are used to transfer transport information. The Alesis ADAT HD24
Alesis ADAT HD24
The Alesis HD24 is a 24-track hard-disk audio recorder. It is the successor to the hugely successful tape based Alesis ADAT.- Advantages over tape-based ADAT :There are numerous advantages that the HD24 has over the tape-based ADATs...
also offers MIDI Time Code for synchronization with MIDI-enabled devices.
Lightpipe Bitstream
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
- 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)
Competing Protocols
There are numerous digital audio transfer protocols. The most commonly used professional interface is AES3, developed by the Audio Engineering SocietyAudio Engineering Society
Established in 1948, the Audio Engineering Society draws its membership from amongst engineers, scientists, other individuals with an interest or involvement in the professional audio industry. The membership largely comprises engineers developing devices or products for audio, and persons working...
and the European Broadcasting Union
European Broadcasting Union
The European Broadcasting Union is a confederation of 74 broadcasting organisations from 56 countries, and 49 associate broadcasters from a further 25...
, which transmits two channels of digital audio over a balanced XLR cable. S/PDIF
S/PDIF
S/PDIF is a digital audio interconnect used in consumer audio equipment over relatively short distances. The signal is transmitted over either a coaxial cable with RCA connectors or a fiber optic cable with TOSLINK connectors. S/PDIF interconnects components in home theaters and other digital high...
(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
MADI
Multichannel Audio Digital Interface, MADI or AES10 is an industry-standard electronic communications protocol that defines the data format and electrical characteristics of an interface carrying multiple channels of digital audio. The Audio Engineering Society standard for MADI was originally...
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
Yamaha
Yamaha may refer to:* Yamaha Corporation, a Japanese company with a wide range of products and services** Yamaha Motor Company, a Japanese motorized vehicle-producing company...
's mLAN
MLAN
mLAN, short for Music Local Area Network, is a protocol for synchronized transmission and management of multi-channel digital audio, video, control signals and multi-port MIDI over a network. It exploits several features of the IEEE 1394 standard, also known as FireWire, upon which it is based, to...
protocol exclusively uses the FireWire interface.