Way back in the all-analog days, just about the only thing engineers had to worry about when connecting gear was whether an audio signal was balanced or unbalanced. These days a single studio might have a dozen different interconnect protocols running simultaneously, so here’s a guide to the ins and out of digital connectivity.
Sometimes referred to as “lightpipe,” ADAT optical is a 24-bit, 8-channel interface originally developed by Alesis for their ADAT machines. ADAT optical employs the same cable and TOSLink connectors used for optical S/PDIF (see next page). Maximum recommended cable length for ADAT optical is five meters, though some studios have had success with longer runs.
The AES/EBU format provides two channels of digital audio over a 110-ohm balanced line using XLR connectors. The two big differences between AES/EBU and S/PDIF are that (1) AES/EBU can be used for cable lengths up to 100 meters (versus S/PDIF’s 15 feet), and (2) AES/EBU doesn’t incorporate SCMS (serial copy management system) — meaning that you won’t have to deal with the copy protection “flag” often added to digital audio when copied via S/PDIF. AES/EBU supports resolutions up to 24-bit/48 kHz.
Ethernet is a protocol for “local area networks” (LAN’s), meaning that the entire network exists in a single building. Ethernet allows computers, printers, and other devices to be connected for access to each other. The common link configuration for Ethernet uses Category 3 or 5 cable terminated with 8-pin RJ-45 connectors. Several types of Ethernet are in use, including 10base-T (10 megabits per second or Mbps), 100base-T (100 Mbps) and 1000base-F (1 Gbps).
With a bandwidth of 400 Mbs, FireWire is an efficient way to transfer digital audio between hard drives and a DAW. FireWire can deliver data via cables up to 4.5 meters in length, and supports 63 devices on a single bus. External power supplies aren’t always required for FireWire devices, since a computer can provide up to 45 watts of power on the FireWire cable.
The newer FireWire 800 (IEEE 1394b) supports data transfer rates up to 800 Mbs, can send data 100 meters, and is backward-compatible with its predecessor. Though FireWire 800 ports differ from FireWire 400 ports, adapter cables allow use of FireWire 400 products on the FireWire 800 port, although this may reduce the speed of the connections.
Fibre channel is a high-performance, bi-directional interface designed specifically for fast data transfer between servers, storage systems, and peripherals. Fibre channel can transfer data at speeds ranging from 1 to 2 GB per second. Up to 126 devices can be connected to a Fibre channel loop (system), and redundant data paths allow information to be stored to multiple drives simultaneously. In a Fibre channel RAID (Redundant Array of Independent Disks) system, the maximum transfer rate can increase significantly depending on the number of drives; the more drives present, the faster the transfer rate.
MADI (Multichannel Audio Digital Interface) is an AES spec for multi-channel digital connection between mixing consoles and multitrack recorders. Using either standard 75-ohm coax or fiber optic cable for runs up to 100 meters, MADI supports 56 digital audio channels at 44.1 or 48 kHz with 24-bit resolution. MADI can also be used to transmit audio at higher sample rates such as 96 kHz with a reduced channel count (26 channels).
Developed by Gibson Labs, MaGIC (Media-Accelerated Global Information Carrier) is a digital audio network for musical instruments, processors, recording, and mixing devices. MaGIC provides up to 32 channels of 32-bit bi-directional audio at sample rates up to 192 kHz. Using standard Ethernet cable, MaGIC reduces latency to 250 microseconds (point-to-point) across 100 meters. The system can work either freestanding or host-based, and is intended as an open-architecture system.
Celebrating its 20th anniversary, MIDI (Musical Instrument Digital Interface) control data is employed in just about all musical instruments and audio gear currently manufactured. In its simplest form MIDI allows remote control of one device from another with messages such as note on/off, and program and volume changes. Sixteen different channels are available for MIDI transmission. MIDI data can be stored in hardware and software sequencers, and may be used for automating mixers and effect devices. Since MIDI allows transmission and reception of clock data and MIDI timecode (MTC), it can also be used to synchronize a drum machine to a sequencer, or lock a sequencer to timecode from an external device such as a tape recorder. MIDI signals are unidirectional, meaning that separate input and out connectors are required.
Developed by Yamaha, mLAN is a digital audio network using FireWire-compatible cables to connect musical instruments, recording gear, and computers. Up to 127 devices may be connected via mLAN, with data transmission rates of 200 Mbps. mLAN devices may be enabled or disabled from a computer (or other controller) without physically unplugging the cable, and devices can be safely hot-plugged. A single mLAN I/O port carries MIDI, audio, serial data, and video information between devices.
Many samplers, DAWs, and digital audio “studio in a box” devices use SCSI (Small Computer System Interface) for connection to external drives. SCSI provides data transmission rates up to 80 MBps, and supports eight devices: the “host” (i.e., the computer) plus seven peripherals. Each must be set to a unique ID number, and the last device in the chain must be terminated to ensure proper operation of all devices connected to the bus.
When most people refer to “SCSI” they mean SCSI-2, which uses an 8-bit bus, supports data rates up to 4 MBps, and supports multiple devices. Wide SCSI uses a different cable (168 cable lines, 68 pins) to support 16-bit transfer with a rate of 10 MBps. Fast SCSI employs an 8-bit bus, but doubles the clock rate to achieve data transfer of 10 MBps. Fast-Wide SCSI uses a 16-bit bus for transmission rates of 20 MBps. Ultra SCSI uses an 8-bit bus for data rates of 20 MBps. Ultra-Wide SCSI (sometimes referred to as SCSI-3) employs a 16-bit bus for transfer rates of 40 MBps. Ultra2 SCSI uses an 8-bit bus for data transfer at 40 MBps. Wide Ultra2 SCSI uses a 16-bit bus and supports data rates of 80 MBps. Got all that?
Primarily implemented in gear manufactured by Sony, SDIF-2 (Sony Digital Interface Format) requires three 75-ohm BNC connections: one each for left and right channels plus a word clock connector. The most frequently used sample rates are 44.056, 44.1, and 48 kHz, although any sample rate can be used with bit depth up to 20.
S/PDIF (Sony/Philips Digital Interface) provides two channels of audio with resolution up to 48 kHz/16-bit. Unbalanced RCA phono connectors are often used for transmitting S/PDIF signals over 75-ohm copper cable, but TOSLink optical connections are also popular. Unfortunately consumer-oriented S/PDIF gear generally employs SCMS (serial copy management system), meaning that a digital copy of a master may not, in turn, be digitally copied at 44.1 kHz. This can be a problem in certain situations requiring safety copies of DAT master tapes.
USB (Universal Serial Bus) is a means for connecting peripherals ranging from printers to MIDI to digital audio interfaces. USB devices can be “hot plugged” and there is no need for termination or device ID numbers on a USB bus. All USB devices use a cable with different connectors at each end — one for the computer, the other for the device — so a USB cable cannot be incorrectly patched. Most computers feature at least two USB ports, but by adding a USB hub (or a series of hubs), up to 127 USB devices may be connected simultaneously to a CPU. USB provides power to devices through the USB data cable, allowing (for example), a removable disk drive to be connected using a single cable.
As you might expect from the name, USB2 (or USB 2.0) is a faster version of USB, upping the transfer speed to 480 MBps. USB2 uses the same cables and connectors and is both forward- and backward-compatible with USB1.
TDIF (TASCAM Digital Interface) is an 8-channel interface developed by TASCAM for use in their digital mixers and DA-series of digital recorders. TDIF is bi-directional (the input and output are carried by the same cable/connector), simplifying connections. There is support for eight channels of 16-bit/48 kHz resolution. Some manufactures have developed “bit-splitting” storage schemes enabling TDIF devices to store four channels of digital audio with 24-bit/96 kHz resolution.
TOSLink (Toshiba Link) is a 2-channel optical digital audio interface, essentially S/PDIF-formatted data transmitted via fiber optical cable. A TOSLink output can be connected to an ADAT Optical input, in which case the output signal shows up on the first two channels of the input.
Also known as word sync, word clock is a very precise way of ensuring that audio and video gear are “on the same page.” Not to be confused with SMPTE code (which determines the location and frame rate of audio or video data), word clock ensures that synced devices start their audio or video samples at the exact same time. When word clock isn’t present, audio from two different devices may run at the same rate (for example 44.1 kHz) but this doesn’t mean that the samples start at the same instant — which results in audible clicks and dropouts.