The Route Less Traveled

Two audio paths - digital and analog-diverge in a personal studio. There was a time when a sound studio, personal or commercial, contained only two types
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Two audio paths - digital and analog-diverge in a personal studio. There was a time when a sound studio, personal or commercial, contained only two types

Two audio paths - digital and analog-diverge in a personal studio.There was a time when a sound studio, personal or commercial, contained only two types of connections: AC power and analog audio. Although a look behind the equipment might have shown a spaghetti of cables, everything was plugged into either a wall outlet or a patch bay. Life was simple, the animals all shared and got along with each other, and Nature reigned supreme.

No longer. Whatever the social state of the animal kingdom, life in the studio is much more involved now, as exemplified by the multiplicity of connections: MIDI, digital audio, computer interfaces such as serial and SCSI, house sync/word clock, time code, and, yes, AC power and analog audio. With each type, the demand for interconnection emerged, and the need for ways to switch, route, and combine these signals and datastreams became acute.

The biggest wrinkle, however, has been the phenomenon of the personal studio, which brought this mess into the project spaces of musicians who previously did not need much more than a way to get multitrack recorder signals into and out of a mixer and route signals to and from a reverb or effects processor. In the course of ten years or so, digital audio tape recorders, DAWs, mixers, and effects processors have proliferated and gotten cheaper.

MIDI patch bays, switchers, and processors appeared long ago and have become commonplace; many personal studios have two or more. But digital audio routing has been a tougher nut to crack. Personal studio budgets are usually much lower than, say, a commercial audio post-production facility, yet many personal studio owners need to route 24 channels or more of digital audio between various devices. The high-end systems used by large audio-post and broadcast houses are out of range for the personal studio budget. Who will save us from being bitten by all those bytes?

BITS AND PIECESThe answer turns out to be less straightforward than in the land of MIDI or analog audio, where manufacturers have responded to the personal studio surge with low-cost 11/44-inch patch bays. This intrepid reporter came face-to-face with the problem after upgrading during the last several years to a studio built around an Alesis ADAT XT20, a Panasonic WR-DA7 digital mixer, and a Mark of the Unicorn 2408 hard-disk I/O system. Although my Toys in the Attic operation has long had DAT decks and a few effects processors with digital I/O, the situation changed with the upgrade, a fact brought into sharp relief the first time I needed to fly audio from the ADAT into the 2408 and bring it back to the DA7 for mixdown.

As I repatched the delicate optical cables used for the ADAT Lightpipe and crawled behind equipment racks to reroute S/PDIF signals going to and from my Lexicon reverbs, I realized I had added a new and large distraction to my creative work. Thus my investigation into affordable solutions for digital audio patching and routing began. (I'm only going to discuss patching and routing here; the subject of format conversion is for another article.)

A VIRTUAL WHO'S WHOFour flavors of digital audio are commonly found in the personal studio: the two 2-channel formats (AES/EBU and S/PDIF) and the competing multichannel formats from Alesis (ADAT Optical, aka Lightpipe) and Tascam (TDIF). To complicate things a little more, some personal studio equipment uses the coaxial form of S/PDIF, while others use the optical format employed by Lightpipe.

Handling the 2-channel formats turns out to be much easier than dealing with multichannel. For a start, AES/EBU and S/PDIF are similar; S/PDIF is a format adapted by Sony and Philips from the then-incomplete AES standard. Although there are differences in the data formats, most notably in how blocks of user data are structured, the two are close enough that most equipment purchased now will accept either format. (Although some frustrating exceptions crop up now and then.)

There are, however, significant electrical differences between the two formats. The biggest difference between the coaxial forms is that AES/EBU is a balanced signal and operates at a higher level than S/PDIF:10V for AES/EBU versus 0.5V for unbalanced electrical S/PDIF. Cable length can reach 100 meters for AES/EBU but only 10 meters for S/PDIF and 5 meters for optical. Several of the routing solutions deal with transforming the electrical characteristics between the two, but no routers convert the data format. You can always add a separate format converter.

Several routers are also capable of the electrical conversion between coaxial and optical S/PDIF. In most cases, S/PDIF coax appears on RCA connectors, but longer cable runs are sometimes achieved by using 70z video cable and BNC connectors (along with a transformer for the signal), so BNCs sometimes show up as S/PDIF coax ports.

Similarly, AES/EBU inputs and outputs most often appear on XLR connectors, but when there are four or more AES connections, DB25 connectors carrying four pairs of inputs and outputs are sometimes used to save panel space.

To complicate matters, every once in awhile, a company goes for a nonspec solution. For instance, Akai samplers use balanced 11/44-inch TRS connectors to carry AES/EBU or S/PDIF signals (switchable), thus saving panel space and money but violating both specifications. Rarely, you find switchable S/PDIF or AES/EBU on an unbalanced RCA connector, which is proper for S/PDIF but out of whack for AES/EBU.

Multichannel routing is another kettle of fish. The starkest news first: if you're looking for a TDIF router, stop now; I found one.

The outlook on ADAT Lightpipe routing is brighter. Lightpipe and optical S/PDIF use the same interface. Because the interface is purely electrical, with no inherent data format, most devices that can route optical S/PDIF can also route Lightpipe. (Note that the signal baud rates are different. Lightpipe works at a higher frequency, and optical transceivers can have different frequency responses.)

Keep in mind that S/PDIF is a 2-channel interface and Lightpipe is an 8-channel interface with a different data format, and the routers examined here do not perform any data format conversion. This means the same router can connect pieces of Lightpipe gear to each other or route one device's S/PDIF output to another's S/PDIF input, but it can't deliver data from a Lightpipe port to an S/PDIF port or vice versa. If you use an optical router with both kinds of interfaces, keep your connections straight.

FROM HERE TO ETERNITYMost options for directing digital audio route the signals electronically, so once connected to the equipment, the user never needs to touch a cable. It is possible, though, to use manual patch bays for some applications. The advantages of manual patch bays are that a large number of connections can appear on a single bay, and you can freely route signals among the connections.

That capability has its drawbacks. First, the manual digital audio patch bays I found are only for use with AES/EBU signals. (I suspect you could use a manual DB25 patch bay for TDIF patching.) Second, the cost is high compared with most routers I mentioned, especially when you factor in patch cord costs, which must be made from high-quality, 110??? cable. Third, more physical connections mean a higher possibility of problems from dirty connectors, which digital audio is more sensitive to than analog audio. Manual patch bays are not usually an optimal choice for personal studios, but they are still a noteworthy option for those with a large number of AES/EBU connections.

Routers use an electrical, rather than a mechanical, means of routing signals, and they offer programmability. In my studio, I have presets for dumping audio from ADAT to the 2408, laying it back in the other direction, bringing either or both into the DA7, or sending analog signals converted by the DA7 (which has excellent A/D converters) to the ADAT and/or 2408. Other presets move audio from the DA7's digital effects sends or the AES I/O option card to the outboard effects processors and DAT machines.

Switchers also can route a single input to multiple outputs, though they do not mix multiple outputs into a single input. Do not mult a digital audio stream with a manual patch bay.

JITTER BUGSThere are some things to be aware of with routers. It is possible for jitter, that old digital demon, to rear its ugly head. According to Z-Systems' Glenn Zelnicker, "For all intents and purposes, jitter is a playback phenomenon only. That is, post-A/D jitter (assuming that it's really post A/D) won't make its way onto a recording. As long as the bits make it from point A to point B, you're in business, and it takes pretty phenomenal amounts of jitter to make complete failure of an AES/EBU link happen. If the bits get to their destination, the jitter will be gone on playback.

"There are a few critical exceptions, though," Zelnicker says. "For example, a jittery router can make for complicated and `cloudy' monitoring if your post-router D/A converter doesn't have good jitter rejection. I wouldn't want to have to make intelligent decisions about dynamics, EQ, and imaging through a jittery monitoring chain. Jitter issues become more critical at larger word widths [for example, 20- and 24-bit words]. That is, to maintain the advantages that increased dynamic range brings, it's necessary to hold the jitter to an even lower level.

"That said, there are two levels of jitter about which to be concerned: first, small amounts of jitter introduced into the AES/EBU or S/PDIF stream, causing variations in the recovered clock; and second, huge amounts of jitter, making recovery of the data bits impossible. Numbers 1 and 2 can be caused by a multitude of things, but just remember that number 1 makes monitoring difficult and number 2 makes everything difficult.

"Accumulated jitter could be indicative of a problem with the design of the router in question," Zelnicker says. "The design flaw could have something to do with common-mode noise propagation, in which case, depending on the architectures of the boxes connected to the router, noise on one unit could make its way to the other units. If one of the units happens to be an A/D converter, it would be quite deleterious to the conversion quality if the common-mode noise from a bad processor made its way back through the router, down through the digital output of the A/D, and on to the A/D's power rail - which the A/D chip's clock reference happens to be using as a voltage source. A good A/D shouldn't behave this way, but I've seen plenty that do."

Routers aimed at personal studios are generally asynchronous; that is, none of the streams passing through them need to sample at the same rate or synchronize to any other. However, properly clocking all the devices from a single master source eliminates the possibility of glitches when switching.

Aside from the jitter issue, format determines the ability of routers to carry data encoded at high sampling rates and word widths (such as 24-bit, 96 kHz audio). The ADAT Lightpipe format, for example, doesn't support sampling rates above 48 kHz.

ROUTES TO TAKEWith that in mind, here's a look at options for routing digital audio in the personal studio:

Digital Labs. Digital Labs manufactures the Fiber-4 ($295), a 4 5 4 optical router. Routing is performed with rotary encoder switches on the Fiber-4's front panel. Digital Labs states that this technology allows routing without adding jitter.

Fostex. The Fostex DS-8 ($389) is a simple router with six optical and two coaxial inputs and outputs. You use the front-panel rotary switches to route data, and the unit converts between coaxial and optical S/PDIF, but the DS-8 is the only nonprogrammable switcher of the bunch. Optical I/O port 8 on the front panel makes it easy to connect carry-in ADATs or S/PDIF gear.

Friend-Chip. Friend-Chip, distributed in the United States by Ucik, offers three routers. The smallest is the Digi-max (DMX) 12/8 ($499), which includes six S/PDIF coax, and six optical inputs and eight output channels. Four of the output channels feature simultaneously available S/PDIF coax and optical; two are coax only, and two are optical only. For $199, the unit can upgrade to a 12 5 12 routing matrix. Ten presets can be stored and recalled from the front panel or via MIDI. You can dump and load the unit's settings via System Exclusive messages. Friend-Chip's remote control software for Windows also uses SysEx.

The Digi-max 16 ($399) and Digi-max 32 ($599) are modular frames that hold four and eight 4-channel I/O modules, respectively. The modules cost $199 each, and four configurations are available: four channels of coax S/PDIF I/O, four channels of optical I/O, four channels of AES I/O (on a DB25 connector), and a "two of each" module featuring two S/PDIF coax channels and two optical I/O channels. Friend-Chip also offers the only TDIF routing module I was able to find.

Like the other routers, the modular systems support MIDI Program Change and System Exclusive dump/load, and offer MIDI remote control with Windows software.

Midiman. Midiman's DigiPatch 12 5 6 ($499) was one of the first digital audio routers to appear at a personal-studio price point. The DigiPatch features 12 inputs - six each of S/PDIF coax and optical - and six output channels, each of which can be directed to S/PDIF coax or optical connections. The unit's 50 presets can be recalled from the front panel or through MIDI. You can also use MIDI to configure the DigiPatch from a Macintosh or Windows computer and to load and dump the unit's contents through System Exclusive messages.

Z-Systems. The modular Detangler line from Z-Systems includes the z-8.8 ($980 to $1,325, depending on configuration) and z-16.16 ($2,200 to $2,550), which host 8 and 16 I/O modules, respectively. Available modules provide AES/EBU on XLR connectors, S/PDIF coax on RCA or BNC connectors; or optical connectors, which can carry S/PDIF or ADAT Lightpipe datastreams. Several stock configurations are available, and custom configurations can also be specified. A module's input and output can be different types. All inputs except optical are transformer-coupled for ground isolation, and the unit performs electrical conversion between AES and S/PDIF. Detanglers store up to 99 presets, which are stored and recalled by front-panel controls. These units cannot be external controlled.

The z-8.8a ($1,500), a versatile optical router, features seven optical I/Os and four pairs of S/PDIF electrical I/O on RCA connectors. In addition to routing ADAT, S/PDIF, or AES/EBU signals over optical lines, the 8.8a converts bidirectionally between an ADAT Lightpipe stream and four coax stereo S/PDIF streams. To accomplish the coax S/PDIF-to-ADAT conversion, the z-8.8a provides eight channels of sample-rate conversion to synchronize the S/PDIF streams. Up to 99 presets can be stored.

The Detangler Pro line contains units accepting 8 ($1,100), 16 ($1,600), 32 ($5,000), or 64 ($6,000 for 64 5 16 and $12,000 for 64 5 64) modules. The Pro units are the same as the Detanglers except they do not accept optical modules, and they make AES/EBU connections available on a DB25 connector instead of XLRs.

All Detanglers except the z-8.8 allow computer control through Mac and Windows software available from Z-Systems. Control protocols are available to Z-Systems users who would like to "roll their own" remote control system.

DOCK OF THE BAYSADC. ADC makes two digital audio patch bays. The DAB3-14MKIINS ($1,355) is a 96-point TT bay. The DAL207-4MKIV ($800) 48-point bay is available in TT or 11/44-inch TRS configurations. The rear of the DAL207 trails a 4-foot umbilical that terminates in ADC's UltraPatch punchdown panel.

Audio-Service. The 4U rack-mount Digital Audio Interconnect System (D.A.I.S.; approximately $9,380 for 56 5 56 with 14 interface cards covering different formats) from Audio-Service of Hamburg, Germany, is designed for studios with extensive digital routing needs and is priced accordingly. Distributed in the United States by X-Vision, it consists of a card cage with internal busing and front-end software. In this cage, you mount up to 14 single-height or 4 double-height Yamaha YGDAI interface cards (the same type used in the Yamaha 02R digital mixer), which provide a variety of I/O options, such as TDIF II, ADAT Lightpipe, AES/EBU, and Yamaha Y2. The unit also supports SDIF-2, AES/EBU with sample-rate conversion, analog, and passive AES/EBU or SPDIF. Signal routing is set using the front end software; according to the manufacturer, the Windows-only application works fine on a Power Macintosh running under FWB SoftWindows 98 or Connectix VirtualPC with Windows 98. Bells and whistles abound.

Neutrik. Neutrik's NPPA patch bay ($1,095 in the standard configuration) is digital-capable. A 96-point TT (bantam) jackfield is available with a variety of rear-panel connection options. The standard configuration is a spring-loaded terminal block, but there are units with 56-pin or 90-pin ELCO, 50-pin D connectors, or solder lugs. The jacks have two gold-plated contacts for each terminal (tip, ring, and sleeve). As with the Switchcraft bay, the NPPA offers flexible normaling schemes.

Switchcraft. Switchcraft internally wires all its patch bays with 110z cabling so you can use analog or AES/EBU digital signals. They are impedance-tested to meet the military specification. The company offers two lines of 48-point patch bays: the MTP and TTP series. Both series include 2U and 1U rack-mount patch bays and are available with the company's Front Access option, which means the unit sits in a slide-out tray to allow reconfiguring from the front. The units offer normaling.

MTP series ($700 to $1,000, depending on the exact configuration) have 11/44-inch TRS longframe jacks. The rear panels are typically outfitted with PPTs; these industry-standard push terminals, also known as "punch blocks" or "punchdowns," provide easy reconfiguration and positive connections. The MTP punchdowns are not spring-loaded. The TTP Series (which cost anywhere from $800 to $1,500, depending on configuration) uses TT jacks. Rear-panel options include punchdown terminals or EDAC/ELCO connectors.

GOTCHER ROUTER RIGHT HEREMost personal studios accommodate a dozen or less digital I/O connections. In such cases, one or two routers probably suffice. If you deal with a variety of digital audio formats, you may be a candidate for a router with a modular architecture, in which you can choose the configuration. On the other hand, if your studio contains mostly S/PDIF coax or optical connections, you can opt for a less expensive alternative. If you have extensive patching needs, such as a number of AES devices spread around several rooms, you may consider a manual patch bay.

For better or worse, narrowing the field of candidates will rarely be a problem, because only a few of these units are priced for the personal studio. However, these products adequately cover most needs. If you plan your routing system carefully, you should find an affordable solution to ensure the back of your rack doesn't become a vacation spot for digital gremlins.