SMR logoDigital Format Converters - MSB, Midiman and Fostex
The proliferation of digital source components has meant that many receiving components simply don't have enough inputs and to make matters worse, two connection formats are used, Toslink optical and coaxial electrical.   Nigel R. Pond investigates products from Fostex, Midiman & MSB which could offer a solution.

Many thanks to Michael 'Hucky' Huckler of Midiman for providing the CO2 sample.

'I Can See The Light At The End Of The Cable'
or...
'How To Coax Your Digital Connections'

Introduction

One of the commonest questions we regulars see on the SMR Forums goes something like this:

Toslink Optical Digital Cable"Help please, I have a big problem!!  I am just getting started in home theater and have a new DVD player and Dolby Digital receiver.  The sales guy told me that the DVD player has a digital output and the receiver a digital input, but I cannot figure out how to connect them.   The connections marked digital on each dont look remotely alike.  One is marked "optical" the other "coax".  Did the store sell me incompatible gear?  What do I do now?."

Well the obvious response is that the store should have ensured that the equipment they sold you was compatible with the gear you already had, or even more obviously with the other gear they were selling you at the time, but that really isnt very helpful.  But all is not lost.  There are several products available to resolve these incompatibilities and I will take a look at three of them, but first a little recap on digital audio interconnection formats.

 

Back to Basics

DVD Player Digital Outputs - Coaxial and ToslinkAt the consumer level there are basically 2 formats for the transmission of digital audio signals (and here I am talking about the physical connections not the format of the digital data being transmitted), coaxial or electrical, using an RCA-type jack and plug (or sometimes a BNC bayonet connector) and a shielded, 75 ohm impedance, cable and Toslink or optical, which transmits the digital data in the form of light down a plastic fiber-optic cable.  The Toslink system was originally developed by Toshiba, hence the name.  Coaxial cable is also widely used for RF transmissions to avoid signal contamination and was originally developed for carrying colour television signals from aerial (antenna) to television.   Both of these connection formats fall, somewhat confusingly, under the wider description of S/PDIF (otherwise S/P-DIF, or S/P DIF) connections, short for Sony/Philips Digital Interface Format, a jointly developed standard for the transmission of digital signals between equipment (IEC-958 defines the standard).  The image to the right shows the rear panel of a standard DVD player, complete with a choice of coaxial or Toslink S/P-DIF outputs.

ST-Glass CableThere are other connection formats usually found on professional gear, but sometimes seen on "higher-end" consumer gear: the AES/EBU, a standardised format for digital transmission developed jointly by the Audio Engineering Society and the European Broadcasting Union and defined by standard IEC-958, which uses a 3 pin, balanced XLR-type connector and 3 conductor, 110 ohm impedance cable; multi-mode AT&T or ST-Glass (pictured left), and single mode laserlinque (a proprietary format developed by Theta Digital, I believe) both using a locking bayonet connector and a glass fiber-optic cable.

Most readers will already be aware, but it is worth mentioning again, that both sorts of S/PDIF formats (and AES/EBU on gear that has them) are now used to carry all the main digital signal formats commonly in use at the moment - PCM (pulse code modulated - the format used on CD); Dolby Digital®; and DTS®.   Coaxial connections are also used to carry the AC-3 RF output from a laserdisc player to RF demodulator.

I ought to mention two other connection formats that you will see mentioned in the press and on the SMR Forums - firewire (IE 1394-1995) and I²S.  Both of these formats have been mentioned as contenders to be the specified format for connecting DVD-Audio players (when we finally see them) to digital preamps/processors.  One of the music industrys Coaxial Digital Cable (RCA)big concerns with the DVD-Audio format is the possibility of piracy.  Audiophiles worry about having the most "clean" and jitter-free connections.  Firewire is currently used in the computer industry but its bi-directional capability has attracted attention for DVD-Audio, the idea being that the DVD-Audio player will be able to interrogate the box at the other end of the cable and pass the digital audio data only when the receiving box does not identify itself as a digital recording device.  I²S has been used by some manufacturers already (Camelot Technologies and the late lamented Audio Alchemy to name but two) and has attracted audiophile attention because the all-important clock signal is carried separately from the audio data.  At the date of writing the current version of the DVD-Audio specification is silent about connection formats.

 

Converters

Ill be looking at three products which convert digital connections between coaxial and Toslink formats; the Fostex COP-1; the Midiman CO2; and, MSBs Digital Director.  There is another product, Core Sounds Digital Format Coaxial Digital Cable (BNC)Translator, but, unfortunately, due to high demand, Len Moskowitz (who is Core Sound) was unable to let me have a sample for review.   If demand slackens off and a review sample does become available, I will update this article.  Other manufacturers have format translation functionality built into some of their products (e.g. Audio-Technica, Sonic Solutions; and z-systems), but these are aimed more at the professional recording studio market so probably not of much interest to the home theater enthusiast.

If you search the web you will find that there are schematics readily available for constructing a converter yourself (see for example the construction projects links at the MiniDisc Community page - but as these require some skill with a soldering iron and circuit layout, they are also outside the scope of this article.  Of course if anyone would like to contribute their experiences of building such a converter, I would be very interested in seeing it.

 

Fostex COP-1

Readers may be unfamiliar with Fostex.   The company specialises in equipment for the pro-audio market - recording studios, sound professionals etc.  Consequently its products are usually only available at music stores and places catering to the music professional.  Fostex has obviously seen a demand for a digital format converter in that market, unlike any of the home audio manufacturers in the home audio market.

The COP-1 is about the size of a cigarette packet, so it will fit in your system rack behind your components with no difficulty, and is a well made unit in a metal case - obviously Fostex realises that gear is subject to abuse in the Fostex COP-1professional environment!  The unit has 1 coax and 1 Toslink input and 1 coax and 1 Toslink output, plus a DC power supply jack (the wall-wart power supply comes with the unit) and a very bright green "power on" LED.  The coax jacks are not gold-plated.  Hmm... I guess that means that the professional market sees no value in gold-plated connectors, or maybe continual plugging and unplugging keeps oxides at bay.  Inside, the circuit board seems neatly laid out.  The COP-1 has no on/off switch so I presume that it is designed to be left on all the time.  The COP-1 is a bi-directional unit, meaning that it can convert between coaxial and Toslink and vice versa at the same time.  This would give the unit maximum functionality in the recording studio and makes it very flexible for home use too.

I tested the COP-1 in my system for several days.  By connecting it to my DVD transports second coax output and by toslink cable to my Lexicon DC-1, I could compare how it sounded side-by-side with a direct coax output from DVD to DC-1.  After much swapping around of different types of source material I could hear little if no difference between the 2 connections, meaning that the unit is not degrading the sound in any way (assuming you believe that anything between digital output and digital input can affect the stream of ones and zeroes that make up a digital signal).  I also tested its capabilities converting from coax to optical and bi-directionally with the same results.

The COP-1 can handle PCM, Dolby Digital and DTS bitstreams but not AC-3 RF from laserdisc.

The only downside of the COP-1, for what its worth, is that it is shipped in a rather drab looking box and the "instructions" do not really instruct at all.  This is obviously a result of the professional market at which the unit is aimed and, because of the basic operation of the unit, not that big a deal.

The COP-1 is an excellent choice if you do not need the extra flexibility of the other units.

 

Midiman CO2

As the name suggests, Midiman is another company in the professional music business.  The CO2 is about the same size as the COP-1 but a slightly different shape.  It has the same number and types of inputs and outputs (2 of each), a DC power supply jack (wall wart included) and a red power indicator LED.  It too has no on/off switch.  As with the COP-1, the circuit board appears to be well laid out and constructed and again we do not find gold plated RCA jacks.   One extra that the CO2 has is an isolation transformer on its coaxial output to prevent any ground loop problems, a thoughtful touch.

The CO2 differs from the COP-1 in that it can operate in 3 modes.  A three-position slider-switch at one end chooses between bi-directional (same functionality as the COP-1) and two "convert-and-pass-through modes".  In the first of these modes it will take an optical input and convert it to coax on the coax output, while at the same time outputting the data on the optical output.  So the unit can function as a line repeater for long cable lengths or as a means to Midiman CO2convert a single digital output from a source component into two, useful for example if you wanted to send the output to an external DAC (digital to analogue converter) for a second rooms system or to a headphone system.  The other convert-and-pass-through mode works the other way taking a coax input, converting it to optical on the optical output and also outputting on the coax output.

I tried the CO2 in my system the same way as I did the COP-1.  As a straight converter, it functions exactly as expected, although I did notice a very slight drop in volume level with the CO2 in the loop, so it must be slightly attenuating the output in some way, though without degrading the sound.   Care also needs to be taken to ensure that the coax and optical cables are plugged into the unit correctly and that the switch is in the correct position.  When I first hooked it up I must have done something wrong, as switching my DC-1 to the input with the CO2 in the signal path, caused the DC-1 shut down.  Unplugging and re-plugging the cables and checking the switch position solved the problem and my DC-1 happily accepted the input (phew!).  The CO2 works as advertised in all three of its modes and, can handle PCM, Dolby Digital and DTS bitstreams but not AC-3 RF from laserdisc.

In contrast to the COP-1, the CO2 comes in a colourful box and has proper instructions.  I did however notice an error in one of the hookup diagrams which I have reported to the manufacturer.

If you have need for the extra flexibility of the extra output that the CO2 can give you, it is an excellent alternative to the COP1.   For conversion-only requirements, the decision will boil down to street price for each unit which can vary between retailers.  As an aside here, I get the impression from checking out a few pro-music retailers that they are more generous than consumer audio retailers in their discounts from list prices.  Has anyone else noticed this?

 

MSB Digital Director

The Digital Director reflects MSBs experience in the digital audio field.  MSB is primarily known for its digital upgrades such as AC-3 RF outputs for laser disc players, Dolby Digital and DTS processor upgrades, and digital output MSB Digital Directorupgrades.  They also market their own surround processor, 24-bit 96kHz DACs (the Link and Link Gold) and 2 new analogue to digital converters (the ADD and the PAD).  The Digital Director performs its conversion duties as an adjunct to its primary role, that of a digital audio switching unit.   Until fairly recently surround processors came equipped with only one or two digital inputs, which was fine when the only digital sources in common use were cd players and laser disc players.  With the advent of more digital sources such as DVD, DSS, and MiniDisc, MSB spotted the need for a product which could accept all these inputs and direct them to the limited number of inputs on a receiver.

In its standard size the Digital Director has 6 digital inputs (4 coax and 2 Toslink) and 3 digital outputs (2 coax and 1 Toslink).   The unit is powered by a wall-wart power supply which is included.  The housing is a very sturdy metal construction and it comes with rack ears for mounting in a standard 19" rack system.  Coaxial inputs and outputs are of the gold plated variety.  The inputs are numbered from 1 to 6 (numbers 6 and 3 are Toslink, the others coaxial) and are switched on a priority basis - a digital source on a lower numbered input takes priority over a higher numbered input.  So the idea is that you connect you most frequently used source as number 6 and the others in ascending order of frequency of use.  For example, connecting your DSS receiver to input 6 would mean that when all other digital sources are off, the DSS signal will be passed through, but when your DVD player on input 1 is on it takes priority over the DSS input.  The priority input is then output on all three digital outputs.  The unit works by detecting the stream of digital zeroes that a digital source emits when switched on (apart from some DVD players which do not emit anything until actually playing a disc).  So if you like all your gear to be on at the same time so you can amaze your friends with the light display, you will need to revise your thinking to make proper use of the Digital Director.  The front panel display indicates by a green LED which inputs are currently active, ie those which are producing digital data that the unit can lock onto, and a single red LED indicates the input that currently has priority.  So all six green LEDs could be lit at one time but only a single red.  Using the appropriate inputs and outputs the Digital Director can convert between digital formats, but, because of its priority system, only in one direction at a time.

The Digital Director also has some jitter reduction circuitry although I have been unable to take a look inside as I do not yet have a torx head screwdriver of the right size - sneaky move there, MSB (they must know that the first thing I do with a new piece of gear is take the top off and have a good look around inside).

MSB Digital DirectorThe Digital Director works exactly as it is supposed to.  I have had one in my system for a few months now and it handles the digital outputs from my DVD and LD players and CD recorder and sends them to my DC-1 and an external DAC for the headphone end of my system, with plenty of room for further expansion (I am currently thinking about DSS and MiniDisc).  After extensive listening, I notice no degradation of sound quality with it in the signal path.   Switching is performed flawlessly and silently.  The Digital Director can handle PCM, Dolby Digital and DTS digital bitstreams, but not AC-3 RF from laserdisc.

So if you need a digital switching device with or without format conversion, the Digital Director is for you, but it comes at a price - $395 list.  If you just need conversion, then this unit would be overkill.   For those not requiring rack-mounting or the Toslink output, there is a smaller model (6" W × 6" D × 1¾" H) - the "Classic" for $349.

As I find out about other converters I will update this page and the following table.

Fostex COP-1 Midiman CO2 MSB Digital Director
MSRP $95.00 $79.95 $395.00
Power Supply 9v, transformer included 9v, transformer included 9v, transformer included
Coax Digital Inputs 1 1 4
Toslink Digital Inputs 1 1 2
Coax Digital Outputs 1 1 2
Toslink Digital Outputs 1 1 1
Simultaneous Bi-directional Conversion? Yes Switchable No
Pass through & Convert? No Switchable Yes
Digital Source Switching? No No Yes
Sample Rate Compatibility 32kHz-48kHz Any (?) max. 48kHz
Colour Black Grey Black or Silver
Dimensions (width × height × depth) 2¾" × 1½" × 3¼" 3½" × 1.2" × 2¼"
Manufacturers Address 15431 Blackburn Avenue
Norwalk,
CA 90650
45 East Saint Joseph Street,
Arcadia,
CA 91006
14251 Pescadero Road,
La Honda,
CA 94020
Phone number (562) 921-1112 (626) 445-2842 (650) 747-0400
Fax number (562) 802-1964 (626) 445-7564 (650) 747-0405
E-mail
Web site URL http://www.fostex.com/ http://www.midiman.net/ http://www.msbtech.com/

 

© 1999.

 

Text © Nigel R. Pond; HTML © SMR Home Theatre and Images © Nigel R. Pond, Jim Wald (ST-Glass) & SMR Home Theatre cannot be reproduced without permission. Dolby Digital is a registered trademark of Dolby Laboratories Licensing Corporation, DTS is a registered trademark of Digital Theater Systems. The images on this page are digitally watermarked: Digimarc http://www.digimarc.com/

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Last updated 14 March, 1999

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