Digital audio and groundloops

Basics of digital audio interfaces

Digital audio connection (AES/EBU and S/PDIF) are not by themselves sensitive to groundloop problems. A small ground loop hummign does not change the digital data at all. If the ground loop problem is really serious, then some problems are possible. Basically in digitla interfaces the signal either get through correctly or not.

Although the digital audio connections themselves are not much disturbed by groundloops, they can be sometimes the connections which form the loop and cause a noise entering to the analogue connections of the equipments on the both ends of the digital connection.

AES/EBU and S/PDIF-connections were originally designed so thet they should be free from groundloop. AES/EBU-interface uses the well known symmetrical connections with transformer isolation and an output impedance of 110 ohm. So an AES/EBU connection on the professional equipmet should not cause any groundloop problems when connected properly.

S/PDIF-connection (also known as IEC60958) was also originally designed to be transformer isolated at the transmitting end. So theoretically ground loops should be impossible, as each coaxial S/P-DIF output should be equipped with an insulating transformer, in order to avoid ground loop problems. But in reality the sitation is not so good, because in some cases because in some equipments the S/PDIF-output are not isolated from the rest of the circuitry.

Reasons for S/PDIF-problems

In reality many coaxial S/P-DIF inputs and outputs of soundcards and such devices lack such transformers. Maybe those products are so cost sensitive that the manufactuers have to cut corners in this way. And, even if the coaxial S/P-DIF output of an audio component is equipped with a transfomer, hat doesn't mean that the output is isolated, because in some equipments the ground of the S/PDIF output connector (RCA-connector) is still permanetly connected to the same ground as all the audio connectors instead of correctly left floating.

Usually the absense of isolation in S/PDIF-connection is not a problem in normal use, but can be a problem among with those audio experts who want the best sound quality without humming. Usually the noise and hummign problems are the reasons to go to digital audio transmission, and it is quite sad that some manufactuers put out products which can't usually fulfill their promises in real life applications, because of the S/PDIF-interface ground loop problem.

Solving the problems with S/PDIF

If your sound card equipped with optical S/P-DIF (Toslink) inputs and outputs, try to use them instead of coaxial ones. The optical cable will mode the digital audio signals but at the same time provides a complete galvanic isolation between equipments. So optical interfaces are free from humming.

If you want to use the coaxial output and your equipment with S/PDIF output lacks the output isolation transformer on S/PDIF-connection you have to make your own extra isolation transformer.

Elector Electronics magazine issue 7-8/1999 recommend building the transformer for S/PDIF isolation in the following way:

The transformer must have good coupling factor, so the transformer core must be a toroidal core made of high permiability material. The prototype described in the magazine uses Philips Type TN13/7.5/5-3E25 core which has permiability rating (yt) of 4500. THe primary and secondary windongs consisted of 6 turns of 0.5 mm diameter enamelled copper wire laid on opposite sides of toroid. The transformer used in this prototype was described to have a bandwidth raged from 50 kHz to 17 MHz, which is more than adequate for an S/PDIF link.

The transformer should be fitted directly to the source and the receiving end must be protperly terminated for reliable operation. The reason for this is that the transformer input and output impedances are not exactly 75 ohm.

Tomi Engdahl <Tomi.Engdahl@iki.fi>