[woodsyi]

Wayne,



I came up on this bit googling spdif.  

Isolation transformer for S/PDIF output
If your equipment (for example sound cards) has an S/PDIF output which does not have an isolation transformer at the output, you can easily add the transformer to it by putting a suitable transformer afther the original output.

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.

Is this a good thing?
Is it already implemented in SB2/3 digital out?
If not, can and would you implement it?

[Wayne1]

In theory, this is a good thing.

In practice, in the SB this can be a bad thing.

There is no transformer in the output of the digital signal path. In order to put one in, you first have to find a place to physically fit it. In both the SB2 and SB3 there is not a lot of real estate near the digital output circuitry. This means you would have to locate the transformer away from the connector. Then you will have to run leads to and from the transformer. This area is also right next to the wireless card and the crystal oscillators. So the transformer and wiring would be constantly bombarded by RFI.

I did build a modded SB2 with a transformer in it. The first SB2 I modded for Zybar had one. I listened to it for a bit and felt the high end was a bit "hot". I let him take it with the knowledge that the transformer may have to be removed. Sure enough, the signal remained very harsh. He sent it back to me and I pulled the transformer and he is happy with it to this day.

Adding a transformer to certain sources of digital signals can be beneficial. I do not believe it would help the SB with all the RFI being created inside it. All the connections to and from the transformer would end up being antennas.