I haven't ever used this Radio Shack box before. But I just did a bit test with it and it passed.
I'm no big fan of the RS brand, but I had a hard time believing that a relatively simple device like this wouldn't be bit perfect.
Here's the method I used. Perhaps it will be of use to others later:
I generated 10 seconds of rectangular waves in Samplitude. 40 hz waves at about -3dbfs. I set the output device to SPDIF and set the software to loop the playback. I left a small portion of silence between each 10 second segment on playback. This was so I could align the samples later.
Recording chain was RME Multiface SPDIF out -> coaxial cable -> radio shack digital signal converter --> toslink output -> PDAudio SPDIF input
Captured using Live2496 with 16-bit resolution at 44.1kHz with the software set for toslink input.
I recorded about 30 minutes of audio then opened the file in Samplitude. Chopped the object in half using the "T" keyboard shortcut. Then moved the second half of the recording onto an adjacent track. I zoomed in and shifted the bottom object to align the samples so that there was a 10 second segment of sound and then a short gap that repeated for about 15 minutes duration. After the objects were in position I trimmed the leading and trailing edges off of the objects so that both objects were identical in size.
I locked the objects in that position on the timeline. Then I checked the "invert" checkbox for both left and right channels of one object. This inverts that channel upon playback or mixdown. Then I bounced (mixed) the resulting objects. The resulting file was null with no artifacts.
Terry, I don't know how to explain your experience with it. Their must be some other factor involved.