01.09.2007 12:00 AM
Connecting 75-ohm AES Outputs to 110-ohm AES Inputs
Digital audio connections found in television facilities seem to be headed towards the 75-ohm BNC-based standard known as AES-3ID or SMPTE 276M. This is likely due to several factors, including the use of the very common and space efficient BNC connector and the use of standard coaxial cable used for carrying other digital signals such as SDI digital video. However, there is still plenty of equipment supporting the 110-ohm AES format, much of it found in the recording and radio industries. There is no difference in audio quality, as the underlying digital bitstream format is identical, just a difference in impedance and signal level.
It might be assumed that simply using a commonly available passive balun (balanced to unbalanced) for conversion would do the trick. After all, this has been carefully defined in both the AES and SMPTE standards, which recommend a transformer with a 1:1.2 ratio to provide the proper impedance match. However, the difference in signal levels also needs to be considered. The 75-ohm format will output a signal that is 1 volt peak-to-peak when terminated into 75-ohms (it is about 2 volts unterminated). The 110-ohm AES specification allows for signal levels between 2-7 volts. Luckily, most 110-ohm inputs have been intelligently designed with a sensitivity that reaches as low as 0.2 volts, and this gear will work very nicely with a passive balun.
Unfortunately, not all 110-ohm equipment has this proper input sensitivity. Once a 75-ohm source signal is passed through the matching balun, very slight changes in signal level on the 75-ohm side are translated to larger changes on the 110-ohm side and signals can fall outside of the range of the 110-ohm input. As these are time varying signals, it is possible to see errors only sporadically.
The cure is to specify gear that supports either 75-ohms or 110-ohms, so that connections are direct without the need for conversion. This is not always possible, so the next best answer is to ensure proper input sensitivity on 110-ohm gear (test it, do not rely on manufacturers claims) and provide active conversion via an AES DA or other device if necessary.