I finally got a reply from Tascam (or rather Teac), so I'll answer my own questions here for the sake of anyone who needs this info.
1. The default digital output format of the UH-7000 is AES/EBU (AES3), for which the electrical specs is 110 Ohms impedance and 4 volts average signal level. If you select SPDIF format as the digital output format of the UH-7000, this changes
only the bitstream encoding from AES/EBU to SPDIF
but not the electrical specs of the output signal, which will remain as 110 Ohms impedance and 4 volts average signal level.
2. The UH-7000's balanced digital output is
transformer balanced.
Using the above info, we can now make the proper cable to connect the XLR digital output to an RCA SPDIF input. For this purpose, please refer to
Rane Note 149:
http://www.rane.com/note149.html
Since we know that the UH-7000's digital output is transformer balanced, we can use the wiring diagram in
Fig. 1 of Rane Note 149 which uses 4 resistors to change the impedance to 75 Ohms and the average signal level to 0.5 volts (the standard electrical specs for SPDIF). Note that for this to work, the cable length must be short, say, 9 ft or less, to be safe. The cable to use in this case would be a 75-Ohm rated coax cable with female XLR termination on one end (which connects to the UH-7000 digital output) and RCA termination on the other end (which connects to the SPDIF input). You can use
Belden 1505A bulk cable to make this cable.
Now, what about going from an RCA SPDIF output to the UH-7000's digital input? How do we make the cable for that? Well, as far as bitstream encoding is concerned, there is no problem. We can send either an SPDIF or AES/EBU bitstream to the UH-7000's digital input because the UH-7000 automatically decodes the input signal's bitstream encoding and configures itself to properly accept the detected encoding. But this is only as far as the bitstream encoding is concerned. The impedance of the digital input will still remain as 110 Ohms with a minimum of 0.2 volts input signal level required in order to be able to process the input signal. Since the SPDIF output of 0.5 volts (as per SPDIF standard specs) can easily supply the minimum of 0.2 volts signal level required, our only problem is the SPDIF output impedance, which needs to be converted from 75 Ohms to 110 Ohms. For this purpose, Rane Note 149 recommends the use of a
transformer to perform this conversion (see Fig. 4). You can use the
Canare BCJ-XP-TRC transformer for this purpose:
http://www.bhphotovideo.com/bnh/con...sCFZJlfgodiqsMIw&is=REG&ap=y&m=Y&A=details&Q=
Note that the cable to use in this case would be a 75-Ohm rated coax cable with RCA termination on one end (which connects to the SPDIF output) and 75-Ohm BNC termination on the other end (which connects to the BNC jack of the transformer). You can use
Belden 1505A bulk cable to make this cable.
The Kludge Approach
For those who do not want to bother with impedance and signal voltage conversions, I have heard that many UH-7000 users are able to get by just fine with simple balanced-to-unbalanced (or vice versa) cables without any conversion components. The only requirement is that the cable length is very short, say, no more than 5 ft. For such a short cable, the impedance does not matter much. The only problem is when going from the UH-7000's digital output, which has an average signal level of 4 volts, to an SPDIF input. If the SPDIF input cannot tolerate such a high signal level then it will distort and not work (although it will not be damaged). Depending on your equipment, some gear are designed to accept up to 5 volts and some are designed to accept only up to 1 volt signal level on their SPDIF input. If you know that your gear can handle the high signal level (ask the manufacturer or review the technical specs), then you can use the wiring diagram in
Rane Note 110:
http://www.rane.com/note110.html
See
Fig. 4 diagram # 6 to make the XLR to RCA cable that goes from the UH-7000 digital output to the SPDIF input.
See
Fig. 4 diagram # 17 to make the RCA to XLR cable that goes from the SPDIF output to the UH-7000 digital input.
To make either cable, you can use
Belden 1800F 110-Ohm bulk cable.
