Sound output issue from a Dolby CP650 with cat790 used for digital cinema projection

[James Norman]

Hi. Thanks so much Carsten and Leo for the help. Im giving it a go. However, regards wiring, forgive my ignorance. Leo, if we take audio channels 1,2 which has 2 RJ45 pinouts (blue and orange wires). The DB25 input pin config only specifies pin1 for L,R Input+. (Blue wire) Where should L,R Input- go (orange wire)?

[James Norman]

Looking at db25 wiring ref images, would the corresponding negative wires follow this pattern? Not sure as the db25 pin diagram for the cat790 does not specifty these pins

[Carsten Kurz]

The ICMP as every server has AES3, which is differential/balanced. The CAT790 inputs are single ended, SPDIF. They are not 100% compatible, but you could try to connect all (-) to GND (DB25 pin7). AES has higher voltage swing, though, and usually a transformer/resistor network is used for attentuation.
I'd first try only with AES pair 1 to SPDIF1 on the CP650/CAT790. This should reveal at least L/R, and you can proceed from there. There is a special adapter board available from Odyssey managing everything. Odyssey closed down recently, but you may still find one of these boards.

https://www.odysseyproducts.com

https://dargcocinema.wixsite.com/mysite

It is possible to wire your own converter cables, depends on your soldering experience.

https://www.ranecommercial.com/legac...S3_&_SPDIF.pdf

The CAT 790 installation manual also has a passage with the necessary impedance/level conversion at the end:

http://mtedigital.com/dolby/CP650/Ca...tallManual.pdf



- Carsten

[Leo Enticknap]

...but you could try to connect all (-) to GND (DB25 pin7).

I've done that (from an Alchemy/ICMP): splicing four conductors into one looked pretty messy when all was done, but it worked.

[Steve Guttag]

Technically (and the way I've always done it (either home made or via the Odyssey board that just became no longer available) is to also convert from AES3 (110Ω) balanced to AES3-id (75Ω) unbalanced too. The CP650 uses 75Ω unbalanced inputs. Technically, the signal should also be attenuated as well since AES3 runs at a significantly higher level. The fact that the CAT790 sort of deals with the mismatch is besides the point. I've always used Canaire in-line transformers.

Odyssey got it right too. Here is the schematic to their board the 650-OPT-IO-AES(-RJ)

Screen Shot 2020-11-25 at 11.18.27 PM.png

[James Norman]

Hi. After rewiring the rj45-db25 connector correctly, sound out is correct and functioning. Thank you all for the great help)