Film-Tech Forum ARCHIVE: CP650 low digital input

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Posted by Leonid Dragilev (Member # 4611) on 03-20-2014, 02:14 PM:

Hi
i have a CP650 with cat790 installed. We used it in external 6 chanels analog mode. recently i tried to switch to digital. I got an AD convertor and AES transformers. But when i send -20 dB signal to AD i get about 12 db level drop comparing to analog chain. AD shows -20 and analog is aligned as well. Any help will be greatly appreciated!

Posted by Pete Naples (Member # 761) on 03-24-2014, 04:26 PM:

What make/model is the D/A convertor?

Check the wiring, if it's not a Dolby DMA8/DMA8+ it may not be pin to pin. Also check the -V arrangement [Wink]

Posted by Louis Bornwasser (Member # 3063) on 03-24-2014, 06:24 PM:

Could he be using a mag input? If so, there is a reversible header that affects gain.

Posted by Leonid Dragilev (Member # 4611) on 03-25-2014, 01:07 AM:

I am using 3rd party AD converter with AES to AES-id adaptor wired to option card connector. What is V arrangement? The same setup works flawlessly at another room..

Posted by Marco Giustini (Member # 4544) on 03-25-2014, 03:45 AM:

If you don't mention the brand/model of the adaptor we cannot help you! Shouldn't a cat 790 be able to accept AES without external converters?

Posted by Dave Macaulay (Member # 813) on 03-26-2014, 06:35 PM:

What he says he's doing is using a A-D converter to convert an analog audio signal to AES, then using the CAT790 to input the AES data. One would tend to ask why.
Anyway, I think any level change is due to a difference in defining the reference level between the Dolby and the A-D converter. The CP650 format 11 analog inputs don't have gain controls (don't think so, anyway) so they have a set reference level (it's in the Dolby manual somewhere) so a known input signal level will give a known SPL in a correctly aligned cinema at fader 7. Someone - AES? EBU? Dolby? DCI? - has defined the 0dB level for the digital audio. I believe that 100% modulation of an AES/SPDIF signal shows as +20 on the Dolby CP650 level bar display. I haven't seen anything go over +20 from digital input, anyway - and a test tone CPL I have, supposedly recorded at 100%, shows as +20 on a 650 w/790.
My experience with A-D converters is that the 0dB reference varies. To avoid clipping, various reference levels are used with various amounts of headroom above "0dB".
I think the issue is that your A-D converter's -20dB corresponds with Dolby's -32dB. Just convert 12dB hotter?

Posted by Leonid Dragilev (Member # 4611) on 03-27-2014, 01:34 AM:

Dave,you are 100% right! i aligned AD with sine wave and got exactly the result you were writing about. thank you for the help!

Posted by Pete Naples (Member # 761) on 03-27-2014, 02:54 AM:

Leonid, apologies, I did read your post the way you wrote it and assumed it was a typo.

You're converting analogue to AES, to put into a CP650. Why not just use the CP650 6 channel analogue input? I think I would invent some switching/patching if I had multiple sources before I went to the trouble and expense of conversion to AES.

Posted by Leonid Dragilev (Member # 4611) on 03-27-2014, 03:14 AM:

Pete, I did it because I need 7.1 at first place and because of noticeable 50hz hum of analog inputs due to unbalanced nature of CP650 inputs. We are using device at mixing stage .

Posted by Pete Naples (Member # 761) on 03-27-2014, 06:23 AM:

OK now that makes sense to me. [beer]

Posted by Manny Knowles (Member # 1171) on 03-27-2014, 02:08 PM:

Does your CP650 show the letter "N" or "U" in the top-right corner of the front-panel display?

I understand that you're using the Cat 790 card but the Cat 778 Digital I/O Option card may be similar.

The 778 offers two settings for Headroom Configuration:

N = Noise Floor Optimization
U = Unity Gain

Never mind -- just read the 790 manual -- looks like this won't help you. But I will leave this here anyway -- some future user can stumble onto it.

Posted by Marco Giustini (Member # 4544) on 03-29-2014, 10:19 AM:

Where does the analogue signal come from in the first place? Any D-Cinema server will output AES, why not feeding it straight into the Cat 790??

Posted by Leonid Dragilev (Member # 4611) on 03-29-2014, 12:24 PM:

We cannot feed cp650 from a server because we are mixing studio and do not own one. Analog signal comes from console, then balanced to unbalanced active adapter. Anyway we are fine now so thanks everyone