Film-Tech Forum ARCHIVE: CAT 69 Dolby Tone?

This is topic CAT 69 Dolby Tone? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Paul Gordon (Member # 3320) on 11-23-2017, 10:59 AM:

Hey all,

Does anyone have a digitized WAV file of the CAT 69 Dolby tone loop? I have a CP65 that we are using to decode Dolby A/SR files that were captured in stereo then fed into the 65 and recaptured. Just need to calibrate the Dolby cards. Thanks.

Posted by Monte L Fullmer (Member # 2797) on 11-23-2017, 03:09 PM:

If you need pink noise, go on YT and find a video with pink noise, record the audio as a Wav file.

Posted by Marco Giustini (Member # 4544) on 11-23-2017, 04:39 PM:

Dolby Tone is not pink noise, it's the 1000Hz (or was it 998Hz?)

You can generate a tone using Audacity (it's free) but the problem is at what level. The dolby tone was giving you a reference level. I do not know what the level should be if you feed it from a different source - you'll also have to consider the level the WAV is played back to, not just recorded.

Posted by Scott Norwood (Member # 30) on 11-23-2017, 06:43 PM:

The Dolby A cards (22 and 350) can generate Dolby tone if you need to record it. You can get a Dolby 363 frame (which will hold two cat. 300s or 350s) on Ebay fairly inexpensively, which has the appropriate buttons and outputs.

The problem is that, as others have mentioned, Dolby decoding is level-dependent, so you need the levels from tone loops supplied with the original recording. In the case of optical tracks, the levels are supposed to be standardized, but that will not be the case with tapes or anything else that you encounter.

Posted by Leo Enticknap (Member # 534) on 11-25-2017, 09:27 AM:

As long as both left and right channels are set to the same input gain in both the recording and decoding process, does it really matter whether that is the Dolby reference level or not?

The actual level was critical in a movie theater, because you needed to maintain the "7 on the fader = reference level" amount of input gain from the preamp. The level on the cat69T test film gave you that reference (though really old cat69T test films on acetate stock may not now be totally accurate, because of dye fading on the emulsion and/or shrinkage of the base over time).

But if what you're trying to do is to remaster an A-type or SR-encoded stereo recording, decoding it into four discrete channels in the destination file, then surely all you need to do in the decoding stage is to ensure that the gain is the same on both left and right input preamps (so that the phase matrixing is done correctly), and then you can normalize the output files later, according to what they will be used for (e.g. AES, or DCI at -20dBFS)? For that, any reference tone generator would do - you don't need the test film, or a recording of it.

Posted by Bill Brandenstein (Member # 7758) on 11-25-2017, 12:41 PM:

Leo, Scott is right. Dolby A and SR noise reduction are not linear companding systems (such as DBX) and thus have trigger levels requiring calibration. If decoding matrix surround were the only issue, then of course, matched channel levels is all that would be necessary. But any form of Dolby noise reduction is level-dependent.

Or perhaps you're suggesting if the digital system was unity gain from capture through playback, the level is already accurate?