Film-Tech Forum ARCHIVE: Dolby Fader Settings to measured dB SPL

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Posted by Tom Glanville (Member # 2978) on 08-18-2005, 02:58 PM:

Has anyone ever seen a chart that showed how the Dolby Fader setting relates to measured volume? If you turned the Fader from 7.0 to 4.2, how many dB are you reducing the volume by? I can't seem to find it on the web or in Dolby docs. From my measurements it seems this is the change.
7 to 6 = 3dB change
6 to 5 = 4dB change
5 to 4 = 5 dB change

Regards,
Tom

Posted by Michael Schaffer (Member # 1204) on 08-18-2005, 07:14 PM:

Each number on the fader corresponds to about 3-3.2 dB, so 5 is -6.4, 10 would be +10dB.
There is a chart somewhere in the Dolby manuals, but I don't remember which one.

Posted by Ken Jacquart (Member # 369) on 08-20-2005, 01:46 PM:

From Fader 0.0 to 4.0, the SPL changes 10dB per whole number.
From Fader 4.0 to maximum, the SPL changes 3.33 dB per whole number.

I'll try and attach a graph of this to this post.

Posted by Michael Schaffer (Member # 1204) on 08-20-2005, 06:39 PM:

From which manual is this graph? "The fader knob rotates continously" tells us it must be CP500 or 650, but I couldn't find the chart when I looked in these manuals.

OK, I found it. It's on page 3-4 of the CP650 installation manual, issue 4.

Posted by Christopher Seo (Member # 7) on 08-21-2005, 11:24 PM:

Do the non-digital Dolby processors also adhere to this fader characteristic?

Posted by David Graham Rose (Member # 1450) on 08-22-2005, 04:33 AM:

No. They are log pots.

Posted by Steve Guttag (Member # 268) on 08-22-2005, 06:44 AM:

Actually, that only defines the CP650 characteristic.

The CP500 neither went to "0" nor went to "10" though the numbers did, the volume level did not change. I want to say it stopped around 9.3 and on the bottom end it was closer to 2...Ken might have the exact range on that one.

As to the older analog systems. They used a linear pot but it operated in a more log fashion with the VCAs (voltage controlled amplifier).

In fact, back in the early days one can find why "7" was such a good choice for norminal fader level. Once upon a time the VCAs were descrete components, once one went away from the calibrated position, the levels would drift from channel to channel (no longer be in absolute balance). "7" placed the whole fader system where variations were kept to a minimum yet allowed for increasing or decreasing volume. Below "5" or halfway, the levels are moving too briskly and at "8" their isn't much headroom left to raise a quiet print. Remember too, when the fader of "7" was established, MONO films were far outweighing stereo films...something of a 90/10 ratio, if not more.

You will find that the analog characteristic curve is similar to the CP650 except it doesn't have those linear regions...they are all "curved" such that if you plot a logarithmic line, it would move briskly at first but start to level off (but never quite be level). The CP650 characteristic is just a single knee approximation of the original fader characteristic. However, I'll bet that the channel to channel balance remains better thoughout the range.