Film-Tech Forum ARCHIVE: third-octave vs. parametric EQ

This is topic third-octave vs. parametric EQ in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Scott Norwood (Member # 30) on 06-30-2008, 07:48 AM:

Why is it that most (all?) cinema processors use third-octive equalization for the stage channels? Wouldn't parametric equalization be more useful for working around room issues?

Is this just some historical artifact due to the measurement capabilities that existed in the 1970s when Dolby first started to work on cinema sound, or is there some reason why third-octave is still better or more useful today?

And why don't all processors have a simple "EQ bypass" switch for those who prefer to use external processing or equalization?

(Apologies if this is a stupid question; my B-chain experience is limited and I've never set up a "good" modern-style room. Maybe EQ is less necessary in a well-designed house.)

Posted by Mark Gulbrandsen (Member # 72) on 06-30-2008, 08:19 AM:

A couple of processors did use parametric eq., Eprad was one. If you have bad room issues that would solely require parametric to be able to "work around" then you should really tending to the room problem itself instead.

Mark

Posted by Sam D. Chavez (Member # 1841) on 06-30-2008, 10:22 AM:

Good questions. You are right that it was built around technology of the time.

It still works pretty well and the filters are pretty gentle so you can't muck up the sound too much unless you are ham fisted.

The way to look at a Cat. No. 64 or other Octave based EQ device, is like the keys on a Piano. If you bang on all 27 keys at once you get a pretty horrific sound. If you play the Piano with some aptitude, you can get a an incredible sound.

Likewise 1/3 Octave EQ

Posted by Robert Minichino (Member # 3495) on 06-30-2008, 10:42 AM:

Parametric EQs have been around for a long time. Third-octave EQs are a bit more intuitive to adjust than paras, especially if you're touching up the response by ear.

Posted by Steve Guttag (Member # 268) on 06-30-2008, 10:42 AM:

Parametrics are certainly a superior way to EQ a room. Certainly and ideal room needs no EQ and that is the best way to be.

1/3-Octave remains primarily due to the level of techs out there setting up the processors. It is a 1:1 relationship between their measurement device (RTA) and the EQ pot. Parametrics would just confuse them...as they seem to do on the subwoofer where Dolby did use a single parametric (and Sony used two bands of parametric). The whole concept of frequency, Q, and Amplitude and how they interract seems to escape many.

You can generally do more with a mere 5 bands of parametric than 30-bands of graphic and cause less harm doing it!

As for bypassing EQ...Panastereo certainly offered it (in fact, each card has a bypass switch in addition to the global System Bypass). In the DSP world, some processors offer EQ bypass.

Please note...not all graphic EQs are the same. Some use variable Q (most) and some use constant-Q (good cinema processors use this type). The problem with variable Q graphics is that they often affect adjacent bands as the desired band is adjusted. Having an analyzer with greater resolution than the EQ will allow one to see just what they are doing to the signal. Too often I think techs are comforted to see their ruler-flat displays when in fact, they have a heavily over-eqed channel with much phase shifting and far less flat than their display indicates.

Steve

Posted by Frank Angel (Member # 248) on 06-30-2008, 10:55 AM:

When I first started out, we used Pultec parametrics in the recording studio. I thought they were the cat's meow. When I first encountered "graphic" equalizers, I though they were just a poor man's parametric and I wasn't impressed. When our CP50 was installed, I quickly found that 1/3rd octaves could be a bit more flexible for whole room eq and the "graphic" nature lends itself to more easily match the waveform anomolies displayed on an RTA.

And as Sam says, if you go about eqing with a gentle hand, GOOD 1/3rd octaves can be very effective. But of course all depends on the quality of the filters, how gently they overlap and what interaction they have with their adjacent controls. We've worked with some cheap 1/3rd octave units which interacted wildly between nodes and introduced harmonic distortions that were worse than any room anomolies. Gotta get the good stuff.

We now eq everything digitally as well as all channel configurations via SoundWeb Digital. It has different configurations for film vs. live vs. simple PA, as well as different eqs for audience size. The CProcessor eqs are flat.

Like Scott, I think it would be nice to be able to take the eq completely out of the CP circuit when using outboard eq. Zero-ing out the eq section can never be the same as bypassing it.

Posted by Louis Bornwasser (Member # 3063) on 06-30-2008, 11:39 AM:

To the contrary; the Dolby EQ is subtractive; set them all at zero and nothing subtracts. Such is the mark of the side-chain design; minimum distortion.

btw: I think the in/out level,of the cat 64 is the same, so if would be possible to remove the cards and jumper around them. Louis

Posted by Sam D. Chavez (Member # 1841) on 06-30-2008, 02:22 PM:

I built a custom Cat. 64 holder with just such a bypass feature, mainly to see what the last tech was trying to "cure", before deciding whether to start all over or leave it mostly alone.

Louis is mostly correct. The Dolby Cat. No. 64 design is side chain, but it is not strictly subtractive.

Posted by Joe Redifer (Member # 3) on 06-30-2008, 06:12 PM:

Could somebody please explain "Q"? I never truly understood that so I rarely touch it, especially since it is only on the subwoofer. I did a Google search for Q but I don't have time to sift through over 2 billion results.

Posted by John Hawkinson (Member # 1135) on 06-30-2008, 06:24 PM:

Q is just the sharpness of the notch. How wide it is (how much of the frequency space it covers). It stands for "Quality factor" in signal processing.

--jhawk

Posted by Joe Redifer (Member # 3) on 06-30-2008, 06:27 PM:

Ah, so if Q stands for Quality, I want to run around and turn the Q all the way up to the highest possible level on all my processors, right? If anyone complains, I'll tell 'em Jhawk told me to do it. [Wink]

Posted by Sam D. Chavez (Member # 1841) on 06-30-2008, 07:47 PM:

First part of the Q explanation was on the money. He must have taken a toke before the second sentence. Please forgive me, I just got back from Amsterdam.

Maximum Q = narrowest notch in this application. There is an explanation in the various Dolby manuals.

Posted by Steve Guttag (Member # 268) on 06-30-2008, 10:13 PM:

Frank...your CSP-1200, if installed, has bypass switches on the back for each and every EQ (including subs and surrounds).

As for the Cat 64 as used on cinema processors does NOT have the same input/output levels...they are close though. 170mV input....150mV output...probably not enough to get worried over.

If you have any Cat 105s laying around (not the 105D...just the old 105), then you can plug them in place of the Cat 64 for bypass.

If you are interested in learning about "Q" or other audio aspects of equalization...I suggest giving the www.rane.com site a check. Look for "Rane Notes." There are many papers on all aspects of audio and they are generally quite good. You will find though, any discussion on Q is going to get somewhat math heavy though calculating the Q of a filter is not terribly complicated.

Steve