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Author
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Topic: Using a Frequency Equalizer
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Paul Adsett
Film God
Posts: 5003
From: USA
Registered: Jun 2003
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posted September 25, 2007 03:37 PM
Thanks for that article Graham. One thing to note is that the frequency response graph they are showing is probably for a 16mm optical sound projector. Super 8 magnetic sound projectors are far superior to anything obtainable on 16mm optical. For instance the Eumig S938 Stereo has a frequency response from 40 to 11,000hz at 18fps, even better at 24 fps.
Even so, I also use a Yamaha 10 band stereo graphic equalizer to boost high frequencies and suppress low frequency rumble on some of my commercial film prints, particularly older mono material. I find that on stereo sound tracks that I have re-recorded from DVD on the 938, the graphic equalizer is really not required, because the re-recorded sound quality is so superb.
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The best of all worlds- 8mm, super 8mm, 9.5mm, and HD Digital Projection,
Elmo GS1200 f1.0 2-blade
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Panasonic PT-AE4000U digital pj
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Graham Ritchie
Film God
Posts: 4001
From: New Zealand
Registered: Feb 2006
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posted September 27, 2007 04:03 AM
Hi Paul
I agree, a good recording on stripe is better than optical, films like the Derann print of "The Lion King" are good examples, I have also found the use of a graphic equalizer in general improves the sound and in particular the high range the sound seems much clearer, using an equalizer is almost a must when using an external amp.
Here are some more details as written from the original 1977 article you may find of interest.
The best way to properly equalize the output of your projector is to play back a recording of an audio oscillator {a radio/TV repair shop could help you with this}. On striped film, you record the oscillator's output slowly sweeping through a frequency range of 100 to 6000 cycles per second. For playback, hook up the equalizer as in Figure 2 and connect a "VU" or volt ohm meter {set to measure AC volts} across the speaker terminals. Run your sound projector with the oscillator sound track. You can adjust the equalizer to boost or attenuate selected bass or treble frequencies so you obtain a near perfect output level throughout the frequency range. If you don't have access to an oscillator or a VU meter, you can improvise and tune by ear for best sound.
I adjusted mine by ear, ![[Roll Eyes]](rolleyes.gif) I spent a lot of time setting it up but it was worth it in the end, certainly if you have a oscillator that would be the more accurate way.
Well its always interesting to read such articles, and I hope it might be of interest to anyone who is thinking of setting up a projector using an external sound system.
Winbert
I havent heard of the 3D-Spalalizer although I am not to keen having sound coming from the rear, I guess I am a bit old fashioned and prefer most of the sound to come from the screen, and side surround speakers.
Graham.
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Brad Miller
Administrator
Posts: 525
From: Dallas, TX, USA
Registered: Jun 2003
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posted September 29, 2007 08:12 PM
Ideally what has to happen to re-record a print is that part of the film must have pink noise recorded on it as a calibration test (all frequencies at the exact same volume playing at once...it sounds like static). Then the playback is checked on a Real Time Analyzer to see what frequencies got boosted and which frequencies are weak, then the EQ is adjusted prior to recording in the audio path, such that when the final print is played back the frequency response is perfectly flat.
Why go to all of this trouble? For recording stereo to later be decoded with Dolby Pro Logic (surround sound) circuitry, the circuit is looking for IDENTICAL sounds as the two tracks are compared against each other to steer them into the center channel. Sounds that are only in the left channel get routed into the left speaker. Sounds that are only in the right channel get routed into the right speaker. Sounds that are out of phase get routed into the surround speakers. That circuit normally works very well, BUT if the frequency response is not identical between the two tracks, the Dolby "steering" mistracks and sends dialogue into the surrounds and the audio isn't locked down into exactly where the audio mixers intended it to be.
Everyone here has noticed that track 1 always sounds better than track 2 due to the extra width, so by Super 8's magnetic track design, a pre-recording EQ, calibrated with pink noise and an RTA, is a definite necessity to TRULY get the best performance out of the format. Play back a print you think sounds great and turn off every speaker except for the surrounds. Do you still hear dialogue? If so, you are mistracking and the sound could be MUCH MUCH better.
Also note that dbx type II noise reduction can't be beat for this task. You will have a noise-free playback when using it with Super 8, although if you play the print back without dbx decoding, it will sound very compressed. What it does is compress everything by a 2 to 1 ratio upon recording and then un-compresses it upon playback, so the noise floor of the magnetic track is now twice as low, etc.
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