Film-Tech Forum ARCHIVE: Exciter voltage DC vs AC ...help?

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Posted by Paul Vollmers (Member # 927) on 05-05-2004, 07:43 PM:

I'm presantly haveing exciter voltage problems...........if I use AC pwr at 6 volts vs 6 volts DC..........in place of the regular 9 volts DC .....will there be a differance in the sound quality?
This is on an SH1000 stereo cell optical track.
Can differance voltages effect the play back?

Thanks in advance,

Paul

Posted by Sam D. Chavez (Member # 1841) on 05-05-2004, 08:02 PM:

Running on AC will definitely produce a 60 Hz hum in the sound. If your speakers will not reproduce the hum it's not too bad. If you have modern loudspeakers, it will be very noticeable.

AC exciter power supplies are suitable only in emergencies or in Drive-ins where the speakers will not pass anything below 150 Hz.

The reason you can get by with running on AC is because the exciter bulb tends to stay incandescent during the swing from positive to negative so the effect is somewhat minimized.

Posted by Steve Guttag (Member # 268) on 05-05-2004, 09:01 PM:

Proper Drive-In exciters are also typically 7.5A rather than the standard 4A versions...again with the larger filliment, there is less hum when running on an AC exciter supply.

Steve

Posted by Paul Vollmers (Member # 927) on 05-05-2004, 09:27 PM:

Thanks for your replies..........however..my question is for low volts say 6 volts inplace of the required 9 volts exciter supply.
Would the lower 6 volts effect the sound pickup, or play back?

Paul

Posted by John Pytlak (Member # 331) on 05-05-2004, 10:27 PM:

Don't forget that all MGM releases will be cyan dye tracks starting at the end of May, and Disney begins conversion this fall and intends to convert 100% by early next year. Now would be the time to consider a red LED reader:

http://www.dyetracks.org

http://www.hollywoodreporter.com/thr/crafts/article_display.jsp?vnu_content_id=1000492312

quote:
MGM and Buena Vista Pictures, the distribution arm of Walt Disney Studios, are opting out of using traditional silver-applicated soundtracks on their film prints in favor of more environmentally friendly pure-dye analog soundtracks.

The moves, announced by MGM on Friday and by Disney today are timed to coincide with the opening of the ShoWest 2004 convention in Las Vegas. The commitment puts the two studios at the forefront of an industrywide effort to eliminate caustic chemicals and conserve some of the water used in the manufacturing and distribution of film prints.

Posted by Lindsay Morris (Member # 1430) on 05-05-2004, 10:45 PM:

Lower voltage will give you lower output from the cell IE less illumination passing thru to the cell.
However running a 9v exciter on 6volts will give you extraordinary long life of the lamp and much less filament sag and thus better overall stereo.
In fact many of the processor makers recommend reducing the exciter volts for those very reasons.
When it comes to running AC exiters the use of the 7.5A lamp is essential in minimizing AC hum as the filament maintains its brilliance thru the AC waveform changes.
A couple of small cinemas where I initially started out had AC exciters and extremely good bass response and you really had to be right up to the bass unit before you could detect any hum from the exciter source using 10v 7.5a exciters.
This is all academic now as with the swing to cyan you will be sunk without switching to the Red light sources be it laser or LED.
Lindsay

Posted by Rick Long (Member # 213) on 05-05-2004, 11:02 PM:

If I understand the question, you are considering using a six-volt supply instead of the usual 9 volts.

Obviously, the exciter will not light up as brightly and, therefore, you will need to increase the gain of the sound system, likely resulting in increased noise.

A second consideration is that the filliament supports act as heat sinks and will draw heat away from the ends of the filliament. This will have the effect of reducing the effictive width of the scanning beam and thus reducing the dynamic range of your reproduced sound.

Posted by Sam D. Chavez (Member # 1841) on 05-06-2004, 12:52 AM:

Well put Rick!

Another way to express it is that you get amplitude compression at the edges of the soundtrack which result in the peaks such as gunshots and other percussive effects sounding mushy. For example a 6dB excursion on the soundtrack results in perhaps a 3 dB excursion from the speaker due to declining light level at the edges.

This is where the reverse scan reader does its best work.

Posted by William Hooper (Member # 71) on 05-06-2004, 02:58 AM:

quote:
Don't forget that all MGM releases will be cyan dye tracks starting at the end of May, and Disney begins conversion this fall and intends to convert 100% by early next year. Now would be the time to consider a red LED reader:

Unless you will be running prints with silver tracks, not cyan.

The fidelity of reproduction of silver tracks on red readers was found to be poor enough that the high magenta track had to be developed to support the red readers.

Now that cyan tracks are being issued to support the red readers, the playback of silver tracks on red readers is being touted as "acceptable", which is quite discontinuous with the reasoning needed for creation of high magenta tracks.

Posted by Edward Jurich (Member # 1787) on 05-07-2004, 11:01 PM:

Having just put in 2 red readers using current non-cyan tracks, I found the fidelity to be outstanding. You do need the pink-noise loop to set phase and focus, otherwise you'll never get the reader set right.

Posted by John Pytlak (Member # 331) on 05-08-2004, 09:24 AM:

quote: William Hooper
The fidelity of reproduction of silver tracks on red readers was found to be poor enough that the high magenta track had to be developed to support the red readers.

The "high magenta" (magenta+silver) tracks have a cross-mod (XM)optimization density that is nearly the same for both tungsten light and red LED readers, so the tracks are more compatible with the MIX of readers out there now. The old (cyan+magenta+silver) tracks had to be XM optimized for one or the other, so the one they were not optimized for would have more cross-mod distortion.

If you optimize an old (cyan+magenta+silver) track for red LED readers, the tungsten readers will have less "fidelity" (more sibilant distortion). If you optimize the cross-mod of a B&W print silver track for a red LED reader, the old tungsten readers will have less "fidelity".

Posted by Josh Jones (Member # 447) on 05-08-2004, 04:38 PM:

quote: John Pytlak
If you optimize an old (cyan+magenta+silver) track for red LED readers, the tungsten readers will have less "fidelity" (more sibilant distortion). If you optimize the cross-mod of a B&W print silver track for a red LED reader, the old tungsten readers will have less "fidelity".

So what exactly are you getting at? both ends of the scale have less 'fidelity' with one extreme resulting in sibilance distortion, what is left at the other end? A HF rolloff?

Don't worry, I'm just confused...

Josh

Posted by Bruce Hansen (Member # 281) on 05-08-2004, 05:35 PM:

Paul,

If you need a cheep excitor supply try All Electronics ( www.allelectronics.com 1-800-826-5432). They have a couple of switching type supplies that will give you nice clean DC.
Stock # PS-655, 6.5 VDC 5 AMP $11.75 (needs an IEC power cord)
Stock # PS-865, 8 VDC 6.5 AMP $9.50
These are "fixed" (not adjustable). Do not exceed current rating. An IEC power cord is a computer type 3 pin power cord. Shipping not included.

Posted by John Pytlak (Member # 331) on 05-08-2004, 07:03 PM:

quote: Josh Jones
So what exactly are you getting at? both ends of the scale have less 'fidelity' with one extreme resulting in sibilance distortion, what is left at the other end? A HF rolloff?

With an old (cyan+magenta+silver) track, you have to optimize the printing for one or the other to minimize cross modulation (sibilant) distortion. So an old track optimized for tungsten light would play best on a tungsten light reader and have some distortion on a red reader, and one optimized for a red LED reader would play best on a red reader and have distortion on a tungsten reader. With the mix of readers in use today, one or the other would have some sibilant distortion.

"High Magenta" (magenta+silver) tracks optimize at about the same density for both types of reader, and are recommended for the mix of readers in use today.

Distributors like MGM and Disney have already decided that there are enough red LED readers in use to allow complete conversion to prints with cyan dye analog soundtracks, to take advantage of the environmental benefits, and eliminate quality defects from sound redeveloper application and washoff problems.

Posted by Rick Long (Member # 213) on 05-08-2004, 11:36 PM:

Sam D. Chavez said "Well put Rick!"

It should be, Sam, it was only one of the many things you taught us during your brilliant seminars here in Toronto. [Smile]

Posted by Paul Vollmers (Member # 927) on 05-09-2004, 06:34 AM:

Hey guys...........thanks for your input.............I'm currently using 6 volts AC, as a temporarily power supply, as my 9 volt DC Kelmar is not setup yet.
The sound does not seem where it should be..............so your info has confirmed my thoughts.
This is a home setup, not corporate theater.
Also, this sound issue is on my Simplex SH1000, not the Norelco as pictured.
Again many thanks,

Paul

Posted by Sam D. Chavez (Member # 1841) on 05-09-2004, 02:58 PM:

Oh, do go on!

The seminars were always great fun. Was Lonny with me then, or was it George Douglas and Bob Warren?

Lonny and I did a seminar for Regal in Seattle two weeks ago. We are polishing our act for the road.

Posted by Mark Gulbrandsen (Member # 72) on 05-09-2004, 03:50 PM:

Yea, those seminars wre lots of fun and also learning some interesting things. The last one I attended was in 1985 and it was when it was Sam and George Douglas. Its even amazing how many people I met at the seminar that I still keep in touch with today. The trip we all made over to Saul Zaentz Film Center was wonderful!

Mark

Posted by Josh Jones (Member # 447) on 05-09-2004, 05:13 PM:

Thanks for the clarification, John. Not all of us are photochemical engineers after all.

Josh

Posted by Phil Hill (Member # 371) on 05-09-2004, 05:44 PM:

quote: John Pytlak
...allow complete conversion to prints with cyan dye analog soundtracks, to take advantage of the environmental benefits...

John, just out of curiosity, what are the economical benefits to the distributor and/or film lab by having only cyan tracks?

(Perhaps this question is more suited for the Cyan Thread... copying over there for reponses.)

>>> Phil

Posted by John Pytlak (Member # 331) on 05-09-2004, 10:08 PM:

The major reason has always been the environmental advantages of greatly reduced water usage, less chemical waste being sewered, elimination of a concentrated caustic developer containing hydroquinone, and eliminating almost all silver from color prints that may be incinerated or landfilled.

Any savings due to reduced water and chemical usage, and reduced waste, may ultimately reduce the cost of prints. As Dominic Case wrote, contracts between labs and distributor are renegotiated periodically.

There is NO CHANGE in the print film raw stock. Film stock manufacturers may actually sell LESS film, since there is less lab waste.

Posted by Jack Ondracek (Member # 1466) on 05-11-2004, 10:21 PM:

This all gets away from Paul's original question, that of AC hum.

As Steve mentioned, there was a higher amperage bulb, sometimes with a different voltage rating. The heavier filament did not respond as quickly to the rise and fall of a 60 cycle waveform, so you were (are) less likely to hear hum when using AC power. The 4 amp bulbs will pass some hum into your system, though.

I assume you only hear the hum when the exciter is on and you have no film in the machine. With a good ear, you can also hear the hum come and go with dialog, since little light gets through to the cell during silent periods. That can be very distracting. If you put a business card between the light and your pickup cell (in an unthreaded soundhead), the hum should also disappear. If it doesn't, you have problems elsewhere in your system... likely between the cell and your amplifier.

Posted by Paul Vollmers (Member # 927) on 05-12-2004, 06:31 AM:

Jack,
Thanks for your reply, yes, I do get hum on AC and the hum does go away when a card is placed between the light and the film, so everything is working as it should.
My question was to the voltage supplied to the exciter........instead of the normal 9 volts DC, what effect would 6 volts AC be with the sound quality ? other than hum .

Paul

Posted by Gordon McLeod (Member # 33) on 05-12-2004, 09:31 AM:

It depends on the lamp if you operate an excitor below 80% of its rating then the wo filament supor rods act like heat sinks and lower the filament temp at the ends of the filament produing very poor edge illumination and one ends up with a compressor to the audio

Posted by Jack Ondracek (Member # 1466) on 05-12-2004, 01:20 PM:

Yah... I don't think I'd drop it quite that low. A volt or so can extend the life of your lamp quite a bit, but there are some trade-offs for going too low... which are listed above.

In any case, you won't hurt anything the way it is now. You appear to have the right kind of power supply on the way, and that will address the concerns you've raised here.

Posted by Bruce Hansen (Member # 281) on 05-12-2004, 05:23 PM:

You could also get a bridge rectifer and a filter cap or two, and you should be able to get about 8 volts of DC. The rectifer must be able to handle the current your lamp drawes (always add a little fudge factor), and you may need to heat sink it. Feed the 6 volt AC output of your xformer (hopefully your xformer can supply enough current for your lamp) to the "~" (AC) input of the bridge. Put the caps accross the "+" and "-" outputs of the bridge (make sure that the "+" of the cap is connected to the "+" of the bridge. Then connect the lamp to the "+" and "-" of the bridge (and cap(S)). The caps must be rated to handle the voltage. Somewhere around 5000 Mfd should do for the caps. Make sure that the bridge does not get too hot. You can possibly heat sink it to the projector caseing.