Film-Tech Forum ARCHIVE: Distorzion in analog sound

This is topic Distorzion in analog sound in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Marin Zorica (Member # 1716) on 04-01-2004, 04:44 AM:

Ok, i've been constructing a pile of preamplifer for optical sound but latest was good, but i got a lot distorzion is sound expecialy in high sound levels on film, can cause of this could be a distance of exciter lamp or lamp optics from film??, i don't use stereo solar cells becose they are very very much expensive (if somebody have a one or pair after instalation of all reverse scan readers and don't know what to do with, send it to me), i use a two photodiode (one for each chanel) they are siemens BPW 34, but i don't think that couse of distorzion is they becose in other cinemas they sound good, and this is not firs amplifier that not work fine, projector is Ernemann IV here's link for datasheet of photodiod

http://www.udt.com/Datasheets/Products/BPW-34.pdf

if you have soem ideas will be cool to sold this problem

Posted by Mike Blakesley (Member # 26) on 04-01-2004, 12:17 PM:

Morin
I can't answer your question but you need to be more descriptive in your thread titles. Using just one word is referred to as a "teaser" and makes it hard to search the older archives later for specific information. There is more on this in the F.A.Q.

One thing you don't want to do is get the moderators upset at you, or they might send Phil Hill to your house. You don't want that to happen. (P.S. I'm kidding with this paragraph....I hope.) [Big Grin]

Posted by Marin Zorica (Member # 1716) on 04-02-2004, 03:29 AM:

Mike thanks for advise!!!!!!!!!

Posted by Phil Hill (Member # 371) on 04-02-2004, 04:40 AM:

"Oh My"...

Dear Diary:

I have been "ordered" to Montana this next week... I will be in Texas in May.

>>> Phil

Posted by Chris Hipp (Member # 1788) on 04-02-2004, 10:22 PM:

Yeah, Phil has to bust Mr Miller for not saying 2004 in the Walking Tall thread. Even though the tail of the print CLEARLY says 2003. [Smile]

Posted by Daryl C. W. O'Shea (Member # 1303) on 04-02-2004, 10:30 PM:

The head says "14 13 12 11 10 9 8 7 6 5 4", would you like that included too?

Or how about we just include the release year.

Marin -- you could be getting distortion from all of the things you mention. Have you ensured that your pre-amp (which it sounds like you built yourself) isn't faulty and can handle the input level?

Posted by Phil Hill (Member # 371) on 04-03-2004, 06:04 AM:

...and then I've been "ordered" to Hamilton Ontario Canada to take care of "business."

>>> Phil

Posted by Gordon McLeod (Member # 33) on 04-03-2004, 09:13 PM:

The distortion is most likely caused by the impedance of the input stage of your preamp

Posted by Marin Zorica (Member # 1716) on 04-04-2004, 04:18 AM:

Gordon, yes the impedance may be coursing distorzion, but what is impedance of input for cell, maybe i can balance this with input transformer or similar, because i use photo diodes don't you think i could bias them with couple of volts, would that help??

Posted by Gordon McLeod (Member # 33) on 04-04-2004, 10:06 AM:

It looks like it is a current not a voltage device so possibly a high impedance transformer in serries with the bias supply
never used one but I beleive the old ernamann preamp was 4.7K for a photodiode some how that sticks in memory from the distant past (no snide comments please)

Posted by Gilbert Travin (Member # 2138) on 04-06-2004, 04:44 AM:

Hi Marin !

Sources of sound distorsion may be :

1 - the photodiodes are not biased correctly : they must be "reverse" mounted with a DC biasing ;
2 - gain of preamp is too much important ; you obtain saturation at great levels ;
3 - impedances don't match ...

good luck !

Posted by Marin Zorica (Member # 1716) on 04-06-2004, 08:53 AM:

>>>>
1 - the photodiodes are not biased correctly : they must be "reverse" mounted with a DC biasing ;
2 - gain of preamp is too much important ; you obtain saturation at great levels ;
3 - impedances don't match ...
>>>
1. i try to reverse polarized photodiodes, but i only get higher signal, but distorzion stay same, how can i biased photodiodes well, anyone have schematic, or advice for this?
2.The gain is ok, i think, i tryed preamp with microphone and it work perfectly.
3.for impendace i don't know, what should be impendance??,
Also soud is quite good at low levels is only distorzin at higher levels.
Can the reason be a light source from exciter lamp??

Posted by Gilbert Travin (Member # 2138) on 04-07-2004, 04:23 AM:

Hi Marin !

The good biasing of photodiode is explainated in ... the data sheet ! You need a resistor in serial with the photodiode for biasing. The value of the resistor is given (with the DC value of biasing) in the datasheet ! The photodiode signal must be connected to the preamp thru a capacitor, of course.
The impedance is the "resistance" of a reactive-component (capacitor or inductor) when used with AC current. Also, there is a difference of phasis between current and voltage.
With DC current the impedance of capacitor is infinity and the impedance of inductor is zero.
For a resistor, impedance = resistance.
To find a solution at your problem, you must use an oscilloscope to verify where the distorsion appears ...
Good luck ! [Big Grin]

Posted by Edward Jurich (Member # 1787) on 04-08-2004, 08:12 AM:

You might try adjusting the bias on the diode but if it's distorting only on high levels your pre-amp input may be overdriven. A simple fix may be to just lower the light source (exciter lamp) some, this will reduce the audio level and operate below whatever is being saturated.

Posted by Marin Zorica (Member # 1716) on 04-08-2004, 08:28 AM:

No, the input is not overdriven because even if I lower gain of pre amp the sound is distorzed, even if i lower the exciter lamp light, but now after set the slit lenses, the sound is ok, and distorzion is small, but even I sold problem i wonder does color of light effect to quality of sound (because all reverse and led readers had red light??), I think the reasons of my problem was in projector reader, but even I think the sound my bee more better, but fact is my projector i Ernemann IV and is very old like slit lenses optic what is in them.

Posted by Richard Fowler (Member # 893) on 04-08-2004, 09:16 AM:

The Ernemann IV was built in the 1930's and used a separate soundhead box.....usually when we upgrade these the slit lens has to be replaced due to the age and the large aperture slit which does not allow for high fidelity reproduction...

Posted by Gordon McLeod (Member # 33) on 04-08-2004, 09:22 AM:

Distortion and colour are unrelated but remember that the old slits were up to 1.5mil so they had in effect an academy rolloff built into them
If you are makeing the preamp boast the highs too much then there will be distortion