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Topic: How do you know if there is noise reduction on a VA track?

Kenneth Wuepper
Phenomenal Film Handler

Posts: 1026
From: Saginaw, MI, USA
Registered: Feb 2002

posted 06-07-2005 06:06 AM

Gentlemen,

Thanks for all of this wonderful information. I knew that the experienced people here could provide not only answers but also insight into this perplexing question.

Although the idea of trying the sound to see what you like (if you have the time) is functional, it is in no way as satiafying as doing it right.

Where can one obtain one of the sound track wall charts as mentioned?

KEN

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Stephen Furley
Film God

Posts: 3059
From: Coulsdon, Croydon, England
Registered: May 2002

posted 06-07-2005 06:22 AM

From the BKSTS. web page

They're expensive, but they are nicely produced. I've seen cinemas that have framed copies on the wall for the public to look at.

I think somebody was distributing them in the US, possibly Boston Light & Sound, but I may be wrong about that.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 06-07-2005 09:44 AM

Sam said "There was no mistaking DBX encoded tracks. If there was no decoder, the pumping made you feel like you were on a roller coaster ride."
Maybe that is why I saw it used on some old Porn Flicks Also I believe Kintek encoded some NR on some special venue stuff a long time ago
I remember on the old Yellow Kintek boxs a Kintek NR function

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Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

posted 06-07-2005 10:30 AM

GNR was used on VA tracks before Dolby and still is used today. The idea is to narrow he bias line to a minimum in the absence of modulation. The trick is not to let it get so small the bias line pinches shut, as you get all sorts of unpleasant artifacts when that happens.

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Stephen Furley
Film God

Posts: 3059
From: Coulsdon, Croydon, England
Registered: May 2002

posted 06-07-2005 12:40 PM

quote: Sam D. Chavez
The trick is not to let it get so small the bias line pinches shut, as you get all sorts of unpleasant artifacts when that happens.

What sort of artifacts? When there's no line there would be no light, and therefore no sound. Where there is a line of constant width there would be light, but unmodulated, therefore no sound other than noise caused by film grain and photocell, therefore I assume that the problems would occur at the point where the line disappears and re-appears; is this the case?

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Martin Brooks
Jedi Master Film Handler

Posts: 900
From: Forest Hills, NY, USA
Registered: May 2002

posted 06-07-2005 02:45 PM

quote: Frank Angel
We have a small mixer in the booth that will take an earphone jack. When in doubt, I listening with earphones to the left and right channels. Having the L/R sound pumped directly into your ears will tell you a lot. Stereo vs mono is almost instantaniously identifable with phones and many times gives you a much quicker answer as opposed to hunting through footage looking at the track with a loupe trying to find differences visually.

Another technique is to wire up a Left minus Right switch. (Combine one channel out of phase with the other channel.) If you hear nothing, it's mono. If you hear something, it's stereo. Won't tell you whether there is NR though. I still use an old Apt-Holman preamp that has this built in. It's great for all kinds of diagnostics (tape head azimuth, FM antenna tuning, etc.)

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Stephen Furley
Film God

Posts: 3059
From: Coulsdon, Croydon, England
Registered: May 2002

posted 06-07-2005 03:22 PM

quote:
I am confused. I thought biasing was done only on VD tracks, as the principle of the VA is to narrow and widen the translucent part of the track anyway. Please explain for the slow-brained. Pictures welcome (or working links!).

Michael, with a VA track without GNR the width of the clear area will vary to follow the waveform of the sound, but on average will be about 50% of the width of the track. Where there is no sound the width will be a constant 50% ish. this large clear area will transmit a lot of light, which will result in noise from the photocell, and any dust, scratches, etc. will also cause unwanted clicks, pops etc. To avoid this the clear area can be reduced by various means. During quiet passages the clear area can also be reduced, but by a lesser amount; as long the modulated edge of the track is not touched the sound will be anaffected, but the noise from the clear film will be reduced. Have a look at the 16mm sound formats thread that I mentioned earlier for some pictures of various methods which have been used.

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Richard Fowler
Film God

Posts: 2392
From: Ft. Lauderdale, FL, USA
Registered: Jun 2001

posted 06-07-2005 03:50 PM

Some mono optical sound recording galvanometers have a mechanical shutter to close the track during long silent passages in addtional to the closing of the audio track electronically....this creates a noise in itself in operation. One of Ray Dolby's early systems for improving sound recording was to use an audio delay to allow these circuits and hardware to react to as not to clip incoming or outgoing audio being recorded.

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Josh Jones
Redhat

Posts: 1207
From: Plano, TX
Registered: Apr 2000

posted 06-07-2005 05:15 PM

To me it would seem that they would use some sort of envelope detector circuit which is sniffing the incoming audio. the detector is only sensitive in one direction, so that it cannot cause the valve to 'crash' or completely close down. the output of the detector is summed with a minimum bias voltage that controls the minimun track width, as well as modulation headroom.

It makes sense to me, does it to the rest of you guys?

In reguards to audio artifacts from the valve closing down during silent passages, when the valve opens, it will cause a thump, or pop, depending upon how fast the transition is, in the playback audio.

JJ

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Richard Fowler
Film God

Posts: 2392
From: Ft. Lauderdale, FL, USA
Registered: Jun 2001

posted 06-07-2005 06:12 PM

More like a wump depending on the audio getting softly clipped [Razz]

I have done plenty of 35mm sound recording when I worked in a film lab that specialized in cinema ads and low budget films...we had little audio tricks, compression and EQ setups depending where the product was shown. On cinema ads for in house production, the sound tracks where recorded on each side of the sound negative and we slugged the picture negative to restore sync; the customers did not own the elements and future reprints where a new order. I think Ray Dolby's set up was like 10ms delay to keep the clipping from happening based on my feeble memory. Josh you should have been there in 1931 to get them to do it right [Big Grin]

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 06-07-2005 07:18 PM

There was a system that used a CCD camera to scan the soundtracks to eliminate the need of noise reduction systems in the galvo But I don't think they ever caught on

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Josh Jones
Redhat

Posts: 1207
From: Plano, TX
Registered: Apr 2000

posted 06-07-2005 07:24 PM

well, they didnt have op amps in the 1930's, and I dont want to design precision DC amplifiers on tubes [Wink]

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Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

posted 06-07-2005 07:29 PM

It looks like Mr. Furley's questions got answered.

You do want a bias line instead of closing all the way to black not just to prevent wumps, but also because the noise floor will be more constant and even with a small bias line vs. the scratch and low frequency background noise you will get from an unbiased dark film.

Try running black stock with no soundtrack at all and you will hear it immediately.

As to driving the GNR, in the not too distant past before Dolby Labs hit the scene, one method used was a preview head slightly ahead of the playback head on the mag dubber that carried the audio for making the printing master. When modulation was sensed by the preview head, it opened up the bias line in anticipation.

Pretty low tech, eh?

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Louis Bornwasser
Film God

Posts: 4441
From: prospect ky usa
Registered: Mar 2005

posted 06-07-2005 08:05 PM

The "wump" was absolutely evident during the slightly-before-Dolby era when I was experimenting with high grade preamps, large power amps, no EQ, and speakers similar to the 4675; all in a small room.

The main complaint was that front row viewers became sick from the constant LF energy from the ground noise reduction gating.

Louis (Now I really feel like an old-timer.)

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Sam D. Chavez
Film God

Posts: 2153
From: Martinez, CA USA
Registered: Aug 2003

posted 06-07-2005 09:36 PM

That's because we are. Oracles of obselete info that is.

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