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Topic: Checking Dolby Level
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Ray Derrick
Master Film Handler
Posts: 310
From: Sydney, Australia
Registered: Sep 2001
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posted 10-14-2002 03:37 AM
Dolby tone is and always has been 850Hz. The distinctive warble is produced by raising the frequency of the tone by about 10% for 30mS duration every 750mS. I suspect 850Hz was originally chosen because it is closer to dead centre of the audio spectrum (on a logarithmic scale)than 1kHz. I suspect for a similar reason 800Hz was chosen as the crossover point for the upper and lower bands of the SR system. Gordon is correct that the warble was added to make the tone distinctive from the normal line-up tones used in studios.Steve, I am not sure how much effect print density will have on level, if any. This is something we are trying to find out. But if there is an effect it will probably be different for red, infrared and "tungsten" light readers. As infrared light is not blocked by the colour or "blackness" of the film, but rather by the silver content, in infra red and tungsten readers presumably variations in the silver content will have more effect than colour density and vice versa in red light readers. Maybe John P can "throw a little light" on this subject (or should I say throw a little LINK at this subject - John is becoming known as The Linkman around here). Keep up the excellent work, John.
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Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935
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Ray Derrick
Master Film Handler
Posts: 310
From: Sydney, Australia
Registered: Sep 2001
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posted 10-16-2002 09:21 AM
Actually I think they put the warble on there to make it distinctive. What do you think Gordy? And they chose 850Hz because it only hurts (your ears that is), but 1kHz is a killer AND it hurts.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 10-20-2002 08:58 PM
Ok...back from a brief hiatus...Yes, I truely dislike the Cat. 222SR/A and would never recommend using it as my front line of analog sound reproduction. As to the various Dolby levels...Ultra's TT-1 was not perfect either. On much of the batches I have the left and right channels are not properly balanced. The pink noise is pretty good though I heard from some that some batches may be clipped. It is recorded loud unlike Cat. 69. Both Cat 69 and TT-1 were products of NTAV. My "method" of determining good test film from the bad is mostly observation when the next batch comes in if it disagrees with the pervious batch. We have also measured and compared using an R2 various batches over the years. When Dolby first went to the color stock test film all hell broke loose! Back to SR....if you have a CP-55 either obtain an SRA-5 or look into the new Panastereo SR unit...they are your best bets...$1995 (list) for the Cat. 222SR/A just isn't such a good bargain...it isn't real SR. Steve ------------------
"Old projectionists never die, they just changeover!"
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Ray Derrick
Master Film Handler
Posts: 310
From: Sydney, Australia
Registered: Sep 2001
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posted 10-21-2002 06:33 PM
Good question Rick. Dolby tone has been around since 1967 or thereabouts and was originally generated by an onboard oscillator on the Cat22 A-type card. The same tone was used in all applications of professional Dolby A-type noise reduction, including studio magnetic tape, video tape, optical film, broadcast systems - you name it. With the introduction of SR a different system was introduced by locating a pink noise generator on the Cat280 card instead of an oscillator and thus was born Dolby Noise, as already mentioned by Gordy and Gunter. However Dolby Noise which is (or rather was) widely used in studios, never made it into the film realm, probably because Dolby SR had to co-exist with Dolby A-type.When Dolby introduced their digital systems, Dolby tone lost it's relevance because the traditional primary line up tone for pro-audio is 1kHz. You will notice that virtually all audio specifications use 1kHz as the primary reference. 1000 is a nice round number and conveniently located in the "most densely populated" part of the audio spectrum. So 1kHz is used as the standard reference for ALL digital audio. ------------------
Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935
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