Push-Pull Variable Density Film – Eric Vast

I worked for a number of years at Denham Studios and during that time quite a bit of the music was recorded using the above technique, It was an outstanding success and it is my firm belief that its quality would compare favourably with that of modern film recording. I have therefore decided at long last to five an account of the methods used and the results obtained. I don’t remember that any public demonstrations of it were ever given in this country but, it was believe, released in some of the US cinemas and in most of the studios over there.

Even when the said recording had to be released after the final dubbing single track, some advantage in the Sound quality could be noticed. However as most of the readers of this article are probably not very well up in the niceties and terms used in film processing and sound recording. I propose first to give a short account of Single Track Sounds more common variations and the meaning of some of the technical terms etc. used in that discipline so that I may write more freely later.

Starting with the blank film this is made from cotton waste in the southern United States where there is a great deal going cheap This is turned into a mixture of various acetates, plasticisers are added to make it pliant and it is spread on enormous heated stainless steel rollers. It is split into the standard 35mm width and rolled up into 1000ft lengths. Later it is coated with its photographic emulsion and perforated. It is not a very good substance but at least it is cheap and does its job fairly well. However it begins to shrink from the moment it is made! After a can of film has been used for six weeks in the West End and six Fleapits out in the Sticks it will have shrunk as mu++ch as ten feet per 1000 ft roll. It then no longer fits the sprocket teeth. Bad flutter caused by the teeth plucking the film gives an audible rough sound to any further listeners. Clear processed film reflects about 20% of any light falling onto it and passes 80% of it.

Film emulsion is made up of silver bromide supported in a gelatin like base. When light falls on it the silver, in the form of grains become released. This produces the silver image. The bromide part of the original is also released arid proceeds to make a nuisance of itself. Bromide slows down development and distorts the intensity of the nearby image. If you expose a large black blob and then fully develop it, keeping the film vertical and the developer is undisturbed the edges of the blob will appear white against the light grey background and below the blob will be an almost clear large white smear. At a lesser degree this is enough to produce harmonics on a recorded Sine wave.

Here I think I must introduce terms which you may not know. OPACITY is the reciprocal of the fraction of light transmitted. DENSITY is the logarithm of opacity. If various exposures are plotted on a log scale against density, the straight part slope plot is known as the GAMMA. Film gammas range from 0.25 to 4.0 commonly (see attached sketch). The curved part of the plot below the straight part is designated the TOE. The toe if plotted separately as transmitted light against original exposure will also produce a straight line. Hence for a variable density track there are alternative techniques each have their uses.

In the recording process light from a lamp is modulated by the signal to be recorded and in the case of variable density, finishes on the film surface as a nearly even line about half a thousandth of an inch wide. The modulator used by Western Electric known as the “light Valve’ consisted of two pieces of Dural foil in the shape of ribbons spaced some six thou apart placed in a strong magnetic field. It opened and closed as directed by the current, thus altering the amount of light.

The developed film would then be printed to produce the POSITIVE or PRINT. The finished article could then be reproduced in a film projector using an illuminated scanning slit one half to one thou wide with the film running at 18 inches a second or a little faster if in a Television Studio. As the Silver on the film is in the form of grains most of the hiss produced by their passage is generated as they pass the two edges of the scanning slit. With good processing a signal to noise ratio over our usual bandwidth, even with great care, was never better than 43 db and accordingly noise reduction techniques were always used.

The favourite one was a device which closed down the light valve ribbons 10 db until a signal comes along, the signal is also fed into a unit which with a small delay produced a DC voltage to widen the ribbons apart. This operated in some 10 milliseconds and lingered for some 30 milliseconds if no more signal appeared. It was of course a fine distorter of transients. Luckily the human ear is not very critical of the sound quality of transients. However this did boost our signal to noise ratio to 53 db but at that level noise could still be heard if your ears are attuned to it.

Anyway that is what the public got. Of the two techniques mentioned we kept Toe recording for frequency Test films. It had a high output level and was rock steady. Its signal to noise level was poor either as negative or a print. As the gamma was unity in either ease you could use either if the source of the signal was sinusoidal. With the Classical Recording (see the section on Gamma) to get no distortion the product of the negative gamma I x the positive gamma must he unity.

As the combined picture and sound negative is traditionally unity to a gamma of 2.2/2.4 the sound negative must be in the range of 0.45. It is an easy mathematical exercise to prove that a sine operated on by a Log function and then by a second one will appear as a sine wave, if the two tog functions when multiplied together make one. A further complication arises whenever light passes through film. The greater the density of the silver image, the greater the scattering of the sight, and the impossibility of collecting it all.

The effect is roughly logarithmic and is in the nature of another gamma which must be allowed for, in printers it is usually small, about 105 but I once found a figure of 1.4 in a certain re-recorder. When a recording chain is nearly lined up there is a dynamic check which can be applied, The light valve is set by DC bias to successive openings of double, normal, half, quarter and one eighth. At each setting a recording at minus 20db of tone is made. The film is processed and printed. The recorded tone is measured on both negative and positive points. The true dynamic gamma is obtainable from the negative and if all is well the five readings on print should read within a level of a quarter of a DB. If they don’t things have to be altered.

Over the years Western Electric were at daggers with RCA, the latter claiming quite correctly that their signal to noise ratio was some four DB better than ours. So it is, both theoretically and in practice. RCA used Variable Area recording in which the waveform of the signal is recorded and blacked-in equally on both sides of the film track. Noise reduction can be applied by squeezing the central clear area. One would expect that in this technique the black area would need to be kept as dense as possible. This is not so due to the effect of unwanted bromide giving distortion to the waveform as it does with variable density during development.

RCA have to do a rather tricky test to find an optimum printing point which does in fact show a minimum distortion at a lower density. With single track film the harmonic content of both Systems at medium level is a few 0 s. However, density mostly produces even and area odd harmonics.

Now with a good knowledge of the art we are all in good heart to appreciate Variable Density Push-Pull. When the Bell Labs invented push-pull and applied it to valve amplifier they also sought other uses. Sound on films was one. They designed and built a new light valve. This one had three ribbons the middle one taking the signal and the two outer ones handling the noise reduction voltages.

The light from the spaces each side of the middle ribbon was recorded on two separate adjacent tracks. When these tracks were made into electric signals they were combined in a transformer usually and received the full benefit.  As nearly all their original harmonics had been even, very good quality sound was left. RCA’s variable area with its odd harmonics mainly, could not benefit. With two balanced tracks greater and faster noise reduction could be employed. This was put up from 10Db to 20Db.

The speed of operation was lowered to 3 milliseconds, film noise was reduced and transients also. We at Denham used this system for internal studio use when recording music. It never got outside the studio but the single-track release prints did seem a bit better. There were also some unexpected gifts.

Film laboratories did sometimes break down. This left any film in situ to be brought along by hand with most unlikely time of development. The new system was hardly bothered and expensive retakes were avoided. Also if we on the operating side accidentally got the balance between
the two tracks wrong we usually got away with it, a gift from the Gods. The overall signal to noise ratio was now 63Db.

At the same time Squeeze Track was brought into service. This was designed to help very quiet passages. The width of the track was squeezed down thus reducing the film noise whilst the signal was increased an equivalent amount – as much as 20Db could be used. This, in practice, was seldom used to the full extent in case the very narrow track encountered a bubble mark on the film. This device was normally controlled by a foot pedal. With all these items nobody needed to worry about film noise.

During this account I have never bothered about the removal of unexposed film material by means of Hypo. However it was around this time that Electrolytic Recovery of the unused silver began. It is quite surprising how much silver a large and busy lab, acquire. Enough to be considered a valuable part of the revenue. It comes out on the electrodes as good solid silver. This rather splendid sound came along when the industry was in decline and as the average cinema goer is seldom interested in good sound. So long as the sound can be understood, that suffices.

I am glad that I was involved. I don’t expect that anybody will resurrect it.

vastEric Vast – December 1997.

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