THERE are many occasions when it is desirable or necessary to make copies of either original discs or commercial phonograph records. Often you will want to keep a copy of a particularly interesting recording made for someone else, or several persons will want copies of a cut. The opportunities for duplication of recordings are many, and no recordist who is interested in the work should be without duplication facilities.
There are 2 methods known as re-recording or dubbing, and the second is the factory pressing process.
Fig. 1301--Setup for dubbing or making a copy of a recording.
Re-recording and Dubbing
Dubbing is a slang term which has come to mean the same as re recording. It is a process of playing back the original disc with a standard phonograph pickup whose output is fed into the recording amplifier, and thence to the cutter. The cutter engraves a new recording which is (ideally) an exact copy of the original (Fig. 1301).
Offhand, the operation appears extremely simple and, as far as the actual physical manipulations are concerned, it is. But here again, we run into the problem of audio quality. In previous Sections we have shown how the amplifier-cutter combination has been equalized to make records of a standard characteristic when sound is fed into the system.
In dubbing and re-recording we must again perform an equalization process since the sound output of the pickup playing back the original record must be of the same quality and character, as the sound the microphone or radio tuner furnished for the original recording. In other words, the pickup characteristics must exactly complement those of the cutter to produce a resultant flat output to be applied to the amplifier.
This brings us up squarely against the problem of record reproduction.
Heretofore, we have concentrated on making the product; now we must play it back properly.
Since the entire system, from input to stylus, has already been equalized on the basis of a flat sound source, the placement of any corrective equalization must be limited to that portion of the circuit which lies between the pickup and the amplifier input. We cannot equalize at any point beyond the grid of the first amplifier tube without upsetting the adjustments already made. Moreover, the input used for the re recording and playback pickup will be used probably for other pickups and for radio tuners ; it would obviously be unwise to tamper with the amplifier itself.
Equalizing the Pickup
From the standpoint of calculating curves, pickup equalization is usually the simplest process involving frequency correction in the en tire recording chain. For dubbing purposes, the object is to obtain from any record a flat output. For playback of records into a loudspeaker, it may be desirable, however, to adjust tone controls by ear to obtain the sound most pleasing to the listener. And pleasing sound, to the average listener, as some listening tests have shown, may not make a full-range frequency curve desirable. However, as stressed in the beginning of Section 11, when producing a disc, the full frequency range should be placed on the record, no matter what the source of the sound. When the disc is played back, the listener is free to adjust his tone controls; but if the full range is not recorded in the first place, the ultimate listener is restricted in his choice of playback response. Discs should represent, as nearly as possible, the original sound. Surface noise is not a consideration. Instantaneous discs introduce very little if any scratch; if, in re recording a commercial phonograph record we try to eliminate noise, we will also eliminate a part of the range. The entire object, therefore, in equalizing a pickup is to obtain a disc as nearly like the original as possible in every respect.
For making copies of original discs made on our own system, the pickup should produce a flat output from those discs. To accomplish this, first make a frequency record. As described in Section 11, this is a process of recording a series of tones from an audio oscillator, keeping the oscillator output constant throughout. This disc will always be valuable for showing the system's characteristic, and it should be care fully preserved.
Fig. 1302--Curve A, output of perfect magnetic pickup from perfect modified
C-V disc. Curve B, required equalization. Curve C, output after insertion
of proper low-boost equalizer.
Now provide a means of measuring while playing this record the output of the pickup by connecting a vacuum-tube voltmeter across the grid of one of the tubes following the amplifier input tube. Select a stage where the voltage is sufficient to actuate the meter, and--this is most important-a stage preceding which there is no equalization.
Connect pickup to amplifier and play the record. Leave all gain controls alone, once set. Note the voltage registered on the meter for each frequency played. Then, following the instructions in Section 11, make a graph of the pickup response.
If the pickup used is a magnetic with a perfect (constant-velocity) frequency response, and if the recording system characteristic is the standard modified-constant-velocity with 500-cycle turnover, the graph will resemble curve A in Fig. 1302. The upper portion will be flat because the characteristic of the perfect magnetic pickup exactly complements that of the perfect magnetic cutter (with a flat amplifier). But in modified-constant-velocity recording, groove width remains constant be--low the turnover frequency ; the pickup would produce flat output only if groove width continued to increase with dropping frequency. There fore, as shown in curve B, equalization equal to the drop will be required at the low-frequency end. With a low-boost equalizer inserted between pickup and amplifier, make new curves by replaying the disc. When the measured output is flat, as shown in curve C, the system is perfectly equalized.
In practical terms, almost no pickup is so perfect as to give a curve exactly like curve A in Fig. 1302, nor is any cutter so faultless as to produce a perfect modified-constant-velocity record. Neither is it possible to design an equalizer which would give precisely the correction shown. But, no matter what the output of the pickup from the test disc, a curve of it may be plotted. If a complementary curve is plotted showing the required equalization (like curve B in Fig. 1302), an equalizer can be chosen which, when properly adjusted, will give a reasonably flat output, regardless of the type pickup used or the curve of the cutting system. If the eventual pickup output is made flat within 2 or 3 decibels throughout the range, a very satisfactory re-recording system is obtained.
Dubbing of commercial phonograph records is a somewhat different matter. The curves used in making these records are not usually standard, although almost all U.S. makers use a 500-cycle turnover. Usually the high range is boosted, often not in a linear manner. If a frequency record could be obtained showing the curve used in these recordings, it would be a simple matter to adjust for flat output, just as was done above with our own test disc, but such records are not available. The best procedure is to use one of the available standard-frequency records, of the type recommended for use as a comparison in Section 11, and equalize the pickup for flat output from this. With these discs, incidentally, just about the same results as those shown in curve A of Fig. 1302 will be obtained, if the pickup is a very good magnetic. An adjustable high-frequency attenuator should be added in addition to whatever equalization is necessary for the original flat output. The phonograph records should then be adjusted by ear for the most natural sound, using a good loudspeaker system and a flat amplifier. Do not try to eliminate surface noise. Just adjust for the most natural sound of all the musical instruments. Re member: while equalization of the pickup for phonograph records is a hit-or-miss proceeding, depending mostly on the ear, adjustment for playing one's own discs definitely is not. Make the test record and the graphs. Just as with the recording amplifier and cutter, correction of the pickup is time-consuming and painstaking, but it is the only guarantee of good copies.
Choice of the pickup is very important. Only a few better grade magnetic units will have the ideal constant-velocity characteristic, and crystals will not have it at all. A good crystal will have a flat constant-amplitude characteristic. However, the type of characteristic is not nearly so important as the frequency range and the linearity of response. The dubbing pickup should have at least the same range as the cutting sys tem, with very small peaks in the range. Most pickups--crystal and magnetic--have an upper and a lower resonance point, and additional ones are present, due to the arm. These peaks should be either outside the range to be reproduced or should be very small. The impracticality of equalizing out these peaks automatically precludes the use of cheap units.
There are a number of high-quality, reasonably priced (under $30) magnetic and crystal pickups or pickup cartridges available for non professional use. Some of these are good enough for professional use.
Most of them have permanent sapphire styli.
If a pickup without a permanent stylus is chosen, be sure to use only the best shadow-graphed steel needles, changing needles after each play. The surface of "acetate" discs is fairly soft and easily plowed up by improperly shaped or worn styli. Most separately sold "permanent" needles will not have the correct angle or the correct shape. Many of them will quickly ruin, not only instantaneous, but also shellac records.
Any pickup used should also have low needle pressure on the disc. This can be determined with a postal scale. If it is realized that a pressure of 3 ounces on the minute area covered by the needle point is equal to several tons per square inch, its importance will be appreciated. Better pickups today exert pressure of only about 0.5 to 1 ounce, sometimes less. Do not counterweight a commercial pickup, however, to reduce pressure, since this will usually make it track improperly.
Fig. 1303-Basic high-booster for crystal pickups.
Fig. 1304-Basic high-attenuator for crystal pickups.
For reduction of angular tracking error (see Fig. 407, Section 4), a long pickup arm is best, but shorter ones with offset heads are accept able. Short, straight arms often damage instantaneous discs to a slight extent and are likely to skid across the surface if the grooves are not deep enough.
Any equalizers must be placed between pickup and recording amplifier, not within the amplifier. Several equalizers for this purpose are shown here.
Since crystal pickups are constant-amplitude devices, they need high-frequency boost when playing standard discs. Fig. 1303 shows one standard method of boosting highs. The selection of value for C and the setting of the 2-megohm potentiometer determines the range and amount of boost. For sharper results, 2 such R-C combinations may be used in series.
For reduction of high-frequency response of crystal pickups, a condenser in series with a variable resistor may be shunted across the input tube grid, provided a series resistor is inserted between pickup and grid (see Fig. 1304). C is again a matter of choice through necessity.
The series resistor is necessary to isolate pickup from equalizer and to keep the impedance across the pickup high, since low termination impedance will reduce output of lows. A crystal pickup is a capacitive de vice and a capacitor shunted directly across it would only reduce output without discriminating against any frequencies.
Fig. 1305 shows a low-frequency booster for magnetic pickups. R1 is the termination recommended by the manufacturer. R2, R3 and C are chosen by experiment. Output will be quite low with this circuit since it is actually a treble attenuator. Many circuits of this type are to be found in radio magazines and guides, and experiment will be fruitful.
For the most reliable equalization, a small equalizing amplifier is recommended. One manufacturer recommends the one shown in Fig. 1306 for the variable-reluctance pickup cartridge, but it is valuable for any pickup. The tube is a 6SC7, and the required B-voltage of about 90 can be conveniently furnished by a 90-volt battery mounted within the playback turntable case, or by the main amplifier.
In this truly versatile unit, any standard R-C or L-C equalizers may be inserted in either the first plate circuit or the second grid circuit, with out causing any undesirable reaction on input or output. If the gain of the preamplifier is not needed, a 0.5-megohm potentiometer may be connected from HI-output to ground and the desired output voltage tapped off. Or, if the main amplifier has a low-impedance input, the output may be taken off a tap the required resistance from ground. If the pre amplifier gain is desired and the amplifier input is low-impedance, an ordinary plate-to-line transformer can be used. The input resistor R should equal whatever termination resistance the maker recommends for his pickup even if it is very low, though gain will be reduced in this case.
If desired, the amplifier may be constructed for use as a feedback equalizer such as that of Fig. 1113, Section 11. In this case, a 6SN7 may be used. It will have no gain except at the frequencies to be accentuated.
Dubbing Procedure
The actual procedure of dubbing is simple. Use a good turntable of correct speed--check with a stroboscope disc, don't estimate--and with no vibration. Start playback and recording tables, position the cutter over the beginning of the blank disc and lower it. Place the pickup needle in the first groove of the original, then open the gain control. The procedure from there on is the same as for any other recording.
The process described in the last paragraph is, according to purists, re-recording. The truer, original meaning of dubbing is the insertion within program material of matter recorded on another disc. For in stance, during a dramatic performance which is being recorded through a microphone, phonograph records may be used, with the dubbing pick up, to furnish background and "bridge" music--an excellent reason for having mixing and monitoring facilities. Music can be faded in under the voices and then brought up to full volume to provide separation be tween scenes. Sound-effects libraries are available, which consist of records of almost every imaginable sound. During performances or dramatized talks, these sound-effect records can be played into the re cording amplifier at the proper moment. They are often superior to effects created manually in the studio because of their very lifelike quality. Most of them were, in fact, recorded from life.
Often composite recordings can be made. One of the many uses to which dubbing can be put was demonstrated during a political campaign when an orator recorded a speech in which he referred often to the late President Roosevelt and several times quoted Roosevelt's words.
Fig. 1305--Bass booster for magnetic pickups.
But, instead of the orator repeating these words himself, he would say, "On July 14th, 1943, Franklin D. Roosevelt said . . . " and then a short excerpt from a record of Roosevelt's voice would be dubbed into the recording amplifier, after which the orator would continue.
Many interesting and--to the uninitiated--strange tricks can be done with dubbing. It is possible for a singer to sing all 4 parts of a barber-shop quartet. First make a record of the melody, and then a disc of the second tenor part, simultaneously dubbing in the record of the first part. Next, record the baritone part, dubbing in simultaneously the 2-voice disc just made; and finally record the bass while dubbing in the other 3 voices. Imagination and ingenuity will convert the dubbing pick up into a tool of many uses.
Fig. 1306--Typical unequalized preamplifier for low-level magnetic pickup.
Pressing
If a substantial number of copies is to be made of a particular disc, the re-recording process is extremely tedious and time-consuming. In addition, of course, the finished copies are not as permanent as a some what tougher material would be and they suffer from the inevitable loss of fidelity. Therefore, the pressing process is used. This is the method by which all commercial phonograph records are duplicated and turned out in mass production. The pressing is done in a factory, and the service is usually available to anyone. It is economical only when a large number of copies are to be made.
The original recording is made in the usual manner, although large producers of phonograph records frequently use wax blanks instead of instantaneous-type materials. The disc used, however, must be somewhat larger than usual, and the technical requirements for quality are strict.
Table 13-1 gives the diameter of the blank discs used for finished pressings of given sizes. Maximum and minimum groove diameters are also shown.
Table 13-1--Dimensions (in inches) of originals for processing.
The cut must be the proper depth, shiny, and generally good, with out any objectionable patterns (see next Section). It is extremely important that no disc intended for processing be played back. Make sufficient test cuts to assure proper system adjustment, then depend on the monitor and the ear to tell whether the disc is satisfactory.
At the factory, the disc is clamped in a retainer ring (the reason for the extra diameter), and a microscopically thin, metal surface, is deposited over it. Next a heavy coating of copper is electroplated onto this surface. The whole is removed from the disc, and the part which contacted the original is a negative reproduction of the grooves. This copper negative is soldered to a heavy backing plate and becomes the initial master stamper. A thin coating of nickel is usually deposited on the grooved surface for additional strength.
The record material, which may be a shellac compound or vinyl acetate (known as vinylite), is prepared by heating, and the grooved face of the master is pressed into it. After cooling, the record is trimmed and labeled.
Selection of the record material is up to the recordist. Ordinary shellac is inexpensive, but rather coarse, containing an abrasive sub stance intended 'to grind steel needles to the shape of the groove, but in the process much surface noise is produced. Shellac is used for almost all phonograph records and is responsible for almost the entire scratch problem which has bothered the industry for years. It is quite hard, though brittle, and will take quite a beating.
Vinylite is much more expensive than shellac, but it is extremely smooth--almost as smooth as the original instantaneous disc coating.
As a result, there is very little surface noise. Commercial record makers are just beginning to put out vinylite discs in limited quantities for public consumption. The rather surprising thing about the attendant advertising is that they are billed as unbreakable rather than scratchless. While it is perfectly true that breaking a vinylite record is a job for a tearer-in-half-of-telephone- guides, the very much lowered surface noise would seem to be the important factor.
The comparative softness of vinylite discs is to some extent a disadvantage. A needle may be dropped on a shellac record without too much damage, if not dropped from too great a height; the same mishap with a vinylite will cause a deep furrow to be plowed up. Vinylites are also easily scratched in ordinary handling. For this reason, vinylite should not be chosen if the users will employ cheap heavy pickups and little care.
Some manufacturers make laminated pressings in which the actual playing surfaces are thin sheets of a high-grade material of better smoothness than ordinary shellac compounds. Between the playing surfaces is sandwiched a cheap paper filler. These discs can be reproduced excellently (with reasonable care) and often cost no more than standard shellac, because the expense of the good surfaces is offset by the cheapness of the filler material. Some British records are made with a very high grade of shellac with low surface noise.
Before making an original for pressing, it is advisable to consult with the company which will do the pressing and learn the detailed requirements. However, if a good re-recording system has been set up, any good record can be re-recorded onto a new oversize blank for pressing with only a small loss in fidelity.
The eccentric groove at the end of the pressings is placed in the master by the pressing firm and, if this is desired, no lock groove should be made in the original.