Nagra 4.2 Portable Analogue Audio Instructions Manual
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In effect, the human ear cannot appreciate the absolute value of a high frequency sinusoidal signal, but, on the other hand, it can determine the harmonic content of a rectangular signal. In practice, this becomes a judgment of a tone and to render it as hard as possible. Procedure a.) Thread a tape onto the Nagra whose playback head has already been adjusted b.) Press the button REF. GEN. c.) Record and listen to the signal played back in the headphones (Line and Phone switch on the position Tape). d.) Adjust the azimuth of the recording head to obtain a sound as rich in harmonics as possible. The sound produced in the headphones should be the same in both positions of the Line and Phones switch. Look for the two points on either side of the optimum, which give a sound equally muffled, and adjust for the midpoint between them. 6.2. MAINTENANCE OF THE MOTOR COLLECTOR The motor of the Nagra 4.2 has a collector in a special alloy, which is perfectly resistant to corrosion. This ensures good operation, even under aggressive climates. The motor brushes are of a graphite silver composition, and their pressure on the collector is ensured by gold plated beryllium bronze springs. The brushes wear by friction on the collector and the product of their wear forms a self-lubricating layer called patina, necessary for the correct operation of the motor. The thickness of this patina depends on the specific pressure of the brushes. If this is too low, the operation will be noisy (squeaking). If it is too high, the insulation between the segments of the collector may become bridged over, resulting in an increase of current consumption. Remedy for Motor Noise It should be noted that by motor noise, the normal noise of the motor due to the ball bearings and the friction of the brushes on the collector segments should be discounted only a squeaking produced by the vibration of the brushes should be taken into account. Experience has shown that this can happen when the pressure of a brush falls below 12 grams. The remedy consists of an increase of the pressure by stretching the brush springs. The normal value is 25 gr ±5 gr. Metalization of the Collector During the running in period, it is possible that the brushes have not taken on the exact form of the collector. Local specific pressure can produce a very thick patina, which can short circuit the segments of the collector. In the worst case, the “SPEED & POWER” indicator on the front panel will warn the operator. It is recommended to check occasionally (every 100 hours of use) the no- load current of the motor. To do this, place the meter in the MOT position. The meter then measures the current taken by the motor. There is no special scale for this measurement, but the lower middle scale graduated 0 to 2 V can be used arbitrarily. With no load, that is to say, without a tape and with the pinch wheel just separated from the capstan, the needle should indicate between 0.2 and 0.3 V. If the needle indicates higher than 0.4 V, the collector probably requires cleaning. Cleaning of the Collector Open the Nagra and pull off the motor shielding (being careful not to deform it). Switch on the motor and clean the collector by rubbing with a rag or absorbent cotton soaked in a solvent. The insulation between the segments of the collector should be the same colour as that on the top of the collector. If the conditions are extremely bad, a very fine abrasive polishing cloth may be used, but it is essential not to use powdered abrasive, which could possibly cause damage within the motor itself.
6.3. LUBRICATION In the case of intensive use, it may be necessary, from time to time, to lubricate the ruby pressure ball on the take-up reel clutch. This ball can be found in the middle of the pulley, which drives the take-up reel. The best grease to use is an Esso grease, but if the recorder is not to be used at very low temperatures, any mineral grease can be used. For other oiling points, use an oil designed for aviation instruments, such as Isoflex PDP65 from Klüber-Munich, or P10 from the Bendix Aviation Corporation, USA; but once again, for recorders not to be used in very low temperatures, any good sewing machine oil should be acceptable.
8.0 MICROPHONES INTRODUCTION There are several different types of microphone pre-amplifiers available for the NAGRA 4.2, in order to accommodate almost all of the different types of microphones presently available. Microphone technology is not stable, and each year new models are put on to the market. Many of them require power supplies, which exist in different forms. The output voltages can vary, according to the type, in the ratio of 1 to 20. This means that it is not advisable to supply a tape recorder with only one type of microphone pre-amplifier. It is preferable to have interchangeable preamplifiers. 5.2. THE MICROPHONES A microphone converts acoustic energy into an electrical signal. Numerous physical principles have been used to obtain this conversion and there are many different types of microphone available: Condenser, moving coil dynamic, ribbon dynamic, microphones etc. Neither the perfect nor the universal microphones exist. Each type has its defects and particular qualities, and the choice depends upon the required effect. MICROPHONE CHARACTERISTICS Sensitivity Placed in a given acoustic field (e.g. µbar R.M.S.), a microphone will give a signal of X mV R.M.S. X represents the sensitivity or, in effect, its efficiency. To give this value sense, it is also necessary to state the internal impedance of the microphone and the load impedance. A classic dynamic microphone may have a sensitivity of 0.2 mV/ìbar from 200 Ohm internal impedance. A model giving 0.25 mV is considered to be sensitive, whereas a model giving 0.1 mV is unsuitable for capturing low level sounds. Condenser microphones always have a preamplifier in the microphone casing, otherwise their high impedance would not allow the signal to be transmitted along a cable. At the output of the preamplifier a typical sensitivity figure is 1-4 mV/µbar with a load impedance of 200-1000 Ohm. It is difficult to produce a very low noise preamplifier capable of receiving (without overloading) a signal given by a condenser microphone placed in a strong acoustic field (100 µbars). For this reason, it is better to have a special preamplifier for condenser microphones. The use of an attenuator between a condenser microphone and a preamplifier designed for a dynamic microphone is not recommended, as the signal-to-noise ratio will be unfavorable. Frequency Response The frequency response represents the sensitivity of the microphone as a function of the frequency. It is possible that the response will be different according to the direction from which the sound comes. This point is very important and will be dealt with in detail later. Microphone manufacturers pay careful attentions to the frequency response, and in general, most of the professional microphones available have a sufficiently good characteristic, at least for sound arriving along the principal axis. Coloration. Transient Reproduction. Reverberation An artificial reverberation chamber may have excellent frequency response, distortion and signal-to- noise characteristics, but it must also modify the signal, which passes it. It adds the reverberation. This shows that the frequency response, distortion and signal-to-noise ratio are not sufficient to describe an electro-acoustic device. A moving coil dynamic microphone makes use of resonances
to render its frequency response flat. With continuous sinusoidal signals it functions perfectly, but when a signal appears suddenly, the resonating device needs a certain time to move. When a sound disappears suddenly, the resonator continues to produce a signal. The result is that the transient signal (e.g. a percussive sound) will be colored by the inherent resonance of the microphones. This explains the difference noted by the ear between microphones with seemingly identical characteristics. In general, condenser microphones use resonators only in the extreme high frequencies, where the coloration phenomenon has little importance. As a result, their fidelity is excellent. Ribbon microphones can colour the low frequencies. Moving coil dynamic microphones colour to the greatest extent, this coloration is not always undesirable. They can improve certain voices, and the experienced engineer will not hesitate to use them under certain conditions. He can also use any defects in the frequency response for filtering, etc. Use at High Sound Levels Ribbon microphones and bi-directional condenser microphones can be damaged by a large air displacement. To record an explosion, a moving coil microphone, or better still, an omni-directional condenser microphone is recommended. A switchable microphone (uni-, bi- or omni-directional) risks the same damage as an ordinary bi-directional microphone. A microphone can be damaged under these conditions whether it is being used or not. It is advisable to place bi-directional and cardioid microphones in sealed boxes if an explosion is likely. Independent of the risk of damage, it is possible that a microphone will not reproduce well at levels greater than a certain value, above which the signal would become distorted. In general, moving coil microphones support the highest levels. Certain condenser microphones are designed so that an attenuator can be placed between the microphone capsule and the preamplifier. Signal-to-noise Ratio The recording of low level sounds can be disturbed by the combination of the microphone and its preamplifier. The word combination is used because the background noise does not come only from the amplifier. Take the case of a dynamic microphone whose impedance is 200 Ohm. As it does not have a temperature of absolute zero (- 273? C) the electron movement in this impedance will produce a noise signal called the thermic noise. The preamplifier adds to the thermic noise its own inherent noise, but in a recorder such as the Nagra 4.2, the thermic noise is by far the most important. The acoustic noise is measured in phons. The phons are decibels whose reference zero has been fixed by convention at 0.0002 µbars. The measuring device is not linear, but has a frequency response similar to that of the ear. For low levels, this frequency response is called the ASA A. It is possible to find out the equivalent acoustic noise level of a microphone and its preamplifier. Take for example a microphone of 200 Ohm having a high sensitivity (0.25 mV/µbar). Its noise level referred to the input will be -126 dBm ASA A (the dBm are decibels whose reference zero has been fixed at 1 mW). Now, 0.0002 µbars is equivalent to 0.005 µV (139 dBm). Therefore the equivalent noise of this microphone will be 139 - 126 = 13 phons. This figure is correct only if the impedance of the microphone is 200 Ohm. Often, certain microphones whose nominal impedance is 200 Ohm have higher impedances, at least in certain parts of frequency spectrum. The effect of this is to increase the equivalent noise. A condenser microphone can also be characterized by an equivalent noise level, thereby making it possible to compare the performance of these microphones with that of dynamic ones. Directional Characteristics Often, when recording sound it is desirable to attenuate certain unwanted sounds, such as echoes coming form the studio walls. Certain microphones have a sensitivity, which varies greatly according to the direction from which the sounds come. In effect, these combine a pressure characteristic with a velocity characteristic. Taking into consideration the air pressure at any given point, a microphone acting as a manometer is called a pressure microphone. The direction from
which the sound comes does not affect the pressure, except at very high frequencies, when the microphone makes its own shadow. On the other hand, the velocity of the air molecules can be used in a microphone. The word velocity implies a combination of speed and direction. A velocity microphone consists of a very light loose diaphragm, which follows the displacement of the air. It will be sensitive to waves, which strike the diaphragm perpendicularly whether they come from in front of, or behind it. Waves coming from the side will have no effect. This is the principle of velocity of bi directional microphones. Such a microphone eliminates an important fraction of the reverberation and if the source of undesirable noise is well localized, it can be placed in the dead zone of the microphone. In combining a pressure microphone with a velocity microphone, a unidirectional, or cardioid microphone is obtained. The two elements are, of course, mounted in a common casing and electrically interconnected. Secondary Characteristics Related to Directional Characteristics Omni-directional microphones (pressure) are much less affected by the wind than bi-directional (velocity) or cardioid microphones (because of their velocity element). The light diaphragms of velocity microphones have a tendency to float in the wind. It has been shown that the velocity microphones are easily damaged by a sudden air displacement (explosion). The response curve of an omni-directional microphone is reasonably independent of the direction. However, sounds coming from behind will have a tendency to become muffled. Bi-directional microphones attenuate the lateral sounds in a relatively uniform manner, but cardioid microphones, and above all, dynamic ones, can have a very bad frequency response in the null directions. In other words, the attenuation varies greatly according to the frequency. If a cardioid microphone is used to eliminate undesired noises, this phenomena is not of great importance. If such a microphone is used to balance the sound, when a very loud source is placed around the null area of the microphone, it is advisable to check the results. The internal impedance of omni-directional dynamic microphones is reasonably constant. They can therefore be used to feed their preamplifier either by voltage or current. On the other hand, the majority of cardioid microphones have an impedance varying greatly with the frequency. In this case only a voltage feed is recommended. Directional microphones only function well if they are sufficiently far from other objects, which can disturb the acoustic field, because an obstacle disturbs the pressure less than the velocity. PRACTICAL ADVICE ON THE CHOICE OF THE MICROPHONES Omni-directional Microphones (pressure) Robust, with low sensitivity to the wind, reproducing ambient sounds well-their price is lower than that of directional. Principal Use: reporting Special Uses: Lavalier microphone. For this use, special units have been created whose frequency response compensates for the perturbation of the body, and which takes into account the very low frequency sounds radiated directly from the chest. Recording music in the open air. Reverberation is non-existent and there are good microphones available - also very robust of low sensitivity, 0.1 mV/bar, which is acceptable as the sound level is reasonably high in these cases. Recording when the microphone is placed in the middle of a sound source (e.g. in the middle of an orchestra). Bi directional Microphones (velocity) These give a very good attenuation of reverberation, and a good fidelity for sounds coming from the null direction. They are very sensitive to wind noise, and they accentuate the low frequencies if the sound source is very close. This phenomenon gives a very Warm effect, which is exploited by certain charm singers. Principal uses: music. Dialogue in the case where the microphone is placed between two speakers. Remarks: Dynamic bi-directional microphones, i.e. ribbon microphones, are either of very low sensitivity, or very bulky. They radiate a magnetic field, which is capable of erasing a tape if placed close to it. Condenser microphones have a normal sensitivity.
Switchable Microphones Certain condenser microphones can function as omni-, bi- or unidirectional by means of a simple switching. Choice between Condenser or Dynamic Microphones Condenser microphones give the best fidelity. In particular their reproduction of transient noises is excellent, but they cost more and are less robust than the dynamic microphones. They require a power supply either from the Nagra or from an auxiliary device. They exist in two types: D.C. polarization and H.F. polarization. The performance and reliability depend, in the long run, more on the competence of the manufacturer than on the chosen system. Dynamic microphones are reputed to be more robust, but here again, the technological level of the manufacturer seems to be more important than the chosen system. The coloration which certain moving coil microphones give can be used to advantage. 5.6. MAXIMUM GAIN OF THE RECORDING CHAIN OR SENSITIVITY OF THE MICROPHONE INPUTS In general, the Nagra 4.2 is used to record the master tape, that is to say, the original from which copies are made. In consequence the recording level should, in certain cases, be lower than normal so that correction can be made during transfer. In the case where a loud sound is recorded, the noise level is that of the tape itself, the microphone noise level being lower, due to the reduced gain of the recording chain. In these conditions, it may be useful to use a high recording level so that the signal-to-noise ratio is as high as possible. In the case where the sound level is very low, the gain has to be increased to a point where the microphone/preamplifier combination noise level becomes greater than the tape noise level. Under these conditions, no advantage is obtained by recording at a high level. If the sound to be recorded is at a still lower level, it is better to adjust the gain to the point where the noise from the microphone clearly predominates, and under record the tape. In any case, an increase of gain will not improve the signal-to-noise ratio, whereas the inconveniences of high level recording will subsist: distortion and a reduced safety margin in the case of a sudden increase of sound level. For these reasons, the sensitivity of the microphone preamplifiers has been limited under normal conditions to 0.2 mV into 200 Ohm to enable a recording to be made at 0 dB. However, there exist applications where a Nagra must be used to obtain a tape recorded at nominal level to avoid having to adjust the playback level. In these cases, it is probably better to use a higher gain, and there are available increased gain preamplifiers. They are recognized by the figure following the letter X in the code, which indicates the number of decibels by which the gain has been increased. Nagra 4.2 Preamplifiers There are three types of microphone preamplifiers: a) plug-in preamplifiers which are fixed inside the Nagra 4.2, but are easily interchangeable. b) cable preamplifiers which are placed close to the microphone and which feed a plug-in preamplifier designed for condenser microphones. c) auxiliary preamplifiers which transform the line input into a third microphone input. These accessories are placed between cable connecting input No 3 and the microphone.
Changing the Plug-in Preamplifiers These preamplifiers are coupled to the rest of the Nagra by a connector. They are physically held in place by a small screw accessible from the bottom of the recorder. On turning the Nagra over, that is, placing it on its cover with the battery compartment upwards, the screw for preamplifier No1 can be seen, on the left viewed towards the front panel. Immediately to its right is the screw for preamplifier No 2. Once these screws have been removed, the Nagra can be opened. To do this, unscrew the two fasteners, which fix the tape deck to the box (on the right- hand side of the recorder). Turn them in the direction Open until the tape deck disengages itself. Open the Nagra. Remove the preamplifiers, which are immediately behind the meter, simply by pulling. Plug-in Preamplifiers LINEAR STANDARD -200 Code: QPSE-200-X0Y0 and STANDARD 50 Code: QPSE-50- X0Y0 These preamplifiers are similar to STANDARD type but have no filter incorporated. Their frequency response is flat from 30 Hz and the attenuation at 20 Hz is in the region of 2 dB. HIGH GAIN STANDARD 200 Code: QPSE-200-X6Y3 and HIGH GAIN STANDARD 50 Code: QPSE-50-X6Y3 These preamplifiers are similar to the Standard type but their gain is double (+6 dB) whereas their attenuation of the low frequencies is slightly greater (3 dB at 50 Hz). The maximum voltage, which they can receive, is 20 mV for 200 Ohm and 10 mV for 50 Ohm. For the application of these preamplifiers see Section 5.6. STATIC 5 Code: QPM-3-5 This preamplifier is designed to receive the signal from the Sennheiser condenser microphones type MKH 105, 405 and 805, Neumann type KM 73, 74, 76 and Schoeps type CMT 40. At the same time, these microphones are powered from the Nagra. This preamplifier is also designed to operate in conjunction with the cable preamplifier type QPLE, which is placed close to a dynamic microphone. The combination of a dynamic microphone plus a QPLE is electrically equivalent to a microphone MKH 105 etc. It is thus possible, when a Nagra is equipped with Static 5 to place either a condenser microphone or a dynamic microphone with the QPLE at the end of the microphone cable. Sensitivity: 2 mV gives 0 dB when the gain is maximum. Attenuation of low frequencies: adjustable by steps of 3 dB at 50 Hz up to -15 dB by a built-in switch. To operate this switch, it is necessary to open the Nagra. Maximum Input Voltage: 200 mV. Distortion and noise level: negligible, compared with those of the microphone. Temperature range: -55? to +71? C (-67? to + 160? F) HIGH LEVEL LINE Code: QPM-6 This preamplifier transforms the microphone input into a symmetrical floating line input. Input Levels: 0.1 to 24 V R.M.S. Impedance: 10 k Ohm. (on special order only) UNIVERSAL Code: QPAUT & QPUT These preamplifiers are designed to accept dynamic 200 Ohm, Phantom +12 V or +48 V and T powered +10 V condenser microphones.
The QPAUT composed of the preamplifier itself and the microphone power supply, is intended for the Mike input No 1, whereas the QPUT intended for the Mike input No 2 is composed of the preamplifier only, then it cannot be installed alone without the QPAUT. The QPAUT is externally switchable and the QPUT internally. Dynamic microphones: Impedance 200 Ohm, frequency response ±1dB 80 Hz to 20 kHz, sensitivity 0.2 mV/µbar, max. input level producing 1% distortion 50 mV. Condenser microphone: Phantom +12 V or +48 V, T powered (+10 V), same frequency response as dynamic ones, sensitivity 1.5 mV/µbar, max. input level producing 1% distortion 640 mV. SPECIAL PLUG-IN PREAMPLIFIERS Filtering Versions of the QPSE On special request, it is possible to supply Standard preamplifiers having a bass attenuation up to as much as 18 dB at 50 Hz or whose gain is different from the normal value. Cable Preamplifiers These preamplifiers are placed near a dynamic microphone. Their power supply is derived from the Nagra and is transmitted along the same cable as the signal. The Nagra should be equipped with a condenser microphone preamplifier (see above). CABLE SEN 5-200 Code: QPLE 200 and CABLE SEN 5-50 Code: QPLE 50 (for 50 ? microphone) These preamplifiers function in conjunction with the Static 5 incorporated within the Nagra. The overall results are equivalent to using the Standard 200 and 50 (see above) The low frequency attenuation can be adjusted on the incorporated Static 5 preamplifier. LOW FREQUENCY ROLL-OFF ATTENUATORS Why Filter? Sound engineers have long known that in certain cases an attenuation of the low frequencies can improve the subjective quality of the recording, because: A) Certain microphones (e.g. ribbon) have a frequency response which is very linear, but only if it is sufficiently distant from the sound source. Placed close to the latter (10 cm), the bass frequencies are accentuated. This gives, for example, a very warm voice, a phenomenon which certain singers exploit, but which diminishes the intelligibility. B) A sound studio is constructed and treated in such a manner as to reflect, in the same proportion, both low and high frequency sounds. When the sound recording is made in any other room, often the low frequencies are exaggerated, the carpets, curtains and other absorbent surfaces attenuating essentially the high frequencies, whereas the low frequencies are integrally reflected. In the two cases above, the attenuation of the low frequencies only re-establishes the linearity. In case A) this is clear, but in B) isnt the reality that which we would have heard if the ear was put in place of the microphone? The ear, however, has the facility of selecting the sounds in function of their direction and to subjectively attenuate reflected sounds. When recording in mono (and even in stereo), the microphone captures, without discrimination, all sound which reaches it. Of course, the directional properties of the microphone can be used, but the reflected bass frequencies can be behind the sound source and reach the microphone from exactly the same direction as the useful sound.
In addition to re-establishing the linearity, it has been found that in certain cases, an attenuation of the low frequencies can, although falsifying the reality, improve the subjective result. In particular, it can increase the intelligibility. On the other hand, it is sometimes necessary to have recourse to the attenuation of stage noises. In this case, choose the lesser evil. When should Filtering be done? Two solutions are possible, filtering during the recording, or the editing. Method comparison: A) In filtering during editing (dubbing) it is easy to start again, if an error is made. On the other hand, if filtering is exaggerated during the recording, the damages are practically irreparable. B) In recording linearly, the tape is loaded with signals, which produce a certain modulation noise. These signals will be eliminated at a later stage, but the noise will remain. C) Before passing to dubbing, it is necessary to listen to the sound during the rushes. An unfiltered sound is unpleasant and the producer may judge the result in a bad light. Conclusion It is recommended to filter during recording, but possibly slightly less than would seem necessary. There is little chance then of over filtering. The filtering will be finished during editing. In any case, the use of very good headphones is strongly recommended. Headphones, which cut the very low frequencies, should be mistrusted as they play the role of filter and mislead the operator.
METERING MODULOMETER OR V.U. METER To measure the level of an electrical signal representing a sound, there are two devices available, the modulometer and the v.u. meter. Both of them are voltmeters whose needle position represents the level. Their construction and use are however different. MODULOMETER The modulometer measures the peak value of the signal, irrespective of the form or the level, the modulometer takes into consideration the strongest positive or negative value. It is equipped with a memory, so the signal can be very brief, but the memory ensures that the meter needle advances and stays there for sufficient time for the operator to read it. The essential advantage of the modulometer comes from the fact that the measurement it gives is that which concerns magnetic recording, in other words it is the signal peak, which saturates the tape. The average value of the signal (as much as it concerns the listener) is of no importance to the tape. In particular, while recording noise, the modulometer indication is always exact, no matter how long the duration of the noise. The scale of a modulometer can be logarithmic, i.e. linear in decibels. In the case of the Nagra, it is possible, for example, to have a range of 70 dB. This allows the exact appreciation of even the lowest sound levels. Nevertheless, it is preferable to limit the range from -30 to +5 dB, to help operators who are used to VU meters, which are not logarithmic. The operator, on seeing the needle move, knows that his level is greater than -20 dB. VU METER In the days of electronic valves (tubes), a modulometer was very costly, and the rudimentary VU meter was often preferred. Later, it was noticed that the VU meter still maintained a certain following and because of habit and standards many radio stations still use them. A VU meter is a simple rectifier voltmeter whose response time has been standardized. If the signal to be measured is continuous, (e.g. a whistle) the VU meter will indicate a value the same as the modulometer, but if the signal is intermittent (e.g. speech) the VU meter will only indicate an average value, i.e. considerably lower than the instantaneous maximum levels. For speech, it has been found that this average value is approximately 8 dB lower than the peak value. By increasing the VU meter sensitivity by 8 dB, an indication of 0 VU. is obtained when the peaks reach the maximum value. This works relatively well in practice. For noise, the indication of the VU meter evidently becomes very inexact, and renders it practically useless. The v.u. meter, however, has certain advantages: a) Speech-music balance. If speech and music are recorded with a modulometer so that the peaks of the signal do not exceed the maximum level, subjectively the music appears stronger. This is due to the more continuous character of music signals. Therefore, in a mixed program, it is necessary to modulate the speech more strongly than the music. This can be done by modulating the music correctly and over modulating the speech or by under modulating the music. It is to be noted that a slight over modulation of speech is not catastrophic: a transmitter is fitted with a limiter, as in the Nagra 4.2, which cuts peaks exceeding the maximum level. The subjective deterioration of the sound quality remains unnoticeable. On the other hand, a strong modulation increases the range of the transmitter and is of direct interest for commercial radio stations. A VU meter under indicates the speech. In modulating a program to 0 VU the speech will be over modulated and the music under modulated. From this point of view, the VU meter seems to be of more interest for mixed transmissions whose quality is not of great importance, but whose range should be as large as possible.