1974 TAPE Recording & Buying GUIDE = ein Verkaufsmagazin
Die Amerikaner hatten immer ein Händchen ffür eine Goldgrube. Und so sprossen aus allen Ecken die Produktübersichten aus den Verlagen, versteckten sich unter dem Deckmantel einer USA-weiten wertneutralen Marktübersicht und waren doch nichts weiter als Anzeigenblätter. Um die Inserenten zu ködern, wurden durchaus seriöse und kompetente Artikel an den Anfang gestellt. Am Ende wichtig waren die Listen mit den Preisen und den minmalen Eigenschaften. Hier geht es zu der einführenden Seite dieser 1974er Übersicht.
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COMPARING NOISE-REDUCTION SYSTEMS
How a variety of noise-reduction methods for home tape machines increases dynamic range of recordings - By JULIAN D. HIRSCH - Hirsch-Houck Laboratories
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COMPARING NOISE-REDUCTION SYSTEMS
One of the major obstacles to achieving realism in the recording and reproduction of sound is the restriction of dynamic range due to the limitations of recording and reproducing equipment and materials. Dynamic range can be defined, for our purposes, as the ratio between the highest signal level that can be handled without excessive distortion and the residual noise level in the system.
Dynamic range of many musical performances exceeds that of even professional tape recording equipment. Maximum levels must be restricted to avoid tape saturation and distortion. When transferring to a master disc, excessive levels can cause groove overlapping or perhaps a recorded velocity beyond the tracking ability of available phono cartridges. As a result of this limitation, the fainter sounds must inevitably be submerged and lost in the background noise which is inherent in the tape coating or the recorder's electronic circuits.
The dynamic-range problem is being attacked on several fronts. Improved tape formulations and head designs allow higher signal levels to be recorded and have lower intrinsic noise levels. With the latest semiconductor devices, electronic circuit noise is approaching the theoretical minimum. Skillful recording techniques and microphone placement can do much to reduce the dynamic range of the program before it is actually recorded. Finally, a number of electronic noise reduction systems have been developed.
The home recordist, lacking the professional's financial resources and extensive experience, nevertheless has a choice of several commercially available noise-reduction systems to assist him in improving the range of his recordings. Although most of these systems are capable of effective performance, they are limited in their compatibility with each other.
It is possible to classify noise-reducing systems as dosed or open. A closed system must be used both when making a recording and when playing it back. The special characteristics of the recording processor are complemented by exactly equal and opposite characteristics in the playback processor. Therefore, at least in the ideal case, there is no effect on frequency response or distortion, but merely an improvement in overall S/N ratio.
Although the most effective noise-reduction systems are of the closed type, there are obvious disadvantages to the requirement of having a specially processed recording. Open systems overcome this objection, since they function with any type of input program. Any special information required to operate the playback processor comes from the program itself, rather than from a complementary recording characteristic. In general, open noise-reducing systems are either less effective than closed types or are more noticeable in their operation.
In a sense, the use of recording pre-emphasis and playback de-emphasis can be considered as a closed noise-reducing system, since it improves the overall S/N ratio of a recording (or broadcast) process. However, it is a static system, operating independently of program level, and will not be considered here. Similarly, a simple low-pass or band-pass filter (such as those found in most amplifiers and receivers) is a static open-type noise-reducing system, but it usually affects the audible program as well as the noise. All the noise-reducing systems we will discuss are dynamic; their action is controlled by the signal characteristics.
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Closed NR Systems
Dynamic closed noise-reducing systems are also known as "compressor/expanders," or "compandors." The recording gain is controlled by the signal level, increasing the amplification of weak signals so that they are above the background noise and decreasing the amplification of loud signals to keep them within the recorder's maximum limits.
The playback transfer function (output versus input) is the inverse of the recording function. Thus, the level of weaker signals is reduced, relative to the stronger ones. Since any noise introduced in the recording and playback process is reduced together with the lower level signals, the net result is the preservation of the original program dynamics, with a reduction in the noise added during the recording and playback process.
Some compandors operate over the entire audiofrequency range. They can reduce low-frequency noise, such as hum or rumble, as well as the usually more audible high-frequency hiss.
The professional "A-Type" Dolby system is of this type, with the unusual feature that it functions as four independent systems, each covering a portion of the audio range.
Another professional compandor, manufactured by Burwen Laboratories, provides an extraordinary 55 dB of noise reduction. Both of these systems are far too expensive for use in consumer products or by hobbyists.
However, a relatively inexpensive compandor, the dbx 117, operates at all frequencies and can provide an overall S/N improvement of as much as 20 dB.
The widely used "B-Type" Dolby system affects only the middle and high frequencies. Depending on the frequency content of the program, its action may begin as low as 500 Hz or as high as several kilohertz. In either case, the amount of recording "boost" (and playback "cut") is a function of program level.
The Dolby "B" system has no effect on strong signals. It provides a progressively increasing noise-reducing action as the signal level drops below - 20 dB (relative to a standard maximum recording level).
The single-band, relatively simple Dolby "B" system is now built into most high-quality cassette recorders, as well as a few of the finest open-reel machines.
It is offered as an external "add-on" accessory by several manufacturers. The Dolby system is also used by a limited number of FM broadcast stations, and a few FM tuners incorporate the Dolby playback circuits.
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The JVC Automatic Noise Reduction System (ANRS)
The JVC Automatic Noise Reduction System (ANRS) is somewhat similar to the Dolby "B" system in its basic concept, although its recording and playback characteristics do not precisely match those of the latter and its circuitry is completely different.
Both the Dolby and ANRS systems can provide a noise reduction of 6 to 10 dB with little or no effect on the audible program. With either system, an encoded program can be played back without decoding. It will sound slightly "bright," but this usually can be corrected with the amplifier treble tone control.
The final result can be very satisfactory, although not exactly as it was meant to be heard (the tone-control action will actually provide some noise reduction).
When a Dolby tape is played back through a ANRS unit (or vice versa), the results are even better, but under certain conditions the differences in filter tracking can be heard as a "swish" or other transient.
This "quasi-compatibility" does not exist when using wide-band compandors, since a compressed program generally sounds less natural than one with unmodified dynamics.
The ANRS system was originally developed for JVC's CD-4 discrete disc quadraphonic demodulators where it is used to reduce noise on the ultrasonic sub-channel signals. It is also built into a number of JVC cassette recorders and is available from JVC as an "add-on" accessory.
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Open NR Systems
Philips DNS
Among the open-type noise-reducing systems, the "Philips Dynamic Noise Limiter" (DNL) is probably the best known. The DNL is a low-pass filter, cutting off at about 5000 Hz. In the absence of program content above that frequency, the filter is "on" and reduces hiss appreciably. Even a small amount of high-frequency program energy causes the filter to "open up" slightly, and the process is smooth and gradual.
Accordingly, the filter is effectively out of the circuit at most musical program levels. In most cases, the Philips DNL operates with little or no audible side effects, but with a fairly noisy background its action can often be heard as a "swish" when the filter is varied.
The Philips DNL system is found in several of their cassette recorders (sold in the United States under the Norelco name), as well as in the deluxe Nakamichi 1000 cassette recorder. While it is not as effective as either the Dolby or ANRS systems, it has the important advantage of being usable with any type of program material.
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Kenwood KF-8011 "De-Noiser
A more elaborate open noise-reducing system is used in the Kenwood KF-8011 "De-Noiser." This accessory unit has four contiguous band-rejection filters covering the range from 3 kHz to 15 kHz. Each filter is controlled by the program content in its own frequency range, attenuating the affected band by as much as 15 dB in the absence of significant program level.
The Kenwood "De-Noiser" operating threshold can be adjusted to suit any requirement. Each of its four bands can be engaged or de-activated at will. It has the unique feature of being usable in the recording path, thereby reducing any noise on the incoming signal (which none of the other systems can do) and then being used again on playback for additional noise reduction. Like the DNL, the Kenwood system can often be heard as it operates, but this effect can be minimized by proper adjustment of its operating controls.
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The autocorrelator in Phase Linear 4000 preamplifier
Recently, we heard of a "new" open-type noise-reducing system which promises to give effective S/N improvement without any audible side-effects. Actually, it is "new" only in its application to a consumer product, having been used for years in processing weak radar returns and photographs received from space satellites.
This is the autocorrelator featured in the Phase Linear 4000 preamplifier. In its present form, it is intended primarily for reducing phonograph record hiss, but would seem to offer hope to tape recordists as well.
The autocorrelator, in effect, splits the signal into two paths and multiplies it by itself. The program portion of each channel is coherent with that in the other, resulting in a reinforced signal after multiplication. Random noise, on the other hand, has no definite phase or waveform relationship in the two signal paths and tends to be reduced in the multiplication process.
The result of the autocorrelation is a substantial improvement in S/N ratio. At this time, we have not had sufficient experience with the Phase Linear system to evaluate it, but a demonstration of its noise-reducing properties was most impressive.
Summarizing, one can say that closed noise-reducing systems are, in general, the most effective although they may limit one's choice of program material. Among these, the Dolby "B" system is so widely used as to be truly universal, as well as doing a most effective job.
For the tape hobbyist who records "live" programs, a full-range compandor such as the dbx 117 offers some interesting possibilities, since its 20-dB improvement factor is the best of any home noise-reducing system and is effective at all frequencies.
- Anmerkung aus 2019 : Die Tester hatten in 1974 irgenwie übersehen, daß das dbx System unter bestimmten Umständen hörbar "pumpt". In den Tascam Semiprofi-Bandgeräten haben wir solch ein (professionellen) 8-Kanal dbx Aufsatz, den wir noch einmessen müssten. Doch die Digitalisierung hat diesen ganzen Aufwand ad absurdum gelegt, es wird nicht mehr gebraucht.
For listening to previously recorded tapes or cassettes, the various open-type systems are all worthwhile (of course, a Dolbyized tape should be played through a Dolby decoder). One thing is certain - there is no need to tolerate background hiss in any tape format. It can be eliminated or greatly reduced at moderate cost
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Im Kasten
HOW THE DOLBY SYSTEM WORKS - genau zur richtigen Zeit
If ever an invention arrived "just in time," it was the Dolby noise-reduction process. Since 48 or even 50 dB of signal-to-noise ratio (achieved with better cassettes with out Dolby) was just marginally acceptable as background hiss on tape, as far as the serious recordist was concerned, another few dB of quieting would push the medium over into the realm of "true" high fidelity.
And that is exactly what Dolby did! The concept is really quite simple (although its successful execution took much work and experimentation). Since tape hiss is basically a high-frequency form of disturbance that is masked by loud program content, Dolby devised circuitry which pre-emphasized high frequencies during the recording process-but only the high frequencies associated with low-level musical passages.
The degree of this pre-emphasis is dynamically regulated, based upon actual musical levels. Note from the illustrations that loud passages are not emphasized at all, while successively lower musical levels are subjected to increasing pre-emphasis.
Recordings made in this way and played back on ordinary machines would sound shrill, particularly when low-level musical passages are reproduced. The playback part of the Dolby system, however, works conversely, rolling off the highs for low-level signals while leaving loud musical passages unaltered with respect to frequency response.
The result of this two-part process is net flat musical frequency response, but with a highly significant reduction of background hiss or noise introduced during the record and playback process by the tape and associated electronics.
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