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VU Meter Standards
Regarding C.E. Moule's article on VU meters in your September, 1976, issue, I agree there is much abuse of the VU meter, but I abhor the consequences of any new standard or scale.
The most important fact about the VU meter is that it is, in fact, a standard. It has standard electro-mechanical response to the input program material. Its chief value is that two differing pieces of equipment may be connected, levels adjusted, and the performance of both will be as nearly alike as the specific equipment will allow.
It is important to recognize that the Volume Unit, like the decibel, is a ratio of powers. As originally defined, 0 VU is 1.228 volts into a 600-ohm circuit (4 dB above 1 milliwatt or 0 dBm). In recording equipment, the exact level impressed across the meter circuit is unimportant as 0 VU in tape recording is defined as that level of a pure sine wave signal at 400 Hz that will produce one percent THD using a standard tape. The original standard was Scotch Brand 111A, 1.5 mil acetate.
However, most recorder manufacturers use 1.0 mil tape because of present user preference. Once having established this reference, all other measurements become meaningful.
Speech in an uncompressed system will produce instantaneous peaks that reach 12 dB above 0 VU (rms), which will not be indicated by the meter.
Good recorder design takes this into account so that the tape used, and not the electronics, becomes the limiting factor as to how much distortion will result from these peaks. It should be noted that the one percent distortion figure, mentioned above, is primarily the result of tape characteristics and the bias signal.
Good recording practice for original material should allow average peaks in loud passages to reach 0 VU with only occasional peaks above, but none higher than +3 VU. Limitations of home listening equipment cause most record companies to use two reference levels, 0 VU for loud passages and -10 VU for soft passage peaks.
It is part of the recordist's art to know what type of material is being recorded, his equipment, and the dynamic range required. By careful placement of the microphone's level setting and work with the director (conductor or individual artist), a recording will be produced with the desired dynamic range.
To gain maximum equipment efficiency for both the listener and the recording devices, the best signal-to-noise ratio, the least distortion, and a 20 dB VU range in peak program levels are desirable.
In summation, do not tamper with the standard VU meter. Learn to use it, understand its performance, and accept it. Make the choice of A or B scale as you prefer, but leave a good standard alone.
-James R. Hougen San Anselmo, Calif.
VU = Volume Indicator
The article by C.E. Moule in your September, 1976, issue is well done and brings up many interesting points.
During my many years in the communications R&D field, I have observed much misuse of the "volume unit." Engineers who should have known better used the VU interchangeably with the dBm on non-steady state signals; they have not recognized that the VU is a VU only when read on a standard "Volume Indicator" bridged across a 600-ohm circuit, and they were as confused as anyone in reading the instrument.
To the best of my knowledge, there has been no work done by experimental psychologists to determine how people average meter deflections over time intervals, and if there is a better way to solve the reading problem.
It would seem to me that with modern circuit knowledge, somebody ought to develop a means for determining volume of speech and program waves which would be applicable to modern tape equipment and would be less of an "after-the-fact" indication of level. After all, the volume indicator was developed using the then available techniques of about 40 years ago, approximately the same time period in which the forerunner of the modern tape system, the German Magnetophone, was being developed.
Certainly, a pilot tone recorded at the beginning of each track would enable the user of a tape to set his tone levels closely. All that would be needed in any playback system would be a standardized level indicator. The remaining problem would be the recording volume measurement and adjustment procedure.
I wonder whether any consumer type audio equipment which uses meters labeled VU comes equipped with certification that the meters meet the prescribed requirements for dynamic characteristics. If not, the manufacturers are missing a bet-think of how much more they could get for equipment with "certified VU meters." The author's thoughts about a different scale and, perhaps, a different unit are fine. In my opinion, let's turn the VU back to the broadcast and long lines people and come up with a new method of volume measurement tied in with the characteristics of modern equipment and using methodologies unknown 40 years ago. Of course, there would have to be a new reference standard correlated with the 0 VU reference volume, but that should be easy. And the basic knowledge developed in coming up with the volume indicator should not be wasted.
-Paul E. Griffith Black Mountain, N.C.
ADDENDA Rumble Filter/ Bass Boost Circuit
I would like to correct my article "Build a Rumble-Filter/Bass-Boost Circuit" which appeared in the July, 1976 issue of Audio.
In Example #2 the ratio of R2/R1 is equal to 2. In the section on Design Adoption, the reference to Fig. 7 is meant to apply to Example #2. In Fig. 1 the values of C1 and C2 are .047 pF, and R2 is 100K. In Fig. 4 the caption for the horizontal axis should read "Ratio of R2/R1." In Fig. 5 the vertical axis should begin at 100 Hz and go up to 1000 Hz.
For those wishing a more detailed analysis of this circuit, I can recommend Active Filter Cook Book written by Don Lancaster and published by Howard W. Sams and Co., Inc.
-Dick Crawford Los Altos, Calif.
(Source: Audio magazine.)
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