VU meter Input

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abbey road d enfer

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Meter movements like to be driven with a current source.
Do they? It depends very much on what aspect of performance you want to optimize.
A galvanometer is a second-order resonant system. There is a particular source resistance that results in no overshoot. The system has then a damping factor of 1. It makes the response a little slow. Making the system a little less damped results in some overshoot and oscillations that are slowly decreasing in amplitude (damped oscillations). By carefully tuning the damping factor, it is possible to increase the apparent response speed with negligible oscillations.
Current drive (infinite source impedance=>undamped system) produces a large overshoot and long oscillations, which is not a desirable effect, particularly for measuring quicly varying signals.
Adding a series resistor with an op amp voltage source sort of approximates this.
It is not the intention. The purpose of resistive drive is to optimize the ballistics of the system.
The VUmeter standard thoroughly describes the target response and how to achieve it.
Alternatively you can use a transconductance amplifier to create a current source output from a voltage source input.
Which would result in a hardly usable system.
 

Brian Roth

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Jay McKnight was one of the "rocket scientists" at Ampex back in the day, then founded and still owns MRL which makes alignment tapes. He has made a variety of posts on this topic via the Ampex mailing list. Here are some links he posted on that list re. the subject of VU (aka SVI) meters. Quoting Jay:


For the straight scoop on vu meters, see a reprint of the original1940 IRE paper at
http://www.aes.org/aeshc/pdf/chinn_a-new-svi.pdf .

For a simple way to measure the meter dynamics, see
http://www.aes.org/aeshc/pdf/mcknight_svi-measurement.pdf .

For my comments on the essence of "vu-ness", see
http://www.aes.org/aeshc/pdf/mcknight_qa-on-the-svi-6.pdf .


Bri
 

moamps

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Today I had some free time so I made measurements on VU meters that I currently have in drawers to see what the real difference is when the value of the resistor connected in series with VU meter is 3k6 or 3k9 and when the output impedance of the source changes between 600 and 50 ohm.
The following VU meters were used in the test:
Sifam AL20, AL22, AL29, R32,
Kyoritsu KM66, KM86,
Teac 5296006000 and
Nissei TR57.
The conclusion is that no difference in rise time or change in overshoot value or other ballistic parameters can be observed. Even when this resistor is reduced to a value of 1k8, these changes are not seen. It is obvious that the damping was predominantly made and achieved by the mechanical design of the VU meter and the internal rectifier and coil resistance.
However, if a 3k9 resistor is used instead of 3k6, then the full scale value is no longer + 7dBu (1.734V) on all Sifams but is higher and there is an error in the measurement, which is about 0.25dB. This is usually not so important, but if VU meters are used in a stereo pair for mastering, this may need to be taken into account.
 

abbey road d enfer

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Today I had some free time so I made measurements on VU meters that I currently have in drawers to see what the real difference is when the value of the resistor connected in series with VU meter is 3k6 or 3k9 and when the output impedance of the source changes between 600 and 50 ohm.
Indeed a difference of 300 or even 600 ohms compared to a total of 7500 is bound to produce minimal differences.
Anyway, considering that VU meters are very inadequate instruments for evaluating quickly varying levels, it doesn't matter much.
However, I believe if you had done the same experiment with either no series resistance or with a current source, teh differences would be more pronounced.
The conclusion is that no difference in rise time or change in overshoot value or other ballistic parameters can be observed.
Note that the McKnight paper resorts to rather sthorough methods for evaluating these mechanical parameters.
It is obvious that the damping was predominantly made and achieved by the mechanical design of the VU meter and the internal rectifier and coil resistance.
I beg to differ on this point. The total damping is the result of the combination of the mechanical and electrical damping. The latter, in particular, cannot be attributed singly to the internal resistance. It is clar that the mechanical damping dominates over the electrical, though, which makes variations in the electrical circuit minor.
However, if a 3k9 resistor is used instead of 3k6, then the full scale value is no longer + 7dBu (1.734V) on all Sifams but is higher and there is an error in the measurement, which is about 0.25dB. This is usually not so important, but if VU meters are used in a stereo pair for mastering, this may need to be taken into account.
This is where the standard gives two contradictory constraints. I guess the fact that very-low impedance line sources were not commonly available at the time is the cause for this inconsistancy.
 

moamps

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I beg to differ on this point. The total damping is the result of the combination of the mechanical and electrical damping. The latter, in particular, cannot be attributed singly to the internal resistance. It is clar that the mechanical damping dominates over the electrical, though, which makes variations in the electrical circuit minor.
Thank you for participating. I honestly don't have a complete picture of the behavior of that electromechanical system (I look at it more or less as a speaker), and especially what effect the fullwave rectifier has on electrical damping (it is located between the source and the moving system itself). One day I will disassemble one VU meter and measure all the transients using DSO.
 

Brian Roth

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Just to toss another test into the mix, a long-time friend has posted these test signals at his website:


As Rick mentions, many/most "VU meters" fail to meet the (admittedly old) ballistic specs.

Bri
 

abbey road d enfer

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Thank you for participating. I honestly don't have a complete picture of the behavior of that electromechanical system (I look at it more or less as a speaker), and especially what effect the fullwave rectifier has on electrical damping (it is located between the source and the moving system itself).
Agreed. A long time ago, at school, I sweated over the analysis of the moving-coil galvanometer, and the effects of the rectifier were more or less neglected, because it would introduce a transcendent function in the resolution. Today, with the aid of a computer, it is certainly possible to have a more precise description of the actual behavior, but what practical value would it have? Nobody seriously relies on VU meters today.
 

Winston OBoogie

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For DIYers, I think Uwe's 51 led meter with switchable ballistics looks to be a good option as far as usefulness:

The pics and info John R posted here of the meter project he worked on for another company looked great too, although I don't believe this is available for mere mortals is it? 😉
 

moamps

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A lot of arguments can still be found in favor of using VU meters. Here is one of them
I personally like to have both a VU meter (mechanical) and a Peak meter (this is a consequence of working as a broadcast engineer). Lately, I recommend students to use the free Orban program where many parameters (VU, peak, LUfs) can be read at the same time.
 

moamps

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Just to toss another test into the mix, a long-time friend has posted these test signals at his website:

In the text, they forgot to mention the fact that 600 ohm output should be loaded with 600 ohms when measured. Hence the difference of 300 ohms.
In practice, all of you who use 1176 and similar 600ohm gear connected to modern high Z input devices (Zin = 10 to 47k) are misled into a small ballistic error ;).
 

moamps

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A long time ago, at school, I sweated over the analysis of the moving-coil galvanometer
... housed in a large wooden box. That’s when I first heard about the concept of parallax and the purpose of the mirror behind the needle. I will always remember an experiment where we tested the effect of EM interference on the accuracy of a measuring instrument. We used a large 1m air coil through which we let AC current from the mains through the variac. The poor instrument was inside the coil. And some of us had a headache for a few hours.
 

abbey road d enfer

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... housed in a large wooden box. That’s when I first heard about the concept of parallax and the purpose of the mirror behind the needle. I will always remember an experiment where we tested the effect of EM interference on the accuracy of a measuring instrument. We used a large 1m air coil through which we let AC current from the mains through the variac. The poor instrument was inside the coil. And some of us had a headache for a few hours.
Some would argue this being assimilated to torture...
 

abbey road d enfer

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A lot of arguments can still be found in favor of using VU meters. Here is one of them
Honest, don't you think some of these arguments are specious?
I don't see how VU meters specifically answer any of the requirements, except maybe some crude form of loudness assessment.
One purpose they have forgotten is tape machine alignment, where the superior resolution is a clear advantage. I understand they don't expect to make a living selling products for the tape recording market... :)
I personally like to have both a VU meter (mechanical) and a Peak meter (this is a consequence of working as a broadcast engineer).
Never been a broadcast SE, but have done (still do) a lot of studio and live work. Since the beginning (1974) I felt the need for a meter that would catch transients. The availability of affordable LED's and various chips was perfect for putting it in practice. Never looked back. Actually, for recording and mixing, I don't need a loudness meter. Loudness is taken care of in the DAW for recording, and with the help of a soundmeter for live.
Lately, I recommend students to use the free Orban program where many parameters (VU, peak, LUfs) can be read at the same time.
That's certainly the way to go in 2021.
 

Brian Roth

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I did some digging to see what the current choices are for VU meters (Sifam is one) and found this:


The prices were shocking <g>.

This is more within my budget:


<G!>

Bri
 

JohnRoberts

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For DIYers, I think Uwe's 51 led meter with switchable ballistics looks to be a good option as far as usefulness:

The pics and info John R posted here of the meter project he worked on for another company looked great too, although I don't believe this is available for mere mortals is it? 😉
I probably sound like a broken record by now... Both of my patents for combo peak/VU meters are expired now so free to copy.

I considered designing a generic microprocessor meter kit but don't have the energy to deal with kit customers. Or any kind of customers.

JR
 

abbey road d enfer

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timgoldsmith

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I did some digging to see what the current choices are for VU meters (Sifam is one) and found this:


The prices were shocking <g>.

This is more within my budget:


<G!>

Bri
Hoyt was expensive but I was able to order a custom panel meter on a VU "A" scale. I could have gotten it vertical or horizontal. Lead time 8 weeks. They can do the "real" VU meter or whatever you want. Great in every way except price.
 

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