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Author Topic: vu needle pinging  (Read 2613 times)

zywot66

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vu needle pinging
« on: April 07, 2006, 02:34:35 PM »

hello.
i was wondering if pinging the vu needle was bad for the parts. commen sence would tell me yes...but
thanks!
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Nid

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Re: vu needle pinging
« Reply #1 on: April 07, 2006, 10:52:37 PM »

...and what could you do to prevent the needle from pinging?  Shocked
Anyway I think that gradually the needle is being decalibrated. Don't panic, this can take years ,depending on the meters build quality and amount of usage. Then you must use the front screw to recalibrate the needle. On how to do this calibration there are many older topics here and there, do a little search. OK, what I say might be bullshit so one of the experts here can help you better.Take care
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John Monforte

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Re: vu needle pinging
« Reply #2 on: April 08, 2006, 01:33:47 AM »

A true VU meter that meets ANSI/NAB specs is designed to be buried continuously with no ill effect. If it is passively hooked to the audio signal (wired in series with 3.6Kohm resistance to the audio line) and the audio line has a finite output impedance (a few ohms after the opamp output or a transformer) the meter will be a nonlinear load on the signal when buried. This will introduce a measurable degree of distortion to the signal.

If the meter has its own driver amp, distortion will not be an issue, but I've seen more than a few of these circuits that drive the meter in an out-of-spec manner that will theoretically expose it to damage. Also, some meters say "VU" yet deviate from true VU character as specified by standards. These can be damaged too.

Modern compressed digital recordings use every inch of headroom. All a VU meter will tell you is if the song is playing!
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Geoff_T

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Re: vu needle pinging
« Reply #3 on: April 08, 2006, 10:25:55 AM »

Hi

You didn't mention the reason for the distortion and non-linearity... VU meters contain a rectifying diode which is across the audio in series with the 3600 ohm resistor and the impedance of the meter coil. This does affect the THD of the audio path albeit the actual degradation is small... I once measured a circuit with 0.05% distorton and noted that it rose to about 0.1% when a meter was placed across the circuit.

You'll notice in Neve test reports that the meters were always switched away from the outputs when distortion measurements were made. "Rolls Royce" consoles like the EMI-Neves had BA386 buffer amps driving the meters that had the equivalent of a 31267 line input transformer and an op amp driving the meter so that the meter was not affecting the audio path.

"Expensive" Sifam VU meters have taught band suspension and can withstand any abuse. The screw on the meter is not for calibrating but frequently is used for that. It's purpose is to set the meter resting point. You set the OVU preferably by means of adjusting the gain of the buffer amp feeding the meter. If you twiddle that screw you have lost the calibration across the scale.

"Less Expensive" Sifam VU meters that significantly have the word VU in bold type to the side of the scale, meet all the level calbration specs but not the timing aspects of the spec. Again, the effect on reading levels is minimal.

"Seriously inexpensive meters" as fitted to budget priced equipment would not meet the ANSI specs and are just simple and tolerably accurate indicators of signal level. Their robustness to abuse is questionable.

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Nid

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Re: vu needle pinging
« Reply #4 on: April 08, 2006, 07:56:28 PM »

Quote:

If you twiddle that screw you have lost the calibration across the scale.


Woops...and how do you set this to spec?
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Geoff_T

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Re: vu needle pinging
« Reply #5 on: April 08, 2006, 08:39:16 PM »

Hi

Put it back where it was! The meter needle should always rest at the -20dB left hand extreme.

Before any twiddling goes on, it's worth checking that the VU is receiving 1.228 volts to read 0VU.

If you only have to twiddle a teeny bit to correct the 0VU... I promise I'll look the other way!

Surprised

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Geoff Tanner
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John Monforte

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Re: vu needle pinging
« Reply #6 on: April 08, 2006, 11:11:23 PM »

Geoff,

The rectifiers are one source of nonlinearity, and you have noticed the distortion that occurs. Also, when the needle is buried there is another distortion source. I am not certain of the cause. It could be that the pegged meter acts like a frozen motor and reflects back the lack of movement to the electrical side of the transducer - like a scraping loudspeaker coil does. Another possibility is that the coil is saturated at that point. Maybe both.

The mechanical screw is there only to determine the proper rest point. It is actually slightly to the left of -20. It is marked on all standard VU meters. This makes -20 and all the other scale marks read properly. Geoff is right to say that adjusting the mechanical zero at 0VU will put a shift on the whole scale. It is OK to tweeze the value of the 3.6Kohm resistance to get 0VU spot on. Manufacturing variations exist and magnets do not last forever, but the tweak should be within 5% or so - which is less than a couple hundred ohms. In fact, the reason for 3600 ohms and 0VU=+4dBu are related. If anyone is interested in that arcana I can be prompted to tell the long version of the story.
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Nid

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Re: vu needle pinging
« Reply #7 on: April 10, 2006, 10:38:31 AM »

Yes John please go on if you like  Cool
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Gold

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Re: vu needle pinging
« Reply #8 on: April 10, 2006, 11:36:39 AM »

If you are a serious VU meter snob you will need a Weston with a copper oxide rectifier. The spec was only what the original designers deemed most important. The original Westons were what was intended. They were so expensive to produce that Weston never thought anyone else would. Then others figured how to meet the spec with a less expensive movement. It wasn't the same as the Weston. I keep meaning to find a pair of Westons to see what all the hub bub is about.
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zywot66

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Re: vu needle pinging
« Reply #9 on: April 10, 2006, 01:36:08 PM »

wow! thanks a bunch. can i allways expect great responses like these. any-hoo ive got this board see. and it sounds great when it distorts but im kinda skerd that its bad on moving parts. i duh-no. its an old midas pro4 with big vu. so im thinking there of "better" quality.
thanks again
zywot
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Teddy G.

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Re: vu needle pinging
« Reply #10 on: April 10, 2006, 09:20:25 PM »

I'm not even sure what "vu meter pinging" is? Just made me think of the broadcast vu meters I've seen that were actually, physically bent - ALOT - from the constant abuse of some DJ's... I chuckle... Such dolts...... I'm sure this is not the case, here, just brought up(to me) that sad and sorry memory flogger...

TG
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dcollins

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Re: vu needle pinging
« Reply #11 on: April 10, 2006, 11:20:33 PM »

Teddy G. wrote on Mon, 10 April 2006 18:20

I'm not even sure what "vu meter pinging" is?


They mean "pinning"  As in hitting the end pin.

It won't break anything but it's annoying, which is why studios have a variable reference for zero VU which may range from, say, -20dBFS to -8dBFS so you can "scale" any program to fit the meter scale.  Even before digital, most studios had this knob.

DC

Teddy G.

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Re: vu needle pinging
« Reply #12 on: April 10, 2006, 11:36:41 PM »

Right, OK... Pinning or pegging - the meter... Pinging would be like something they used to do with the sonar on the Seaview, wouldn't it??? I'm not too sure about the "not hurt anything", part..? Again, I've seen more than one "hurt" meter needle! And some of those meters cost some serious coin. I remember a few engineers who wanted to "hurt" the people who allowed such a thing to happen, fer shure...


Ahh, I say: "Don't do it."(Of course, that's just my opinion...)


TG
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John Monforte

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Re: vu needle pinging
« Reply #13 on: April 11, 2006, 02:16:03 AM »

OK Nid! It takes little to get me started and lots more to get me to stop. You can tell this tale to audio folks at a bar sometime. It is worth a free beer.

HOW +4 BECAME OPERATING LEVEL

Many audio standards can be traced back to early telephone systems which used balanced lines and 600 ohm circuits. You don't need electronics for telephony, but sooner or later line losses force you to add amplification which makes you need to know gain which makes you need a meter.

Line level (the word line comes from telephone line) was around 1mW as they set up the system - thus 0dBm is defined as 1mW in 600 ohms. To meter this they needed to measure things around that level with an AC voltmeter that was fast enough to respond to speech but not so fast as to measure the pops and clicks that were pervasive on the system. The VU meter was designed to respond to the ballistics of speech, and speech only, so you could measure signal loudness on the lines. Units of Volume, you might call it.

Unfortunately, the little motors that threw around the needle's mass required a bit of current - enough to load down the line a slight amount. {An old fashioned passive voltmeter like a Simpson is only accurate for 60Hz AC and the needle swings slowly, but there is extremely little current drawn which is essential for an accurate voltmeter}. If you added 3600 ohms in series with with the VU meter, the signal level would not perceptibly change as you switched it on and off a line. Of course this resistance desensitizes the meter - by exactly 4dB! Thus, +4dB became the standard for line level!



Epilogue:

Remember that this meter is good for speech. Transient things will get by it. When you add this to the fact that analog tape EQ makes it so tape saturates easier at high frequencies than low ones, you will understand why savvy recording engineers will bury a kick drum channel's meter and barely tickle the meter on hi-hat or direct Fender Rhodes. You gotta know how to work AROUND the meters!

Now you know why British console manufacturers gag when you ask them to remove their nice PPM's (which REQUIRE complex electronics to measure instantaneous peak amplitudes and hold it long enough for the human eye to see) in order to put in VU's which miss musical signals and will cause distortion if not buffered.
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Geoff_T

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Re: vu needle pinging
« Reply #14 on: April 11, 2006, 12:26:42 PM »

John Monforte wrote on Mon, 10 April 2006 23:16


Now you know why British console manufacturers gag when you ask them to remove their nice PPM's (which REQUIRE complex electronics to measure instantaneous peak amplitudes and hold it long enough for the human eye to see) in order to put in VU's which miss musical signals and will cause distortion if not buffered.


Hi

Indeed! I'd be at my desk at Neve, reading magazines during my lunch break, and see these adverts for USA manufactured broadcast consoles with VU meters for level indication.

Our hands were tied by the BBC and IBA specs that would never allow anything but a PPM to measure levels in a broadcast environment.

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Geoff Tanner
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NB Please do not pm me if you want a fast response... please email me.
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