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Old 9th June 2015, 03:21 PM   #1
akis is offline akis  United Kingdom
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Default VU meter indicator signal levels

I have figured out the following regarding VU meters labelling:

Hi-Fi Alignment Level = -8dBu = 0.31V RMS = 0.872V p-p (~300 mV RMS)
0 dBu = 0.775V RMS = 2.19V p-p
+4 dBu = 1.23V RMS = 3.47V p-p corresponds to 0 VU


Does that mean that if I am looking at some "VU meter" the "0" may either be 0.775mV or 1.23V ?
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Old 9th June 2015, 08:24 PM   #2
Mooly is offline Mooly  United Kingdom
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I think it was all 'open to interpretation' on typical home equipment from how I recall it back in the day. One products '0db' level was always different to another's. With reel to reel and cassette there was often reference to a tape of known magnetic strength, whats the word, fluxivity (lol) of so many nano or microwebers or whatever the units were. That would become the reference level that the machine was calibrated to.

This might go some way to explaining it all, because as voltage and power levels its all accurately defined... except everyones products always seemed different

Sorry, that's probably all not much help at all.

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Old 9th June 2015, 09:37 PM   #3
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You must define if this meter is going to be used in a amplifier or at a cassette tape deck.
For example a cassette tape deck has an output of 150 mV , the VU meter measures the level of the recorded tape, but this does not translate in watt.
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Old 9th June 2015, 09:43 PM   #4
sreten is offline sreten  United Kingdom
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Hi,

VU meters can be calibrated with sine waves but they don't
respond to peak to peak levels of music, they average.

From the numbers you have found you can assume they will read
at least 4dB below actual peak level for typical radio program.

In reality for most signals most of the time peak level is 6dB to 10dB higher
than VU. VU or volume units is an an attempt to indicate apparent volume.

Typically the levels to be considered when designing systems using a VU meter are:
- Reference Level (typically +4dBu, valid with tones only)
- Standard Output Level (10dB above Reference, typical peak levels)
- Clip Level (6dB above Standard Output Level, "headroom" to allow for unusual conditions)

The zero VU level can be adjusted for purpose, e.g. in a cassette deck.
Read the manual carefully regarding indicated recording levels.
Cassette decks moved to peak hold meters due to lack of understanding of
VU, or dual displays than indicated VU levels and peak hold levels together.

rgds, sreten.

Last edited by sreten; 9th June 2015 at 09:53 PM.
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Old 9th June 2015, 10:45 PM   #5
M Gregg is offline M Gregg  England
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I had a play with tube VU meters,

I tried them after the input volume control for showing clip level of a preamp.

I tried them at the output of amp for watts indication..

And in the end I found it more useful showing input signal level, setting the tube to show 2V input. I didn't think this would be any use, however when you get a sudden change in volume a quick glance at the VU meter and you can instantly see if its program material level or something else. So it stayed there. I think a simple LED that lights at clip for an amp is OK with perhaps even 3 LED's green OK yellow close to clip and red clip..so they could be on green as soon as signal is input to show its working..no flashing lights.

So they are really more use for tape saturation..but I don't use tape. Or a tuning meter for radio..but we now have DAB.

Regards
M. Gregg

Last edited by M Gregg; 9th June 2015 at 10:51 PM.
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Old 10th June 2015, 09:34 AM   #6
AndrewT is offline AndrewT  Scotland
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red led = -1dB ref just about clipping level
orange = -3dB ref clip
yellow = -5dB ref clip
grn bargraph for all levels -7db and lower ref clip.

No meter can read the peak of a very short transient.
Accept that some of the peaks will not register and if the display stays green then you have 6dB of headroom for the unseen peaks and hopefully not many of them actually clip the system.
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