LED VU/PPM-meters -metering scales and their use

[TTL]

I'm building a vocoder which has two LM-3915 based LED PPM meters in conjunction with its two audio input amps.



The project article gives no explanation of what the 10 LEDs represent so I searched around and came across an article entitled "LED graph circuits" which shows the following scale:

LED 1 = -27 dB
LED 2 = -24 dB
LED 3 = -21 dB
LED 4 = -18 dB
LED 5 = -15 dB
LED 6 = -12 dB
LED 7 = -9 dB
LED 8 = -6 dB
LED 9 = -3 dB
LED 10 = 0 dB

Fine, so now I know how to mark the front panel
But comparing it to the EMS Vocoder 3000 I see a slightly different scale:

It appears the readings have been shifted so the lowest level starts at -21 dB while it adds two levels above 0 dB (+3 and +6 dB).
So I'm trying to make sense of this, wondering if the EMS design is a more useful one than the one I'm building or if these are just two different ways of achieving the same thing? Are there standards to what kind of metering scale should be use in (professional) audio applications? How about the LED colour -is there some sort of standard for that as well? The vocoder I'm building says the last 3 LEDs should be red while the one by EMS has just two red LEDs and a yellow one before that.

 

[Audioguru]

+3dB sounds only a littler louder, +6dB is again a little louder than +3dB. +6dB is a voltage gain of 2.
Must people think that an amber or yellow light is a warning (here it might warn that distortion is beginning to happen at this level and red might mean that severe distortion is happening.
You need to calibrate the circuit so that the LEDs tell you warnings of the levels.

 

[Harald Kapp]

dB is a logarithmic measure of relative power (or voltage). What marks 0 dB is up to the designer of a system. The relevance is in the differences in dB as Audioguru explained.Typically a system designer could set 0 dB as the reference level where no distortion exists. Or to a lower signal level in order to gain some headroom for overdriving without distortion.

Another use is with defined reference levels, e.g. 1 mV equiv. 0 dBmV (note the suffix mV). Here the system designer is free in his choice of reference level only within the confines of the definition (dBmV, dBµV etc.). This notation is used to make signal levels comparable between systems.
See for example this Wikipedia article (sub-section 'suffixes and reference values') to learn more.

 

[TTL]

I'm not sure if I understand correctly, but are the meter readings not necessarily defined by an already set reference?

In other words, the LED input meters in the vocoder I'm building might not have any specific dB meaning (i.e. the lower led means -27dB), but rather just shows that one or two lit LEDs means "just a low level input", some more LEDs obviously means "higher level audio input" while having the last couple of red LEDs lit means "warning! danger of overload"?

So, in order to make the LEDs actually mean something I need to calibrate the system?
From the LM-3915 (dot/bar LED display driver IC) datasheet I believe it's already calibrated to give some actual meaning, but perhaps it's the input to the LED meter circuitry which needs to be calibrated?

I assume the actual calibration involves feeding a stable audio source (i.e. a signal generator with a sine wave or whatever at a known voltage (using a digital multimeter to simultaneously read the voltage), then in part of the LED display circuit insert a trimpot so as to make say the lower LED light up only when the input voltage is at whichever standard you choose that -27dB should mean?

Many words for something that can likely be explained a lot simpler, but did I understand the basic concept?

 

[Harald Kapp]

I think you got it. If you want to calibrate a system, you need to consider the whole signal chain, including any amplifiers, attenuators, filters etc.
The nice thng about using decibels is that it makesrelative measurements easy (add/subtract a few dB). If you want to convert relative dB values into absolute values, you need a defined reference for 0 dB.
What are you going to use for 0 dB reference?
Why do you want to calibrate the system at all? The designers of this vocoder will (or at least should have) designed the VU meter such that for any level at or below 0 B (according to their display) the signal is undistorted. That's the important part.
What do you gain by knowing that this 0 dB level is equivalent to say 1 V or any other voltage?

 

[Audioguru]

Some of my electronic recorders have an LED display of level that is calibrated so that the highest levels are when they produce overload distortion.
I have a "VU" meter with microphone in my family room with a range of 50dB and it is not calibrated, but the highest LED shows very loud levels. The lowest LED does not light if I hold my breath and the furnace and refrigerator are not running. I do not accurately measure sound levels. I simply have enough common sense to use earplugs when I run my leaf blower. My gas lawn mower has a good muffler and its sound level is low enough that I do not need to accurately measure it. I am curious about the blasting noise level from some motorcycles and cars that I might accurately measure some day. I do not play with guns so I won't measure the sound level from them.

 

[TTL]

My main reason for asking is because I want to know how to label the front panel next to the LEDs, like done with the EMS vocoder in the photo I posted earlier. Apart from looking good I also want it all to make sense of course

I took a closer look at the project instructions and it says (for the LED meter schematic I posted in the beginning):

"IC1 is a full wave rectifier with a peak detector charging C2 to the peak voltage of the input signal. IC2 is a logarithmic display driver, the sensitivity of which is determined by R7, 8. The LEDs are at 3dB spacing. The red LEDs illumintae when the filters overload. The LEDs have their own power supply."

So I assume that since R7 and R8 are fixed resistors, the LED meter display board has already been pre-calibrated with those specific resistor values to match the rest of the vocoder's circuitry.

 

[Audioguru]

Correct.

 

[TTL]

So at least I likely don't need to look into calibrating the meters to match the rest of the circuitry.
I assume the next thing I have to do (when the vocoder is completed and working) is to measure the audio input voltage of the input amplifiers, compare that with which LED lights up and use that as a base for what the actual dB values are (perhaps matching the numbers in my first posting).

 

[davenn]

So at least I likely don't need to look into calibrating the meters to match the rest of the circuitry.

you do if you want it to read the same and correct values ... other wise the labels for your dB scale are meaningless

 

[TTL]

Yes, I'll check to see which values I get when the vocoder is done, then take it from there to label something meaningful on the front panel

 

[Audioguru]

You want the input levels to be high enough that the circuit noise is not heard and you want the input levels to be low enough that there is no overload distortion. The VU meter will show you how high and how low the levels are and if they should be increased or decreased. You calibrate the VU meter so that you can see when the signal is beginning to produce overload distortion which might be shown as 0dB. If you do not care about audio quality you can adjust the input levels so that peak levels are distorted.

Many power amplifiers have an LED light when clipping distortion occurs. It is good for deaf people who cannot hear the distortion. I made a little amplifier and included a clipping indicator LED circuit. I adjust its maximum input level so that the LED never lights. If the LED lights only on a bass beat then distortion is not noticed because it might be a normal part of the beat sound.