Another question about VU

[Tim Ressel]

My dearest Mr. Goodfortune,

No need to be embarrased. This whole decibel thing is hard to get one's brain
around. Lucky for you I am doing the same type of project at the moment. 

The major problem is that fact that VU displays power ratios, but we will be
measuring voltages. 0 dBm, as you know, is 1 mW into 600 ohms. We use E = SQRT (
P * R ) to calculate the voltage. E=SQRT(P*R) = SQRT(0.001*600) = 0.775 volts.
Now here is the tricky part: Calculate the voltage for 1 dBm: First turn the dB
number into a ratio: Ratio=10^(dB/10) = 10^(1/10) = 10^(0.1) = 1.2589. By the
way, by nature this ratio is true for any 1 dB step, no matter what the level.
That is the beauty of dB. So, 1 dB is 1.2589 times the wattage as 0 dB (1 mw),
or 1.2589 mW. Now we use E=SQRT(P*R) to calc the voltage: SQRT(1.2589*600) =
SQRT(0.7734)=0.879V. Repeat the procedure for each alue you want. A spreadsheet
comes in r-e-e-e-a-l handy when doing this. I have attached the spreadsheet from
my project. It is intended as an example. 

The spread also shows the method I use for determining the A/D counts for given
levels. You may have to play games with the voltages because of the A/D
reference voltage.

You may be interested in the method I am using for my VU meter. Instead of an
A/D converter, I am using an D/A to generate voltages cooresponding to the dB
levels. Then I use comparators to test the input voltage against the D/A output.
Since you need only one voltage per displayed level (i.e. 40 LEDs = 40 levels),
the conversion goes pretty quick. I will put a schematic up as soon as I figure
out how to do so.

I hope this helped. Questions are always welcome.

Tim Ressel--Compliance Engineer
Hewlett-Packard
Verifone Division
916-630-2541  
tim_r1 at verifone.com                     



-----Original Message-----
From: Batz Goodfortune [mailto:batzman at all-electric.com]
Sent: Wednesday, February 23, 2000 6:36 PM
To: synth-diy at mailhost.bpa.nl
Subject: Another question about VU


Y-ellow Y'all.
	While we're on the subject, and as embarrassing as it is for me to ask.
I
need to know how to render a VU scale using an 8 bit A-D. In the simplest
possible terms. I was away the day they did math at school. I know it's log
20 something but I can't rap my brains round a ratio right now.

I'll explain the problem.

I need to build multiple bargraphs. The simplest way to do that is to use a
multi-channel A-D chip and a micro. The micros can drive LEDs directly and
multiplex them. So it would be a 2 or 3 chip solution. Small in size (very
important) and able to do all the nice bits like peak/average in software.

The way I imagined it would work is that there might be a look up table Or
at least a scale table, Where if a value was higher that a certain point it
would turn a LED on. Etc.

Now the problem is that In some cases this might be an array of the usual
10 LEDs. In other cases there are some 36 LED displays I can use. In yet
more cases I could use an LCD. On the LCD it could be showing the two main
channel outputs across the display in detail. In the other modes it could
be showing multiple channels in less detail vertically. So I'd like to have
some universal scale that divides the 256 step linear scale into the log 20
VU scale. Then I could apply this to any given set of bargraphs.

I just can't wrap my brain around it at the moment. And if you tell me,
you'd have to tell me in the simplest possible forms. "Do this, do that
then you get this..."

I know the proper VU (dB) scale is a ratio devised by nWatts into 600 ohms.
1mW thanks Tim. But how do you map that against a scale which could be
totally arbitrary and is actually measuring voltage?

I've been asking round without trying to embarrass myself in public like
this but I just had to bite the bullet in the end. <he says grimacing>

Thanks in advance.

Be absolutely Icebox.

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