dBm to dBV true RMS - Space Echo Cal

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corgan4321

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Nov 30, 2008
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I'm working on a Space Echo RE201 and wondering about converting dBm values to dBV values. The original service manual says to feed a -50dBm signal into the mic input. How would I convert this number into dBV or RMS voltage for measuring on my multimeter? I understand that in converting from dBm the impedance is required, but which impedance do you plug into the formula? What impedance value would I use to do the conversion? I have the same question in regards to measuring the output level too.
 

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dBm comes from the days of 600 ohm interfaces and in the audio world means 1 milliwatt of power into a 600 ohm load. Later, when 600 ohm matching was abandoned, the term dBu was introduced to mean the same voltage level. Since 0dBm into 600 ohms required a voltage of 0.775V rms this means that 0dBu = 0.775V rms. dBv is just signal level relative to one volt rms so 0dBV = 1V rms. The difference between dBu and dBV is the ratio of 0,775 to 1 in dB which is which is -2.21dB.

So dBu = dBV + 2.21

Cheers

Ian
 
Since 0dBm into 600 ohms required a voltage of 0.775V rms this means that 0dBu = 0.775V rms. dBv is just signal level relative to one volt rms so 0dBV = 1V rms.
Is this relative to 600ohms only? When would it be necessary for the impedance used in the formula to change? Confused as to what impedance would go into this calculator on the page tt listed above? For audio is it just safe to assume 600 ohms when trying to get at the op question? So .002 Vrms?
 

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..generally, originally, the measure was dB(mili-Watt) - relative-to-a-miliwatt-of-power-dissipated
For 600 ohms transmission lines: 775mV into 600 Ohms sinks 1mW of power..

you can set your fluke45 tester to refer to anything from 2 Ohms to 8K Ohms as reference for its dB measure. Messes with my brain if set to anything but 600

/Jakob E.
 
Is this relative to 600ohms only? When would it be necessary for the impedance used in the formula to change? Confused as to what impedance would go into this calculator on the page tt listed above? For audio is it just safe to assume 600 ohms when trying to get at the op question? So .002 Vrms?
Impedance is not used when computing dBu or dBV. When computing dBm, 0dBm == 0dBu only when the impedance is 600 ohms. If the impedance is not 600 ohms, dBm will vary accordingly.
 
0dBm == 0dBu only when the impedance is 600 ohms. If the impedance is not 600 ohms, dBm will vary accordingly.
So would corgan4321 use the impedance of the generator or the impedance of the mic input to figure out the Vrms of -50dBm? Or am I not understanding? Or maybe that's the wrong calculator?
 

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I’m thinking I’d strap a 600 ohm resistor across the input and the output of the DUT, and then I’m safe to say that dBm=dBu. Is this a safe practice?
 
You're waaay over thinking this. A Space Echo is a glorified guitar pedal. The input is going to be some impedance that is correct for any numbers they use dBm with (although the graphic says -50dB with no reference to dBm wrt the input). The output is designed to work with guitar pedal type impedances which is at least 10k so you don't need to load the output. The Space Echo is almost certainly not designed to drive 600 ohms. So when they say -12dBm, that's just some engineer trying to use tape spec type lingo where dBu would have been perfectly appropriate. So just hook it up to your meter or scope and look for -12dBu = 0.1946 Vrms = -14.22 dBV = -12 dBm.

Oh, and don't forget to use the "blank" to disable the dry signal. That's important or you're readings are going to be off.
 
And BTW, it is very possible that adjusting the bias current will do precisely nothing. A Space Echo is not a precision instrument compared to even a pro-sumer R2R. A much more practical method for setting the bias current would be to just get the right lubricated tape, feed in white noise at -50dBu, look at the FFT of the output and then just fiddle with the bias current to get the best LF performance. If it doesn't seem to have much of an effect at all, I would terminate the input and look at the noise floor and see if the bias current changes that. If it actually does nothing, just set the trimmer in the middle and forgettaboutit.
 
And BTW, it is very possible that adjusting the bias current will do precisely nothing. A Space Echo is not a precision instrument compared to even a pro-sumer R2R. A much more practical method for setting the bias current would be to just get the right lubricated tape, feed in white noise at -50dBu, look at the FFT of the output and then just fiddle with the bias current to get the best LF performance. If it doesn't seem to have much of an effect at all, I would terminate the input and look at the noise floor and see if the bias current changes that. If it actually does nothing, just set the trimmer in the middle and forgettaboutit.
Really appreciate your input here! I’ll follow your advice with regard to the space echo.

I understand the Space Echo is just a big guitar pedal - I was mostly asking as an example because the manual brought the question up for me. I’m just trying to learn more generally about these unit conversions for when/if they come up in the future.
 
+1 to don't overthink this....but

[TMI] Decibels became popular for engineering terminology because their logarithmic relationship allowed them to represent vary large power ratios using only modest numbers. Because of this convenience they are widely used to represent signal levels. To square with being "power" ratios, the original reference level was 0dBm=1 mW. But modern audio deals with voltage levels not power levels so to keep it simple they normalized on standard 600 ohm terminations to establish that 1mW at 600 ohms = 0.775 Vac. Back in the day there were many common audio interfaces with 600 ohm terminations. Modern audio further evolved to think in terms of voltage. 0dBm @ 600 ohms =0.775V to use it without the 600 ohm stipulation we changed the nomenclature to 0dBu= 0.775 irrespective of termination impedance. Another popular voltage reference is 0dBV or relative to 1VAC. Digital audio usage has further evolved to dBFS or voltage level relative to saturation, or decibels below full scale (clipping). [/TMI]

JR
 
Is this relative to 600ohms only? When would it be necessary for the impedance used in the formula to change? Confused as to what impedance would go into this calculator on the page tt listed above? For audio is it just safe to assume 600 ohms when trying to get at the op question? So .002 Vrms?
If you are dealing with dBu and dBV only, the impedance does not matter because we are measuring voltages not powers. The only one you need to know the impedance for is dBm.

0dBm into 600 ohms is 0.775V rms. For any other impedance it will be a different voltage for the same power. But don't worry 99.99% of the time you can just assume dBm and dBu are the same.

Cheers

Ian
 
dBM is a power measurement. dBV and dBu are voltage measurements. They are not directly translatable. The good thing is that the vast majority of the time dBM is a misused term. What is actually meant is dBu.
 
I wrote a set of octave scripts that I keep handy when I need to convert things like that:

my dbu2v.m script to convert dBu values to voltage:

function levelV = dbu2v (leveldBu)

# usage: dbu2v(level_in_dBu)
# Returns the voltage given level in dB relative to 0.775V

if (nargin <1)
print_usage();
endif

# 0.775 is voltage of 1mW through 600 Ohm load, i.e. sqrt(0.6V)
levelV = sqrt(0.6)*(10**(leveldBu/20));

endfunction

my dBu and dBV scripts to convert voltage to dBu or dBV:

function leveldBu = dBu (levelV)

# usage: dBu(voltage)
# Returns the level in dB relative to 0.775V

if (nargin <1)
print_usage();
endif

# 0.775 is voltage of 1mW through 600 Ohm load, i.e. sqrt(0.6V)
leveldBu = 20*log10(levelV/sqrt(0.6));

endfunction

function leveldBV = dBV (levelV)

# usage: dBV(voltage)
# Returns the level in dB relative to 1V

if (nargin <1)
print_usage();
endif

leveldBV = 20*log10(levelV/1);

endfunction

You can just use those as-is if you want to put Octave on your machine (or have access to Matlab), or use them as examples to program into your calculator.
 
On page 2 of the RE201 service manual, they list the nominal level of the mic input as -50dBm and 2.4mV, into an input impedance of 900 ohms. This is close (2.4mV into 900 ohms is -52dBm), but the other values given in the table are quite wrong - unless Roland is using dBm here to mean what we'd now call dBu. Then all the numbers make sense; -50dBu is 2.4mV and -24dBu is 44mV, as stated.

1675559698988.png
 
Heres the parent directory with loads and loads of good stuff , calcs for almost anything electronic,audio or acoustics related . Many great articles explaining audio concepts etc .

http://www.sengpielaudio.com/Calculations03.htm
I was able to hash out the silly gifs on the page and clean it up a bit ,
Its an easy matter to store the links to each of the calculators in favourites in browser or even just in a text file for quick reference .
 
The contributions made by Tonmeister Eberhard Sengspiel for his students are indeed one of the most valuable things on the internet. Unfortunately he passed away (2014); his son Alexander ensures that the site is not shut down, for which one must be very grateful.

As far as dBu, dBV, etc. are concerned, it is specifically the follogwing page that offers help.

Unfortunately the English version is currently 404, so I have to quote the working German page here. However, I found a captured version of the English page (on which the calculators don't work) so that you can use it for the translation.

http://www.sengpielaudio.com/Rechner-db-volt.htm
https://archive.ph/j2xW
Nick Salis
 
The contributions made by Tonmeister Eberhard Sengspiel for his students are indeed one of the most valuable things on the internet. Unfortunately he passed away (2014); his son Alexander ensures that the site is not shut down, for which one must be very grateful.

As far as dBu, dBV, etc. are concerned, it is specifically the follogwing page that offers help.

Unfortunately the English version is currently 404, so I have to quote the working German page here. However, I found a captured version of the English page (on which the calculators don't work) so that you can use it for the translation.

http://www.sengpielaudio.com/Rechner-db-volt.htm
https://archive.ph/j2xW
Nick Salis

Apparently he had to take the calculator down due to copyright claims from a third party. Unbelievable, no?

Now, it's been online again but I'm not sure whether I'm allowed to post the link here...
 
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