What in blue blazes is dBm?? - AVS Forum
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post #1 of 17 Old 01-24-2014, 04:44 PM - Thread Starter
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I know we have quite a few experienced pro-audio folks here, so I’m hoping I can get some help with this (Arny, FOH, I’m looking at you!).

I’ve noticed that on occasion Yamaha will give noise specs as “dBm” for their pro audio components. What’s up with that? As far as I can tell, dBm is no longer an industry-accepted designation for audio specifications and these days is more typically seen in radio, microwave and fiber optic networks.

Is there any way to translate their dBm spec to a more traditional dBu?

Regards,
Wayne A. Pflughaupt



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post #2 of 17 Old 01-24-2014, 05:26 PM
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dB referenced to 1mW into 600Ω (for audio).
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post #3 of 17 Old 01-24-2014, 05:28 PM
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dBm is dB relative to 1 milliwatt. In order to convert to Volts RMS, it's necessary to know the reference impedance for this power computation. In the RF world where I come from, it's almost always 50 Ohms. According to this Wikipedia article:
Quote:
Originally Posted by Wikipedia 
In audio and telephony, dBm is typically referenced relative to a 600 ohm impedance, while in radio frequency work dBm is typically referenced relative to a 50 ohm impedance.

If we assume 600 Ohms, what is the reference RMS voltage?

P = 10-3 = VRMS(ref)2 / R
VRMS(ref) = sqrt(P * R) = sqrt(10-3 * 600) = sqrt(0.6)

But dBu is exactly dB relative to VRMS(ref) = sqrt(0.6), so in the case of a 600 Ohm reference resistance, dBu and dBm are the same.

In the case of a 50 Ohm reference resistance:

VRMS(ref) = sqrt(P * R) = sqrt(10-3 * 50) = sqrt(0.05)

In the latter case, the reference voltage is lower, so for the same voltage, dBm(50) is higher than dBm(600) as follows:

dBm(50 Ohms) = dBm(600 Ohms) + 10 * log10(0.6 / 0.05)
for the same RMS Voltage.
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post #4 of 17 Old 01-24-2014, 07:43 PM
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^^^ All that agrees with my experience and training in audio and RF/mW/mmW circuits.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #5 of 17 Old 01-25-2014, 05:49 AM
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Quote:
Originally Posted by Wayne A. Pflughaupt View Post


I know we have quite a few experienced pro-audio folks here, so I’m hoping I can get some help with this (Arny, FOH, I’m looking at you!).

I’ve noticed that on occasion Yamaha will give noise specs as “dBm” for their pro audio components. What’s up with that? As far as I can tell, dBm is no longer an industry-accepted designation for audio specifications and these days is more typically seen in radio, microwave and fiber optic networks.

Is there any way to translate their dBm spec to a more traditional dBu?

http://en.wikipedia.org/wiki/Decibel

covers dBm, dBv, a nd dBu

http://www.animations.physics.unsw.edu.au/jw/dB.htm

does all that and also adds dBA and dBc

http://designtools.analog.com/dt/dbconvert/dbconvert.html

is an interactive converter for dBm, dBv, and dBu
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post #6 of 17 Old 01-25-2014, 05:00 PM - Thread Starter
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Quote:
Originally Posted by andyc56 View Post

dBm is dB relative to 1 milliwatt. In order to convert to Volts RMS, it's necessary to know the reference impedance for this power computation.

The hum and noise spec for the component in question is <-76 dBm, with input impedance of 20k ohms, and output of 150 ohms. Is that enough information to translate <-76 dBm to a dBu value?

When I plug in 20,000 ohms and 76 dBm into the calculator Arny linked here, it shows 91.23 dBu. Does that sound right?

Why on earth would any manufacture use such a specification? They even show maximum input level as dBm (+24 dBm).

Regards,
Wayne A. Pflughaupt


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post #7 of 17 Old 01-25-2014, 05:14 PM
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Given the specified max input level is +24 dBm, it seems reasonable to assume the reference impedance is 600 Ohms, giving +24 dBu. That seems typical of the pro equipment I've come across.

It does seem like a dumb way to specify signal levels, especially since in the small-signal audio world, people rarely give a hoot about power, only voltage.
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post #8 of 17 Old 01-25-2014, 07:54 PM
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Input-referred noise depends upon the driving source impedance, not the 20k input impedance spec. I'd use 600 ohms. Is -76 dBm the specified output noise floor? If full-scale output is +24 dBm, that gives you 100 dB dynamic range.

"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley
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post #9 of 17 Old 01-26-2014, 07:32 AM - Thread Starter
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Quote:
Originally Posted by DonH50 View Post

Is -76 dBm the specified output noise floor? If full-scale output is +24 dBm, that gives you 100 dB dynamic range.

Not output specifically, just a general “noise and hum” spec. Dynamic range is spec’d at >100 dB, so you nailed that one!

So there’s no way to translate 76 dBm “hum and noise” to a traditional S/N spec?

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Wayne A. Pflughaupt


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post #10 of 17 Old 01-26-2014, 07:34 AM - Thread Starter
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Quote:
Originally Posted by andyc56 View Post

If we assume 600 Ohms, what is the reference RMS voltage?

P = 10-3 = VRMS(ref)2 / R
VRMS(ref) = sqrt(P * R) = sqrt(10-3 * 600) = sqrt(0.6)

But dBu is exactly dB relative to VRMS(ref) = sqrt(0.6), so in the case of a 600 Ohm reference resistance, dBu and dBm are the same.

So with a 600 source impedance, 76 dBm is going to equal 76 dBu? That’s nuts. Not your math, which I’m sure fine. In fact, Arny’s calculator page comes up with the same thing. I mean it’s nuts for Yamaha to give this thing such a poor noise spec. It’s dead silent! I’ve seen components with a 95 dBu noise spec that weren’t as quiet.

Regards,
Wayne A. Pflughaupt


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post #11 of 17 Old 01-26-2014, 07:48 AM
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Quote:
Originally Posted by Wayne A. Pflughaupt View Post


So with a 600 source impedance, 76 dBm is going to equal 76 dBu? That’s nuts. Not your math, which I’m sure fine. In fact, Arny’s calculator page comes up with the same thing. I mean it’s nuts for Yamaha to give this thing such a poor noise spec. It’s dead silent! I’ve seen components with a 95 dBu noise spec that weren’t as quiet.

I dunno. smile.gif Looks like it might be an unweighted vs. weighted thing, with the spec of the Yammy being unweighted and the others weighted?
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post #12 of 17 Old 01-26-2014, 08:16 AM
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Well, it does say "less than" -76 dBm, so maybe it's a lot les than... smile.gif Andy's idea that Yamaha may specify unweighted instead of A- (C-, whatever) weighted makes some sense.

As for your earlier question, assuming S/N does not include distortion components, and given they are using dBm for output as well, then the S/N ratio is >100 dB from max output.

I am surprised Yamaha chose dBm, not sure I have seen that in audio (or have just forgotten) -- except in tuners, which sometimes used dBm or dBf. The pro equipment I recall off-hand all used dBu or dBV. As Andy said, in the RF world we use dBm all the time, and power transmitters use dBW, referenced to either 50 ohms (most RF) or 75 ohms (video systems). (And dBc, dBHz, dBi, dBK, dBmV, dBZ, etc. etc. etc.) The 600-ohm reference comes from Bell telephone days (the B actually stands for "bel", a unit named in honor of Alexander Graham Bell, but a bel is just too big for most practical use so the standard is decibels, dB, one-tenth of a bel).

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post #13 of 17 Old 01-26-2014, 08:51 AM
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Quote:
Originally Posted by A9X-308 View Post

dB referenced to 1mW into 600Ω (for audio).

Which means that 1 millwatt is defined as 0 dbm, and all other values are relative to that.

For example, 30 dbm would mean 20 millwatts and -30 dbm would mean 0.05 milliwatts.
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post #14 of 17 Old 01-26-2014, 09:15 AM
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Quote:
Originally Posted by commsysman View Post

For example, 30 dbm would mean 20 millwatts and -30 dbm would mean 0.05 milliwatts.

eek.gif
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post #15 of 17 Old 01-26-2014, 11:43 AM
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Quote:
Originally Posted by commsysman View Post

Which means that 1 millwatt is defined as 0 dbm, and all other values are relative to that.
Well, duh.
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post #16 of 17 Old 01-26-2014, 02:09 PM
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What? 0 dBm = 1 mW, yes, but 10 dBm = 10 mW, 20 dBm = 100 mW, and 30 dBm = 1 W. At least when I went to school...

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post #17 of 17 Old 01-27-2014, 07:08 AM
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Quote:
Originally Posted by commsysman View Post

Which means that 1 millwatt is defined as 0 dbm, and all other values are relative to that.

For example, 30 dbm would mean 20 millwatts and -30 dbm would mean 0.05 milliwatts.

Like many of your posts, you are, once again completely wrong.

You claimed to be a teacher....did you actually teach electronics?
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