To add to EC8010 . . .
The way I learned dB's are the ratio of 2 powers, where the reference power happens to be 1 mw into 600 ohms. The generic equation for power in dB's is
power dB generic = 10 Log (P1 / P2)
where P1 = V1^2 / R1, and P2 = V2^2 / r2,
so
Power dB generic = 10 * Log [V1^2/R1] / [ V2^2 / R2]
power dBm = 10 * Log [V1^2/R1] / [ .775V^2 / 600]
Note that 0.775^2 / 600 = .001 or 1 milliwatt.
when measuring a power in dBm, and R1 happens to be 600 ohms, then the formula simplifies removing the 600, and you only need to measure the voltages.
Looks like dBu uses 0.775V as the reference, so
power dBu = 10 * Log [V1^2/R1] / [0.775V]
and remember that dB without any suffix doesn't tell you much. dBm means referenced to 1mW into 600 ohms. dBuV references 1 micro volt, dBu references 0.775 V, etc. Too often the term dB gets thrown around and it's hard to know what the author really means.
The way I learned dB's are the ratio of 2 powers, where the reference power happens to be 1 mw into 600 ohms. The generic equation for power in dB's is
power dB generic = 10 Log (P1 / P2)
where P1 = V1^2 / R1, and P2 = V2^2 / r2,
so
Power dB generic = 10 * Log [V1^2/R1] / [ V2^2 / R2]
power dBm = 10 * Log [V1^2/R1] / [ .775V^2 / 600]
Note that 0.775^2 / 600 = .001 or 1 milliwatt.
when measuring a power in dBm, and R1 happens to be 600 ohms, then the formula simplifies removing the 600, and you only need to measure the voltages.
Looks like dBu uses 0.775V as the reference, so
power dBu = 10 * Log [V1^2/R1] / [0.775V]
and remember that dB without any suffix doesn't tell you much. dBm means referenced to 1mW into 600 ohms. dBuV references 1 micro volt, dBu references 0.775 V, etc. Too often the term dB gets thrown around and it's hard to know what the author really means.
Hey - you beat me to the answer
moamps said:
in your case, the reference is still 1mw into 600 ohms, or 0.775V. But, 0dBm is no longer 0.775V. For example, at 50 ohms:
0dBm = 10 Log [V1^2 / 50] / .001
which works out to around 0.2236V at 50 ohms.
gearheadgene said:
Nope. It's just 0dBm. Or 1mW into "whatever" impedance.
In audio, the impedance is 600ohms; in RF circuitry it is 50 or 75 or 200 etc. ohms.
Exactly what I said.
In balanced audio systems, the nominal level is normally +4dBm, rarely +6dBm. In digital systems, the nominal level is between -18 and -12dBfs, which is around +4dBm.
In unbalanced professional systems (tape returns, inserts etc.), the nominal level is usually 0dBm, and only occasionally -10dBV or +4dBm.
In nonprofessional systems, the nominal level is not strictly specified so it can be whatever the designer chooses in the range of between -10dBV and +6dBm.
Regards,
Milan
Sorry, I was obviously wrong, but that was how I once learnt the dBm unit. So from what some of you said, and from some extra googling, it seems 0 dBm simply refers to 1 mW into whatever impedance we have in the particular case. Interestingly, I found that it is even used for optics, where 0 dBm is 1 mW of optical power (obviously there is no impedance there). Note however that the unit originated from the US telecom industry, where 600 Ohm was the standard impedance.
Edit:
Perhaps I could add to my defense that I did take a course in physical measurements at univeristy and there we were actually taught that 0 dBm = 775 mV. The professor, however, had no idea where it came from, and used it as an example of a stupid arbitrary unit. However, I did know already then about the 1 mW into 600 Ohm origin, and enlightened him about this.
Edit:
Perhaps I could add to my defense that I did take a course in physical measurements at univeristy and there we were actually taught that 0 dBm = 775 mV. The professor, however, had no idea where it came from, and used it as an example of a stupid arbitrary unit. However, I did know already then about the 1 mW into 600 Ohm origin, and enlightened him about this.
ENVIRONMENT
The UK broadcast industry uses 0dBu* almost exclusively (-18dBfs).
*Note I use the term dBu not dBm, as lines are almost never terminated, and source impedance is usually low, not 600 ohms.
Hi guys.....
0 dBm = 1mW. Period. In whatever impedance might be available.
The main reason is that it makes calculaltions on transmission networks very simple, - all gain/loss elements in a chain is just about adding/subtracting the dBs..... be it telephones or RF.
The 600 ohm stuff is from the very childhood of telephone networks, being the charactetristic impedance of a telephone line suspended on poles. This then transferred into radio transmission in the very early days, and then got stuck there.
Today there are several "line levels". +4dBu seems to be todays standard of analog broadcast. 0 dBV = 1V is another, and 0 dBu = 0.775V - u for Unterminated, or impedance independant.
Classical stereo gear used to be around 150-200 mV, as stated, but seemingly it was the japanese equipment boom in the late 60ies that raised the signal level closer to the full volt, or thereabouts. Except for the CD red book standard, there is no proper definition of line level for consummer equipment, that I know of. Anything around 1V seems to be the thing today.
The only way to cater for the mess is to design for some extra gain and then use attenuators to level out.
0 dBm = 1mW. Period. In whatever impedance might be available.
The main reason is that it makes calculaltions on transmission networks very simple, - all gain/loss elements in a chain is just about adding/subtracting the dBs..... be it telephones or RF.
The 600 ohm stuff is from the very childhood of telephone networks, being the charactetristic impedance of a telephone line suspended on poles. This then transferred into radio transmission in the very early days, and then got stuck there.
Today there are several "line levels". +4dBu seems to be todays standard of analog broadcast. 0 dBV = 1V is another, and 0 dBu = 0.775V - u for Unterminated, or impedance independant.
Classical stereo gear used to be around 150-200 mV, as stated, but seemingly it was the japanese equipment boom in the late 60ies that raised the signal level closer to the full volt, or thereabouts. Except for the CD red book standard, there is no proper definition of line level for consummer equipment, that I know of. Anything around 1V seems to be the thing today.
The only way to cater for the mess is to design for some extra gain and then use attenuators to level out.
Hi gearheadgene,
Having done time designing broadcast mixers as well, and having escaped in the 80's, I think you need to focus on the application. If it's not broadcast , maybe 1V or 2.2V ( just above CD level) is appropriate, cognisant that your bit rate defined resolution is dictated by the max signal handling. You have to flex res against sig handling, for most practical situations within expected applications.
Cheers,
Greg
Having done time designing broadcast mixers as well, and having escaped in the 80's, I think you need to focus on the application. If it's not broadcast , maybe 1V or 2.2V ( just above CD level) is appropriate, cognisant that your bit rate defined resolution is dictated by the max signal handling. You have to flex res against sig handling, for most practical situations within expected applications.
Cheers,
Greg
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