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US and UK Audio levels
US and UK Audio levels
A colleague recently came up to me and asked me why he was measuring 7.2 dBu coming out of a brand new
Yamaha audio desk when he had set tone reference audio level to -18dBfs. I had to think, and asked him to wheel
the unit in where measurement could take place with my Neutrik A2 audio test set.
It took a moment but we worked it out and I thought I would share what we learned.
The UK ( + Europe ) and US have always worked to slightly different audio level standards. It was only relatively
recently that all US equipment wired equipment XLR Pin 3 hot. We have aligned since, so now generally on new
equipment, Pin 2 HOT, + Phase, but on audio levels, we still differ by quite a bit.
Europe works to a standard digital audio reference level of -18dBfs, the US works to -20dBfs.
On the face of it, -20 dBfs seems to be more logical level, humans being familair with the base 10 numbering
system. Where did -18dBfs come, what is in 2dB ? I will come to that later.
By convention ( and standard ) in Europe, digital to analogue convertors are aligned so that -18dBfs is converted
to 0dBu audio level, with low output impedance, ( 50 Ohm or less ) into equipment that will have an input
impedance of 10KΩ ( or more ).
In the US, audio equipment is often aligned so that digital to analogue convertors are aligned to output +4dBu into
a 600 Ohm load from a -20dBfs digital signal. This causes a 6 /7 dB discrepency in European and US equipment.
Yamaha audio desk when he had set tone reference audio level to -18dBfs. I had to think, and asked him to wheel
the unit in where measurement could take place with my Neutrik A2 audio test set.
It took a moment but we worked it out and I thought I would share what we learned.
The UK ( + Europe ) and US have always worked to slightly different audio level standards. It was only relatively
recently that all US equipment wired equipment XLR Pin 3 hot. We have aligned since, so now generally on new
equipment, Pin 2 HOT, + Phase, but on audio levels, we still differ by quite a bit.
Europe works to a standard digital audio reference level of -18dBfs, the US works to -20dBfs.
On the face of it, -20 dBfs seems to be more logical level, humans being familair with the base 10 numbering
system. Where did -18dBfs come, what is in 2dB ? I will come to that later.
By convention ( and standard ) in Europe, digital to analogue convertors are aligned so that -18dBfs is converted
to 0dBu audio level, with low output impedance, ( 50 Ohm or less ) into equipment that will have an input
impedance of 10KΩ ( or more ).
In the US, audio equipment is often aligned so that digital to analogue convertors are aligned to output +4dBu into
a 600 Ohm load from a -20dBfs digital signal. This causes a 6 /7 dB discrepency in European and US equipment.
So, this seems pretty straight forward, two continents, two slightly different audio level references.
Imagine we have a piece of equipment in a U.S. environment, the equipment aligned to UK levels
( -18dBfs = 0dBu ).
When the reference file, encoded at -20dBfs is played we will measure a signal of amplitude -2dBu analogue
audio output, with is 6dB lower than the +4dBu of analogue audio level that the studio equipment expects.
Reverse the situation, we have equipment aligned to US levels operating in Europe. Reference tone of -18dBfs
digital level will be play out +6dBu analogue audio level, 6dB higher than we expect again.
I have only talked about converting digital to analogue levels. The situation is equal and opposite, converting
the analogue audio signal to digital levels
Imagine we have a piece of equipment in a U.S. environment, the equipment aligned to UK levels
( -18dBfs = 0dBu ).
When the reference file, encoded at -20dBfs is played we will measure a signal of amplitude -2dBu analogue
audio output, with is 6dB lower than the +4dBu of analogue audio level that the studio equipment expects.
Reverse the situation, we have equipment aligned to US levels operating in Europe. Reference tone of -18dBfs
digital level will be play out +6dBu analogue audio level, 6dB higher than we expect again.
I have only talked about converting digital to analogue levels. The situation is equal and opposite, converting
the analogue audio signal to digital levels
| The drawings below show equipment aligned to the -18dBfs and -20 dBfs reference levels operating in studio environments that are working in the opposite configuration |
| The above picture is an example of where equipment aligned to European levels of 0dBu = -18dBfs is operated in the US where +4dBu = -20dBfs. On the A/D side, +4dBu reference tone will be encoded not to -20dBfs, but to -14dBfs, 6dB higher than we expect. On the D/A side, if reference tone is digitally generated internally from the equipment, the encoded analogue signal will be -2dBu, 6 dB lower than we want. |
| The above picture is an example of where equipment aligned to US levels of +4dBu = -20dBfs is operated in Europe where 0dBu = -18dBfs. On the A/D side, 0dBu reference tone will be encoded not to -20dBfs, but to -24dBfs, 6dB lower than we expect. On the D/A side, if reference tone of -18dBfs is internally digitally generated from the equipment, the encoded analogue signal will be +6dBu, 6 dB higher than we want. |
So far so good. Most equipment these days has a check box or menu to set the digital reference level ( -18 / -20
dBfs ) and to set the analogue output levels,( though some equipment still inexplicably doesn't ) problem solved.
Audio equipment Input and output Impedance
At the top of this article I mentioned that my colleague was puzzled that the Yamaha CL1 outputs +7.5 dBu
analogue audio level when internal digital reference tone is set to -18dBfs.
With reference to Fig 5, we see that if we generate -18dBfs, equipment aligned to U.S. levels equipment should
output +6dBu. On our test bench with the Yamaha,connected to my audio meter, set to have an input impedance of
100 kΩ, the output audio level when measured was 7.2 dB. When I adjusted the input impedance of the meter to
600Ω, the Yamaha output dropped to 6.21dBu.
We now knew that the Yamaha CL1 was aligned to US levels, adjusting the oscillator output to -20dBfs the
analogue output level into a 600Ω output to load was now 4.1 dBu, perfectly on specification.
Most equipment used in a professional audio or broadcast application these days has an input impedance of
greater than 10kΩ, and an output impedance of 50-100 Ω. We can calculate the output impedance of the Yamaha
by placing selected loads onto the output and measuring the result.
dBfs ) and to set the analogue output levels,( though some equipment still inexplicably doesn't ) problem solved.
Audio equipment Input and output Impedance
At the top of this article I mentioned that my colleague was puzzled that the Yamaha CL1 outputs +7.5 dBu
analogue audio level when internal digital reference tone is set to -18dBfs.
With reference to Fig 5, we see that if we generate -18dBfs, equipment aligned to U.S. levels equipment should
output +6dBu. On our test bench with the Yamaha,connected to my audio meter, set to have an input impedance of
100 kΩ, the output audio level when measured was 7.2 dB. When I adjusted the input impedance of the meter to
600Ω, the Yamaha output dropped to 6.21dBu.
We now knew that the Yamaha CL1 was aligned to US levels, adjusting the oscillator output to -20dBfs the
analogue output level into a 600Ω output to load was now 4.1 dBu, perfectly on specification.
Most equipment used in a professional audio or broadcast application these days has an input impedance of
greater than 10kΩ, and an output impedance of 50-100 Ω. We can calculate the output impedance of the Yamaha
by placing selected loads onto the output and measuring the result.
Fig 1
Fig 4
Fig 5
Fig 3
Fig 2
We would like to calculate the total source output impedance, represented by the total of the resistance in each
leg of the phase. The calculated source impedance will be 2*Rs.
If we make an initial measurement with Rl at say 100 kΩ, the maximum input impedance of my Neutrik A2 audio
test set, we measured an output of 7.21 dBu when reference tone was set to output 1kHz at -18dBfs
When I set Rl to be 600 Ω, the measured output fell to 6.13dBu
Lets calculate the Source impedance.
Thus V load = R load * V source
R load + R Source
If R Load is very high, say 100 KΩ, and R source is less than 100 Ω then 100,000 / ( 100,000 + 100 ) = 0.999,
The measured Voltage Vload is V source
Now if we set R load to be 600 Ω the measurement dropped to 6.13 dBu.
7.21dBu - 6.13dBu = 1.08dBu drop when a 600 Ω load is used
dB = 20 log10 ( V1 / V2 )
thus -1.08 dB = 20 log 10 (0.883 )
V load = R load * V source
R load + R Source
0.883 = R load : Rload = 600Ω
R load + R Source
We calculate that Rsource is thus 78 Ω. Rs in the diagram is half this value at ~ 40Ω each
Lets cross check, I set the load impedence to be 1200 Ω, the output was 6.65 dBu, a drop of ( 7.21 - 6.65 )
- 0.56 dB. -0.56 = 20 log 10 ( 0.937 )
0.937 = R load : Rload = 1200Ω
R load + R Source
1200 / ( 1200 + 78 ) = 0.938 : lets guess that Rs = 40 Ω
1200 / ( 1200 + 80 ) = 0.9375
So again : with a different load ( of 1200 Ω ) we calculate the total output impedance of the Yamaha CL 1
analogue audio outputs to be about 80 Ohms with a good tolerance.
leg of the phase. The calculated source impedance will be 2*Rs.
If we make an initial measurement with Rl at say 100 kΩ, the maximum input impedance of my Neutrik A2 audio
test set, we measured an output of 7.21 dBu when reference tone was set to output 1kHz at -18dBfs
When I set Rl to be 600 Ω, the measured output fell to 6.13dBu
Lets calculate the Source impedance.
Thus V load = R load * V source
R load + R Source
If R Load is very high, say 100 KΩ, and R source is less than 100 Ω then 100,000 / ( 100,000 + 100 ) = 0.999,
The measured Voltage Vload is V source
Now if we set R load to be 600 Ω the measurement dropped to 6.13 dBu.
7.21dBu - 6.13dBu = 1.08dBu drop when a 600 Ω load is used
dB = 20 log10 ( V1 / V2 )
thus -1.08 dB = 20 log 10 (0.883 )
V load = R load * V source
R load + R Source
0.883 = R load : Rload = 600Ω
R load + R Source
We calculate that Rsource is thus 78 Ω. Rs in the diagram is half this value at ~ 40Ω each
Lets cross check, I set the load impedence to be 1200 Ω, the output was 6.65 dBu, a drop of ( 7.21 - 6.65 )
- 0.56 dB. -0.56 = 20 log 10 ( 0.937 )
0.937 = R load : Rload = 1200Ω
R load + R Source
1200 / ( 1200 + 78 ) = 0.938 : lets guess that Rs = 40 Ω
1200 / ( 1200 + 80 ) = 0.9375
So again : with a different load ( of 1200 Ω ) we calculate the total output impedance of the Yamaha CL 1
analogue audio outputs to be about 80 Ohms with a good tolerance.
