[Adrian Tuddenham[_2_]]

davew wrote:

On Nov 22, 7:14*am, PStamler wrote:
[...]
1V into 1k, at the input, means 1mA, so the power is 1mW. 10V into
100k at the output means 0.1mA, so the power is 1mW again. There's 0dB
power gain, but there's voltage gain of 10x, which is coded +20dB in
the voltage-gain realm of decibel calculation. This deviates from the
"real" standard of what decibels are, by divorcing the voltage and
power gains, but using the same unit for them, dB.
But, if I were to double the input voltage I would see a 6dB increase
in power output would I not? In context, the use of dB for voltage
gain is entirely correct and acceptable.

It can be used that way as long as the two voltages being compared are
across the same impedance and both relate to the power in the same way.
This was the case in your example.

You can compare voltages at two different places as long as the
impedances at those places are the same. So an amplifier working with
600-ohm input and output terminations could have its power gain
expressed in dB by just measuring the voltage gain.

What you must not do (which is all-too frequently done) is to use the
same calculation to compare voltages in different impedances. For that,
you must use the voltages to calculate the power at each point and then
compare the powers to get a result in dB. If you want to use a
logarithmic scale to compare voltages, call it something different (dBu,
dBV) because it is not "dB".

The convenient shortcut dB formula relating voltages is often taught
with insufficient emphasis being placed on the conditions for which it
is valid. After a while, people begin to believe that the voltage
formula is the true representation of dB until a situation arises where
it generates nonsense results - then they get confused and sometimes
adamant. At least one well-known textbook has got it wrong.



--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

[Don Pearce[_3_]]

On Mon, 22 Nov 2010 12:41:10 +0000,
lid (Adrian Tuddenham) wrote:

davew wrote:

On Nov 22, 7:14*am, PStamler wrote:
[...]
1V into 1k, at the input, means 1mA, so the power is 1mW. 10V into
100k at the output means 0.1mA, so the power is 1mW again. There's 0dB
power gain, but there's voltage gain of 10x, which is coded +20dB in
the voltage-gain realm of decibel calculation. This deviates from the
"real" standard of what decibels are, by divorcing the voltage and
power gains, but using the same unit for them, dB.
But, if I were to double the input voltage I would see a 6dB increase
in power output would I not? In context, the use of dB for voltage
gain is entirely correct and acceptable.

It can be used that way as long as the two voltages being compared are
across the same impedance and both relate to the power in the same way.
This was the case in your example.

You can compare voltages at two different places as long as the
impedances at those places are the same. So an amplifier working with
600-ohm input and output terminations could have its power gain
expressed in dB by just measuring the voltage gain.

What you must not do (which is all-too frequently done) is to use the
same calculation to compare voltages in different impedances. For that,
you must use the voltages to calculate the power at each point and then
compare the powers to get a result in dB. If you want to use a
logarithmic scale to compare voltages, call it something different (dBu,
dBV) because it is not "dB".

The convenient shortcut dB formula relating voltages is often taught
with insufficient emphasis being placed on the conditions for which it
is valid. After a while, people begin to believe that the voltage
formula is the true representation of dB until a situation arises where
it generates nonsense results - then they get confused and sometimes
adamant. At least one well-known textbook has got it wrong.

The full equation is

20 log(v2/v1 *sqrt(z1/z2))

That gets the power gain right for any impedance. (V1 is across
impedance z1, and v2 is across z2).

d

[davew]

On Nov 22, 12:41*pm, (Adrian
Tuddenham) wrote:
davew wrote:
On Nov 22, 7:14*am, PStamler wrote:
[...]
1V into 1k, at the input, means 1mA, so the power is 1mW. 10V into
100k at the output means 0.1mA, so the power is 1mW again. There's 0dB
power gain, but there's voltage gain of 10x, which is coded +20dB in
the voltage-gain realm of decibel calculation. This deviates from the
"real" standard of what decibels are, by divorcing the voltage and
power gains, but using the same unit for them, dB.
But, if I were to double the input voltage I would see a 6dB increase
in power output would I not? *In context, the use of dB for voltage
gain is entirely correct and acceptable.

It can be used that way as long as the two voltages being compared are
across the same impedance and both relate to the power in the same way.
This was the case in your example.

You can compare voltages at two different places as long as the
impedances at those places are the same. *So an amplifier working with
600-ohm input and output terminations could have its power gain
expressed in dB by just measuring the voltage gain.

What you must not do (which is all-too frequently done) is to use the
same calculation to compare voltages in different impedances. *For that,
you must use the voltages to calculate the power at each point and then
compare the powers to get a result in dB. *If you want to use a
logarithmic scale to compare voltages, call it something different (dBu,
dBV) because it is not "dB".

The convenient shortcut dB formula relating voltages is often taught
with insufficient emphasis being placed on the conditions for which it
is valid. *After a while, people begin to believe that the voltage
formula is the true representation of dB until a situation arises where
it generates nonsense results - then they get confused and sometimes
adamant. *At least one well-known textbook has got it wrong.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)www.poppyrecords.co.uk

This is all well and good, but in a lot of small signal territory and
in DSP, we don't care about power or impedances. We may worry about
output impedance and power drain of a small signal stage, for example,
but we would usually say that a given stage has a voltage gain, not a
power gain i.e. where voltage gain, not power gain, is the object.
So, a gain of 10 in the signal chain would result in an end output
power increase (assuming we were feeding some form of power amplifier)
of 20dB.

Consider a FET source follower as an example. Tremendous gain i.e.
power gain in dB but voltage gain of a little under unity or 0dB. Now
you ask a hundred analogue electronics engineers what the gain is in
dB and (yes it's a trick question) 99/100 will say a bit less than
0dB.

[Mike Rivers]

On 11/22/2010 7:41 AM, Adrian Tuddenham wrote:

The convenient shortcut dB formula relating voltages is often taught
with insufficient emphasis being placed on the conditions for which it
is valid.

This is why it's only been used over the last 30 or maybe 40
years, while the "power dB" goes back 80 years or so. It's
only been since the adoption of the nearly universal
voltage-not-power scheme of interfacing. This is primarily a
result of the use of solid state electronics which
characteristically have nearly zero output (source)
impedance but without the ability to supply any significant
amount of current. It's just how things work now.

After a while, people begin to believe that the voltage
formula is the true representation of dB until a situation arises where
it generates nonsense results

Not a lot of confusion, because, in general, audio people
TODAY work with dB of gain or voltage levels, and absolute
(not relative) watts when dealing with power levels. Someone
will understand a 100W or 200W amplifier, but won't think of
one having 3 dB more (power) gain than the other.

--
"Today's production equipment is IT based and cannot be
operated without a passing knowledge of computing, although
it seems that it can be operated without a passing knowledge
of audio." - John Watkinson

http://mikeriversaudio.wordpress.com - useful and
interesting audio stuff

[Arny Krueger]

"Mike Rivers" wrote in message


Someone will understand a 100W or 200W amplifier, but
won't think of one having 3 dB more (power) gain than the
other.

That would be their loss. If you compare amp power in dBs, and remember that
it takes about 10 dB to create the perception of "twice as loud", then a lot
of things in the real world that may seem mysterious become understandable.

[Adrian Tuddenham[_2_]]

davew wrote:

On Nov 22, 12:41*pm, (Adrian
Tuddenham) wrote:
davew wrote:
On Nov 22, 7:14*am, PStamler wrote:
[...]
1V into 1k, at the input, means 1mA, so the power is 1mW. 10V into
100k at the output means 0.1mA, so the power is 1mW again. There's 0dB
power gain, but there's voltage gain of 10x, which is coded +20dB in
the voltage-gain realm of decibel calculation. This deviates from the
"real" standard of what decibels are, by divorcing the voltage and
power gains, but using the same unit for them, dB.
But, if I were to double the input voltage I would see a 6dB increase
in power output would I not? *In context, the use of dB for voltage
gain is entirely correct and acceptable.

It can be used that way as long as the two voltages being compared are
across the same impedance and both relate to the power in the same way.
This was the case in your example.

You can compare voltages at two different places as long as the
impedances at those places are the same. *So an amplifier working with
600-ohm input and output terminations could have its power gain
expressed in dB by just measuring the voltage gain.

What you must not do (which is all-too frequently done) is to use the
same calculation to compare voltages in different impedances. *For that,
you must use the voltages to calculate the power at each point and then
compare the powers to get a result in dB. *If you want to use a
logarithmic scale to compare voltages, call it something different (dBu,
dBV) because it is not "dB".

The convenient shortcut dB formula relating voltages is often taught
with insufficient emphasis being placed on the conditions for which it
is valid. *After a while, people begin to believe that the voltage
formula is the true representation of dB until a situation arises where
it generates nonsense results - then they get confused and sometimes
adamant. *At least one well-known textbook has got it wrong.



This is all well and good, but in a lot of small signal territory and
in DSP, we don't care about power or impedances.

There is nothing wrong with that approach, but you must not then express
the ratios as 'dB'.

You may include the letters 'dB' to suggest a logarithmic scale as long
as they also have some sort of qualifier prefix or suffix to warn the
student or the pedant (or anyone in between) that you are not comparing
powers and these measurements do not follow the correct definition of a
decibel.

Much confusion has been caused over the years by the omission of that
qualifier. Try explaining to a student who is used to thinking of dB in
terms of voltage alone where the gain occurs in the following example:

http://www.poppyrecords.co.uk/other/images/Gain.gif


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

[gs]

In article , "Arny Krueger" wrote:
"Mike Rivers" wrote in message


Someone will understand a 100W or 200W amplifier, but
won't think of one having 3 dB more (power) gain than the
other.

That would be their loss. If you compare amp power in dBs, and remember that
it takes about 10 dB to create the perception of "twice as loud", then a lot
of things in the real world that may seem mysterious become understandable.

You got to consider that.
But, for understanding basics, knowing 6 dB is twice the voltage is what
people should know and use, and talk about. The only thing power
dB is good for is speaker dissapation and cost of amplifiers, and
indirectly, mains power. Forget volume levels.

greg

[Adrian Tuddenham[_2_]]

GS wrote:

In article ,
"Arny Krueger" wrote:
"Mike Rivers" wrote in message


Someone will understand a 100W or 200W amplifier, but
won't think of one having 3 dB more (power) gain than the
other.

That would be their loss. If you compare amp power in dBs, and remember that
it takes about 10 dB to create the perception of "twice as loud", then a lot
of things in the real world that may seem mysterious become understandable.

You got to consider that.
But, for understanding basics, knowing 6 dB is twice the voltage is what
people should know....

They should first know that dB is a power measurement, this is the
fundamental fact on which the rest is based. When they have grasped the
basics, then they can be shown that the voltage is a handy way of
comparing two power levels in the right circumstances (and they can
remember some handy voltage ratios if appropriate).


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

[Mike Rivers]

On 11/22/2010 11:36 AM, Adrian Tuddenham wrote:

They should first know that dB is a power measurement, this is the
fundamental fact on which the rest is based. When they have grasped the
basics, then they can be shown that the voltage is a handy way of
comparing two power levels in the right circumstances (and they can
remember some handy voltage ratios if appropriate).

But most people working in audio don't need to compare power
levels, they need to compare voltage levels. I'd teach them
the "20" formula first. But then you may be more into theory
and less into practice than I am.

--
"Today's production equipment is IT based and cannot be
operated without a passing knowledge of computing, although
it seems that it can be operated without a passing knowledge
of audio." - John Watkinson

http://mikeriversaudio.wordpress.com - useful and
interesting audio stuff

[Adrian Tuddenham[_2_]]

Mike Rivers wrote:

On 11/22/2010 11:36 AM, Adrian Tuddenham wrote:

They should first know that dB is a power measurement, this is the
fundamental fact on which the rest is based. When they have grasped the
basics, then they can be shown that the voltage is a handy way of
comparing two power levels in the right circumstances (and they can
remember some handy voltage ratios if appropriate).

But most people working in audio don't need to compare power
levels, they need to compare voltage levels. I'd teach them
the "20" formula first. But then you may be more into theory
and less into practice than I am.

I remember the struggles I had in my earlier days, trying to get a grasp
on how to use the dB system. There was so much misinformation and so
many things that didn't add up when you came to match theory with
practice. Nobody seemed to be able to explain what was really going on.

Now most of my recording chain is of my own manufacture and I design
specialist equipment for others. I am in a good position to look back
at my earlier mistakes and misunderstandings and see where I went wrong
(and where I was sent up the wrong path by others). I am also in a good
position to spot when other people are in the same quandry for the same
reasons as I was.

That's why I tend to be a bit pedantic about the use of dB; I know from
experience that a little pedantry at the outset will pay-off later, when
the student moves on and becomes a true professional.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

[Arny Krueger]

"Mike Rivers" wrote in message

On 11/22/2010 11:36 AM, Adrian Tuddenham wrote:

They should first know that dB is a power measurement,
this is the fundamental fact on which the rest is based.
When they have grasped the basics, then they can be
shown that the voltage is a handy way of comparing two
power levels in the right circumstances (and they can
remember some handy voltage ratios if appropriate).

But most people working in audio don't need to compare
power levels, they need to compare voltage levels. I'd
teach them the "20" formula first. But then you may be
more into theory and less into practice than I am.

Agreed, since in my world, voltmeters vastly outnumber true watt meters.

QED.