The truth about decibels

[Pooh Bear]

decibel Abbr. dB Equal to one-tenth of a bel. [After Alexander Graham
Bell.]

1. A measuring system first used in telephony (Martin, W.H., "DeciBel --
the new name for the transmission unit. Bell System Tech. J. January,
1929), where signal loss is a logarithmic function of the cable length.

2. The preferred method and term for representing the ratio of different
audio levels. It is a mathematical shorthand that uses logarithms (a
shortcut using the powers of 10 to represent the actual number) to
reduce the size of the number. For example, instead of saying the
dynamic range is 32,000 to 1, we say it is 90 dB [the answer in dB
equals 20 log x/y, where x and y are the different signal levels]. Being
a ratio, decibels have no units. Everything is relative. Since it is
relative, then it must be relative to some 0 dB reference point. To
distinguish between reference points a suffix letter is added as follows
[The officially correct way per AES-R2, IEC 60027-3 & IEC 60268-2
documents is to enclose the reference value in parenthesis separated by
a space from "dB"; however this never caught on, probably for brevity
reasons if no other.]:


0 dBu Preferred informal abbreviation for the official dB (0.775 V); a
voltage reference point equal to 0.775 Vrms. [This reference originally
was labeled dBv (lower-case) but was too often confused with dBV
(upper-case), so it was changed to dBu (for unterminated).]
+4 dBu Standard pro audio voltage reference level equal to 1.23 Vrms.

0 dBV Preferred informal abbreviation for the official dB (1.0 V); a
voltage reference point equal to 1.0 Vrms.

-10 dBV Standard voltage reference level for consumer and some pro audio
use (e.g. TASCAM), equal to 0.316 Vrms. (Tip: RCA connectors are a good
indicator of units operating at -10 dBV levels.)

0 dBm Preferred informal abbreviation of the official dB (mW); a power
reference point equal to 1 milliwatt. To convert into an equivalent
voltage level, the impedance must be specified. For example, 0 dBm into
600 ohms gives an equivalent voltage level of 0.775 V, or 0 dBu (see
above); however, 0 dBm into 50 ohms, for instance, yields an equivalent
voltage of 0.224 V -- something quite different. Since modern audio
engineering is concerned with voltage levels, as opposed to power levels
of yore, the convention of using a reference level of 0 dBm is obsolete.
The reference levels of +4 dBu, or -10 dBV are the preferred units.

0 dBr An arbitrary reference level (r = re; or reference) that must be
specified. For example, a signal-to-noise graph may be calibrated in
dBr, where 0 dBr is specified to be equal to 1.23 Vrms (+4 dBu);
commonly stated as "dB re +4," that is, "0 dBr is defined to be equal to
+4 dBu."

0 dBFS A digital audio reference level equal to "Full Scale." Used in
specifying A/D and D/A audio data converters. Full scale refers to the
maximum peak voltage level possible before "digital clipping," or
digital overload (see overs) of the data converter. The Full Scale value
is fixed by the internal data converter design, and varies from model to
model. [According to standards people, there's supposed to be a space
between "dB" and "FS" -- yeah, right, like that's gonna happen.]

0 dBf Preferred informal abbreviation of the official dB (fW); a power
reference point equal to 1 femtowatt, i.e., 10-15 watts.

0 dB-SPL The reference point for the threshold of hearing, equal to 20
microPA (micro Pascals rms).

Since 1 PA = 1 newton/m2 = .000145 PSI (pounds per square inch)

Then 0 dB-SPL = 2.9 nano PSI (rms) -- an unbelievably small value.

This means that since 1 atm = 14.7 PSI, it is equivalent to a loudness
level of 194 dB-SPL! [Thanks to Bob Pease for pointing out these
enlightening facts!]

dBA Unofficial but popular way of stating loudness measurements made
using an A-weighting curve.

dBC Unofficial but popular way of stating loudness measurements made
using an C-weighting curve.



http://www.rane.com/par-d.html#decibel

Note that *voltage* ( or pressure in the case of acoustic dBs )
reference related decibels are now the accepted norm.

The dBm ( 600 ohms ) is all but extinct in real usage.


Graham

 

[Joerg]

Hello Graham,

... Since modern audio
engineering is concerned with voltage levels, as opposed to power levels
of yore, the convention of using a reference level of 0 dBm is obsolete.

Absolutely not. It is alive and kicking in serious RF engineering.

The dBm ( 600 ohms ) is all but extinct in real usage.

In telephony maybe but certainly not in the RF world. Look here:
http://www.mini-circuits.com/dg03-80B.pdf

dBm all over the place...

So, were you named after Alexander Graham?

Regards, Joerg

http://www.analogconsultants.com

 

[martin griffith]

On Mon, 27 Jun 2005 20:50:49 +0100, in sci.electronics.design Pooh
Bear <rabbitsfriendsandrelations@hotmail.com> wrote:

decibel Abbr. dB Equal to one-tenth of a bel. [After Alexander Graham
Bell.]

1. A measuring system first used in telephony (Martin, W.H., "DeciBel --
the new name for the transmission unit. Bell System Tech. J. January,
1929), where signal loss is a logarithmic function of the cable length.

Vinyl sounds nice.

Tic, tic. tic, pop, pop, pop, rumble, rumble, rumble.


martin

 

[Pooh Bear]

martin griffith wrote:

On Mon, 27 Jun 2005 20:50:49 +0100, in sci.electronics.design Pooh
Bear <rabbitsfriendsandrelations@hotmail.com> wrote:

decibel Abbr. dB Equal to one-tenth of a bel. [After Alexander Graham
Bell.]

1. A measuring system first used in telephony (Martin, W.H., "DeciBel --
the new name for the transmission unit. Bell System Tech. J. January,
1929), where signal loss is a logarithmic function of the cable length.

Vinyl sounds nice.

Tic, tic. tic, pop, pop, pop, rumble, rumble, rumble.

martin

Been playing some of the old black stuff recently ?

If you want click, hiss etc try shellac !


Graham

 

[Pooh Bear]

Joerg wrote:

Hello Graham,

... Since modern audio
engineering is concerned with voltage levels, as opposed to power levels
of yore, the convention of using a reference level of 0 dBm is obsolete.

Absolutely not. It is alive and kicking in serious RF engineering.

The dBm ( 600 ohms ) is all but extinct in real usage.

In telephony maybe but certainly not in the RF world. Look here:
http://www.mini-circuits.com/dg03-80B.pdf

dBm all over the place...

Yeah - but they aren't 600 ohms are they ?

I'd love to know when the RF world adopted the dB. Since RF circuits are
invariably matched impedance it makes some sense to keep using the dBm there
since it doesn't have the inherent limitations it did in audio.

So, were you named after Alexander Graham?

Nah ! Graham's just a popular Scottish name.

http://www.rampantscotland.com/clans/blclangraham.htm

My middle name is Stewart ! Which together with my surname gives no less than
12 possible spelling combinations.


Graham

 

[martin griffith]

On Mon, 27 Jun 2005 21:49:52 +0100, in sci.electronics.design Pooh
Bear <rabbitsfriendsandrelations@hotmail.com> wrote:

martin griffith wrote:

On Mon, 27 Jun 2005 20:50:49 +0100, in sci.electronics.design Pooh
Bear <rabbitsfriendsandrelations@hotmail.com> wrote:

decibel Abbr. dB Equal to one-tenth of a bel. [After Alexander Graham
Bell.]

1. A measuring system first used in telephony (Martin, W.H., "DeciBel --
the new name for the transmission unit. Bell System Tech. J. January,
1929), where signal loss is a logarithmic function of the cable length.

Vinyl sounds nice.

Tic, tic. tic, pop, pop, pop, rumble, rumble, rumble.

martin

Been playing some of the old black stuff recently ?

If you want click, hiss etc try shellac !


Graham

Nope, I just play the 15ips 1/4" tape, much more satifying .

martin

 

[Tim Wescott]

Pooh Bear wrote:

Joerg wrote:


Hello Graham,


... Since modern audio
engineering is concerned with voltage levels, as opposed to power levels
of yore, the convention of using a reference level of 0 dBm is obsolete.

Absolutely not. It is alive and kicking in serious RF engineering.


The dBm ( 600 ohms ) is all but extinct in real usage.

In telephony maybe but certainly not in the RF world. Look here:
http://www.mini-circuits.com/dg03-80B.pdf

dBm all over the place...


Yeah - but they aren't 600 ohms are they ?

No, but you said "real" usage. Now some RF design is pretty unreal...

I'd love to know when the RF world adopted the dB. Since RF circuits are
invariably matched impedance it makes some sense to keep using the dBm there
since it doesn't have the inherent limitations it did in audio.

Not always impedance matched -- without playing some very interesting

tricks the best noise figure of an amplifier is generally achieved at an
impedance that is _not_ equal to the impedance of the source. Yet noise
figures are still measured in dB.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Pooh Bear]

Tim Wescott wrote:

Pooh Bear wrote:

Joerg wrote:


Hello Graham,


... Since modern audio
engineering is concerned with voltage levels, as opposed to power levels
of yore, the convention of using a reference level of 0 dBm is obsolete.

Absolutely not. It is alive and kicking in serious RF engineering.


The dBm ( 600 ohms ) is all but extinct in real usage.

In telephony maybe but certainly not in the RF world. Look here:
http://www.mini-circuits.com/dg03-80B.pdf

dBm all over the place...


Yeah - but they aren't 600 ohms are they ?

No, but you said "real" usage. Now some RF design is pretty unreal...

That much I can believe !

I'd love to know when the RF world adopted the dB. Since RF circuits are
invariably matched impedance it makes some sense to keep using the dBm there
since it doesn't have the inherent limitations it did in audio.

Not always impedance matched -- without playing some very interesting
tricks the best noise figure of an amplifier is generally achieved at an
impedance that is _not_ equal to the impedance of the source. Yet noise
figures are still measured in dB.

Interesting about the noise figure. That's precisely one of the reasons the audio
world dropped matched impedance working too. When you lose half the signal volts
just by connecting 2 circuits together that isn't brilliant for s/n ! Not to
mention there was no *need* to be impedance matched since you weren't going to get
cable problems at audio frequencies.

Graham

 

[Joerg]

Hello Tim,

No, but you said "real" usage. Now some RF design is pretty unreal...

ROFL!

... Yet noise figures are still measured in dB.

Except in Germany where they also use the "noise number". They call it
"Rauschzahl". The interesting thing is that "Rausch" is also the word
for the condition after way too many beers and Schnapses.

Regards, Joerg

http://www.analogconsultants.com

 

[Joerg]

Hello Graham,

Nah ! Graham's just a popular Scottish name.

http://www.rampantscotland.com/clans/blclangraham.htm

That confirms what I thought, that it's really spelled Graeme.

My middle name is Stewart ! Which together with my surname gives no less than
12 possible spelling combinations.

At least I hope they gave up all the vendettas between the clans...

Regards, Joerg

http://www.analogconsultants.com

 

[Guy Macon]

martin griffith wrote:

Nope, I just play the 15ips 1/4" tape, much more satifying .

Half track, I presume...

 

[Pooh Bear]

Guy Macon wrote:

martin griffith wrote:

Nope, I just play the 15ips 1/4" tape, much more satifying .

Half track, I presume...

If in pursuit of ultimate performance, Ampex offered 1/2" tape stereo
head stacks for the superb ATR 100 series. Probably the finest
professional analogue tape recorder ever built and I've been lucky
enought to have had my hands on a few.

http://www.precisionmotorworks.com/atr100.htm

Graham

 

[Robert]

"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:11c0sf3jouflndb@corp.supernews.com...

Pooh Bear wrote:
[snip]

Not always impedance matched -- without playing some very interesting
tricks the best noise figure of an amplifier is generally achieved at an
impedance that is _not_ equal to the impedance of the source. Yet noise
figures are still measured in dB.

Also Power Amps don't always work into matched loads for efficiency (or

other reasons).

Robert

 

[Winfield Hill]

martin griffith wrote...

Nope, I just play the 15ips 1/4" tape, much more satifying.

I've forgotten, how long does a 7" 15ips reel of tape play,
say compared to a CD? Or do you keep and play larger reels?


--
Thanks,
- Win

 

[Pooh Bear]

Robert wrote:

"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:11c0sf3jouflndb@corp.supernews.com...
Pooh Bear wrote:
[snip]

Not always impedance matched -- without playing some very interesting
tricks the best noise figure of an amplifier is generally achieved at an
impedance that is _not_ equal to the impedance of the source. Yet noise
figures are still measured in dB.

Also Power Amps don't always work into matched loads for efficiency (or
other reasons).

I know of *no* audio frequency power amplifier that works into a *matched
load* since the dissipation would be astronomical !

The RF guys tackle the issues in a different way since they're less worried
about waveform distortion as a rule.

The 'toob nuts' probably think that when they select 15, 8 or 4 ohms on the
transformer output tap that they're 'matching' the load. Nothing could be
further from the truth ! They're simply adjusting a voltage ratio. Even toob
amps have a damping factor ! That means that their output impedance is
significantly less than the load impedance. Hopefully by a factor of 10 or
more ( for toobs ) and likely to be >> 100+ for solid state.

Even 'toob amps' have voltage matched outputs. Matched to the allowable plate
/ anode dissipation for a given load.


Graham

 

[Bob]

The dBm ( 600 ohms ) is all but extinct in real usage.


Graham

Well, until the world is rid of plain-old telephone service (POTS) and
analog trunks, dBm referenced to 600 and 900 ohms will be alive-and-well.

Nice try, though. Thanks for playing.

Bob

 

[Pooh Bear]

Bob wrote:

The dBm ( 600 ohms ) is all but extinct in real usage.

Graham

Well, until the world is rid of plain-old telephone service (POTS) and
analog trunks, dBm referenced to 600 and 900 ohms will be alive-and-well.

Nice try, though. Thanks for playing.

I never said telecoms didn't use it. Indeed I referred to the fact that it
was precisely telecoms that first used it ! Just that *audio* generally moved
on.from a kludgy solution to a non-problem. Maybe you didn't read the post ?

I seriously doubt that there's any even remote need for 600 ohm working in
telecoms now since the advent of microwave, optical and satellite links.

600 ohms working was only ever needed for long 'land lines' - and how many of
them still exist ? It's all concentrated at the local exchange and
distributed digitally via optic fibre these days !

If telecoms want to stick with 600 ohms - so be it. The rest of the world
moved on though.

Btw - is my DSL connection 600 ohms ? I doubt it !

Graham

 

[Bob]

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:42C0BCC1.9B92F93D@hotmail.com...

Bob wrote:

The dBm ( 600 ohms ) is all but extinct in real usage.

Graham

Well, until the world is rid of plain-old telephone service (POTS) and
analog trunks, dBm referenced to 600 and 900 ohms will be alive-and-well.

Nice try, though. Thanks for playing.

I never said telecoms didn't use it. Indeed I referred to the fact that it
was precisely telecoms that first used it ! Just that *audio* generally
moved
on.from a kludgy solution to a non-problem. Maybe you didn't read the post
?

I seriously doubt that there's any even remote need for 600 ohm working in
telecoms now since the advent of microwave, optical and satellite links.

600 ohms working was only ever needed for long 'land lines' - and how many
of
them still exist ? It's all concentrated at the local exchange and
distributed digitally via optic fibre these days !

If telecoms want to stick with 600 ohms - so be it. The rest of the world
moved on though.

Btw - is my DSL connection 600 ohms ? I doubt it !

Graham

No offense intended, Graham.

600 ohms reference is used in POTS -- and POTS is still ubiquitous. If your
home phone is a POTS phone then its designers have used 600 ohms in their
design calculations. Not too many people have digital wired phones, at home.
Until there is a solution to "emergency power for phones" then POTS service
will remain. ISDN has gone nowhere. Perhaps power over Ethernet (POE) may
lend some hope, but I doubt it offers a near-future solution, and VOIP
quality is worse than cellular (is that possible?). POTS and the 600 ohm dBm
will be with us for many years-to-come.

DSL is 100 ohms because the characteristic impedance of twisted pair
approaches this value at the higher frequencies (MHz). This technology uses
units of dBm (100) and dBm/Hz (100).

Also, don't microphones and their cables use 600 ohms reference impedance?

Bob

 

[James T. White]

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:WZYve.421$0V3.386@newssvr13.news.prodigy.com...

Hello Graham,


dBm all over the place...

Plus dBi and dBd for antennas!

--
James T. White

 

[Pooh Bear]

Bob wrote:

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:42C0BCC1.9B92F93D@hotmail.com...
Bob wrote:

The dBm ( 600 ohms ) is all but extinct in real usage.

Graham

Well, until the world is rid of plain-old telephone service (POTS) and
analog trunks, dBm referenced to 600 and 900 ohms will be alive-and-well.

Nice try, though. Thanks for playing.

I never said telecoms didn't use it. Indeed I referred to the fact that it
was precisely telecoms that first used it ! Just that *audio* generally
moved
on.from a kludgy solution to a non-problem. Maybe you didn't read the post
?

I seriously doubt that there's any even remote need for 600 ohm working in
telecoms now since the advent of microwave, optical and satellite links.

600 ohms working was only ever needed for long 'land lines' - and how many
of
them still exist ? It's all concentrated at the local exchange and
distributed digitally via optic fibre these days !

If telecoms want to stick with 600 ohms - so be it. The rest of the world
moved on though.

Btw - is my DSL connection 600 ohms ? I doubt it !

Graham


No offense intended, Graham.

None taken.

600 ohms reference is used in POTS -- and POTS is still ubiquitous.

Indeed for old-fashioned voice circuits this is true. Regardless of the *actual*
impedance, there's this time warp that certain ppl are stuck in

If your
home phone is a POTS phone then its designers have used 600 ohms in their
design calculations. Not too many people have digital wired phones, at home.
Until there is a solution to "emergency power for phones" then POTS service
will remain.

Sure. POTS is *legacy* in effect.

ISDN has gone nowhere.

Didn't ever deserve to !

Perhaps power over Ethernet (POE) may
lend some hope, but I doubt it offers a near-future solution, and VOIP
quality is worse than cellular (is that possible?).

You reckon ? I know both, and cellular is inferior IMHO to a good VOIP
connection.

*NO* - I'll correct that. I subscribe to a UK telecoms provider that routes my
international calls via IP. The quality is *vastly* SUPERIOR to a 'normal'
circuit that has echo and stuff !

Common carriers watch out !

POTS and the 600 ohm dBm
will be with us for many years-to-come.

I'm sure that few will abandon it totally.

I just doubt that my actual impedance to the local exchange is anywhere even
remotely near 600 ohms ! I'm < 1km away !

DSL is 100 ohms because the characteristic impedance of twisted pair
approaches this value at the higher frequencies (MHz).

I *know* ! It's actually difficult to make a practical twisted pair that *isn't*
around 100 ohms at HF !

This technology uses
units of dBm (100) and dBm/Hz (100).

I'll look at the diagnostics page of my router ! I'm sure you're right.

Also, don't microphones and their cables use 600 ohms reference impedance?

Not any more since about 40 yrs ago !

A typical microphone of decent quality measures around 150 ~ 200 ohms. Good
practice is to load it with 10x the source impedance. This ensures maximum
voltage transfer - essential for maximum s/n ratio and avoids overdamping the
capsule. Hence most mixing desks have 2k ohm inputs.

The cables have around 110 ohms impedance at VHF ( see earlier discuusion re
twisted pairs ) but in the audio band they simply make a connection !

Join the AES ( Audio Engineering Society ) and learn !

I could give you a typical example of how audio working dropped the 600 ohm
fiasco and bettered its performance by leaps and bounds overnight but it's a
long story !

Graham