[Iain Churches[_2_]]

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

I have added a page to my website on the
subject mainly constructed from my dog-eared
student notes from the third quarter of the last century.
The page cannot yet be accessed
from the index page, but can be found at:

http://www.kolumbus.fi/iain.churches...heDecibel.html

Any comments or suggestions/additions would
be welcome. I am grateful for the help of my
pals Jim and Richard in the UK for their contributions.

Regards to all
Iain

[Eeyore]

Iain Churches wrote:

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

You abused it straight off.

"0dBm = 0.775V or 1mW into 600 Ohms is a traditional analogue reference level
in studios and broadcast."

It may have been 40 years ago bur it bloody well isn't any more except by ppl
who are ****wits.

600 ohms is NOT used in such equipment any more and hasn't been for many
decades.

Graham

[jim Gregory]

Unkind things to have said....

Yes, most analogue routes typically flow efficiently from low Z into high Z,
where
dBU are applicable in measurement, yet all studio/broadcast audio engineers
remember 1mW is
still the bog-standard, steady calibration/reference level for a PPM (peak
4) or -4 for a VU indicator at line-up time.
Even nowadays, 600 Ohm terminations are regularly applied to feeds via
studio audio distribution amps, or the gain control at each destination gear
may have
to be dropped a few dB if sourced from, say, 40 Ohms - to re-correspond with
unity-gain line-up.

Noise and THD figures are generally measured with a 600R term.
It is surely still revered as a standard in the voice comms industry
from where it emanated.

"Eeyore" wrote in message
...



"0dBm = 0.775V or 1mW into 600 Ohms is a traditional analogue reference
level
in studios and broadcast."

It may have been 40 years ago bur it bloody well isn't any more except by
ppl
who are ****wits.

600 ohms is NOT used in such equipment any more and hasn't been for many
decades.

Graham

[Eeyore]

jim Gregory wrote:

Even nowadays, 600 Ohm terminations are regularly applied to feeds via
studio audio distribution amps, or the gain control at each destination gear
may have

Simply not true. I've been in pro-audio some 34 years now and 600 ohm
terminations were already 'old hat' back when I entered the industry. They were
preserved for a while by some manufacturers to maintain compatability with old
kit but everyone I know rejoiced to see the back of them.

In any case, the characteristic impedance of typical twisted pair cable is ~ 100
ohms, NOT 600 ohms. '600 ohm working' serves no useful purpose today WHATEVER !

600 ohms is the characteristic impedance of long distance TELEGRAPH cables.
That's where it came from and that's where it belongs, not in the studio or
home.

Graham

[Iain Churches[_2_]]

"Eeyore" wrote in message
...


jim Gregory wrote:

Even nowadays, 600 Ohm terminations are regularly applied to feeds via
studio audio distribution amps, or the gain control at each destination
gear
may have

Simply not true. I've been in pro-audio some 34 years now and 600 ohm
terminations were already 'old hat' back when I entered the industry.
(snip)

Only 34 years, Graham? I can beat that by eight. Now pull yourself
together lad, and fetch us a cup of tea. Milk and two sugars:-)

Iain

[Andre Jute]

On Aug 16, 11:42 am, "Iain Churches" wrote:
"Eeyore" wrote in message

I've been in pro-audio some 34 years now

Only 34 years, Graham? I can beat that by eight. Now pull yourself
together lad, and fetch us a cup of tea. Milk and two sugars:-)

Iain

Iain, I'll remind you again that it is company policy, when you send
the junior apprentice to make the tea, to instruct him to wash the
mugs first!

Andre Jute
Time-served master craftsman

[Arny Krueger]

"Iain Churches" wrote in message
i.fi...

"Eeyore" wrote in message
...


jim Gregory wrote:

Even nowadays, 600 Ohm terminations are regularly applied to feeds via
studio audio distribution amps, or the gain control at each destination
gear
may have

Simply not true. I've been in pro-audio some 34 years now and 600 ohm
terminations were already 'old hat' back when I entered the industry.
(snip)

Only 34 years, Graham? I can beat that by eight. Now pull yourself
together lad, and fetch us a cup of tea. Milk and two sugars:-)

Based on Iain's technical opinions, he didn't so much have 42 years of
experience as 1 year's experience 42 times.

[Iain Churches[_2_]]

"Eeyore" wrote in message
...


Iain Churches wrote:

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

You abused it straight off.

Hello Graham.

Thanks for your friendly and informative reply. There
seems to be some truth in the rumour that you are working
hard to replace the now departed Stewart Pinkerton:-)


"0dBm = 0.775V or 1mW into 600 Ohms is a traditional analogue reference
level
in studios and broadcast."

It may have been 40 years ago bur it bloody well isn't any more except by
ppl
who are ****wits.

Please note the use of the word "traditional". I did not claim
it was a contemporary term, but nonetheless it is a common one,
and worthy of definition. The definition I used is the standard one,
and will be found in many a text book.

If you disagree, please post your definition of 0dBm.
That was my reason for starting this thread, to ask for
suggstions or improvements to the page.

Simply calling one a f*ckw*t doesn't really help much.,
does it?


Regards
Iain

[Eeyore]

Iain Churches wrote:

"Eeyore" wrote
Iain Churches wrote:

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

You abused it straight off.

Hello Graham.

Thanks for your friendly and informative reply. There
seems to be some truth in the rumour that you are working
hard to replace the now departed Stewart Pinkerton:-)

Sorry I was a bit 'sharp' but it annoys the feck out of me that the myth of the
600 ohm audio dBm is perpetuated in this way. As a result you forever get posts
in the electronics groups from ppl woried about mythical 'matching' impedances
presuambly because they heard this story that it was once the done thing.


"0dBm = 0.775V or 1mW into 600 Ohms is a traditional analogue reference
level in studios and broadcast."

It may have been 40 years ago bur it bloody well isn't any more except by
ppl who are ****wits.

Please note the use of the word "traditional". I did not claim
it was a contemporary term, but nonetheless it is a common one,
and worthy of definition. The definition I used is the standard one,
and will be found in many a text book.

If you disagree, please post your definition of 0dBm.
That was my reason for starting this thread, to ask for
suggstions or improvements to the page.

I've been giving some thought to it. Will come back on this.

There is another point btw. The dB is defined originally as a POWER ratio. The
voltage and current ratios are derived from that, not the other way round.

Graham

[Iain Churches[_2_]]

"Eeyore" wrote in message
...


Iain Churches wrote:

"Eeyore" wrote
Iain Churches wrote:

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

You abused it straight off.

Hello Graham.

Thanks for your friendly and informative reply. There
seems to be some truth in the rumour that you are working
hard to replace the now departed Stewart Pinkerton:-)

Sorry I was a bit 'sharp' (snip)

OK. Good.
Now we are back to the Graham we all know and love:-)

If you disagree, please post your definition of 0dBm.
That was my reason for starting this thread, to ask for
suggstions or improvements to the page.

I've been giving some thought to it. Will come back on this.

Thanks. much appreciated.

There is another point btw. The dB is defined originally as a POWER ratio.
The
voltage and current ratios are derived from that, not the other way round.

Yes. I thought I made that clear, when I wrote:

"The first thing to notice about the dB is that it not an absolute quantity
or unit of measurement in itself, as are the Volt, Watt, Ohm Inch, or
centimetre, but a logarithmic ratio to indicate the difference between two
levels in power or voltage and current, for which there are two separate
equations"

The worked examples start with power from which the dB ratio
is defined. Is my text not clear on that point? Could it be put in
a better way?

Cheers
Iain





Graham

[KeithR]

Iain Churches wrote:


"Eeyore" wrote in message
...


Iain Churches wrote:

"Eeyore" wrote

There is another point btw. The dB is defined originally as a POWER
ratio. The
voltage and current ratios are derived from that, not the other way
round.

Yes. I thought I made that clear, when I wrote:

"The first thing to notice about the dB is that it not an absolute
quantity or unit of measurement in itself, as are the Volt, Watt, Ohm
Inch, or
centimetre, but a logarithmic ratio to indicate the difference between
two levels in power or voltage and current, for which there are two
separate equations"

The worked examples start with power from which the dB ratio
is defined. Is my text not clear on that point? Could it be put in
a better way?

Cheers
Iain


Lets put it this way - the dB scale is a logarithmic ratio between power
levels, therefore, if you wish to compare voltage or current levels, you
need to double the values as power is proportional to voltage or current
squared and multiplying a logarithm by 2 is the same as squaring a linear
value.

When you use a dB value against an absolute value, then the result is an
absolute value (similarly x3 is a ratio, 3x3 is a value). When you compare
a power level to 1mW then you use dBm ie 3dBm = 2 milliwatts, -3dBm = 500
microwatts, no impedance comes into the equation. There may be some
specialised dB scales that rely on specific impdances, but dBm is not one
of them.

[Iain Churches[_2_]]

"Eeyore" wrote in message
...

Iain wrote:
If you disagree, please post your definition of 0dBm.
That was my reason for starting this thread, to ask for
suggstions or improvements to the page.

I've been giving some thought to it. Will come back on this.

Looking forward to your version.


The Crowhurst reference to dBu is from circa 1973. It
may have been he who suggested the term.

I have modified the page to read:

dBu

"When 600 ohm source and load impedances were no longer common in
professional audio, an alternative term, the dBu, came into wider use after
much discussion within the audio industry during the 1970s. The dBu may be
used when working into an unknown or unspecified load impedance. It is
referenced to 0.775 Volts into any impedance"

Regards
Iain

[Paul Babiak]

Eeyore wrote:

Iain Churches wrote:

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

You abused it straight off.

"0dBm = 0.775V or 1mW into 600 Ohms is a traditional analogue reference level
in studios and broadcast."

It may have been 40 years ago bur it bloody well isn't any more except by ppl
who are ****wits.

600 ohms is NOT used in such equipment any more and hasn't been for many
decades.

Graham


By definition, dBm is a power ratio, referenced to 1 mW into 600 ohms,
which is 0.775v across that same 600 ohm load.

When 600 ohm source and load impedances were no longer common in
professional audio, a new term was introduced, the dBu.

The dBu is a voltage ratio, referenced to 0.775v, and since it is a
voltage ratio, it is impedance-independant.

This handy calculator shows how they are related - if the load is set to
600 ohms, dBu=dBm. Change only the impedance, only the dBm values change.

http://www.analog.com/Analog_Root/st...dbconvert.html

And I was using 600 ohm equipment in post-production last year.

And I'm not a ****wit.

Paul

[Phil Allison]

"Paul Babiak"

By definition, dBm is a power ratio,


** Fraid "dBm " is not a *ratio* at all - but a way of expressing a
power level.

ie 20 dBm = 100 milliwatts

" dBs " are power ratios

ie a 3dB increase = double power.



The dBu is a voltage ratio,


** Fraid " dBu " is also not a ratio - but a way of expressing a signal
voltage level.

ie 20 dBu = 7.75 volts rms.


And I was using 600 ohm equipment in post-production last year.


** But not source and load matched at 600 ohms.


And I'm not a ****wit.


** Really.

Then you are a freakish exception among sound equipment operators.



....... Phil

[Chris Hornbeck]

On Fri, 17 Aug 2007 14:17:09 +1000, "Phil Allison"
wrote:

And I'm not a ****wit.


** Really.

Then you are a freakish exception among sound equipment operators.

Very cool to see ya back 'round. Where ya been?

Remember, there's no vacation from r.a.t !


Thanks, as always,

Chris Hornbeck
"It's just this little Chromium Switch.
You people are SO superstitious."

[Iain Churches[_2_]]

"Paul Babiak" wrote in message
...

By definition, dBm is a power ratio, referenced to 1 mW into 600 ohms,
which is 0.775v across that same 600 ohm load.

When 600 ohm source and load impedances were no longer common in
professional audio, a new term was introduced, the dBu.

The dBu is a voltage ratio, referenced to 0.775v, and since it is a
voltage ratio, it is impedance-independant.



Hi Paul. Two points.

You seem to have fallen into the usual trap.
Unlike the dB, from which it is derived, dBm is *not* a
ratio but a unit, with a specific reference.

dBu was certainly not a *new* term.

Regards
Iain

[Eeyore]

Iain Churches wrote:

"Paul Babiak" wrote:

By definition, dBm is a power ratio, referenced to 1 mW into 600 ohms,
which is 0.775v across that same 600 ohm load.

When 600 ohm source and load impedances were no longer common in
professional audio, a new term was introduced, the dBu.

The dBu is a voltage ratio, referenced to 0.775v, and since it is a
voltage ratio, it is impedance-independant.


Hi Paul. Two points.

You seem to have fallen into the usual trap.
Unlike the dB, from which it is derived, dBm is *not* a
ratio but a unit, with a specific reference.

dBu was certainly not a *new* term.

Eh ?

Yes it was. It dates from the 1970s. 'What to use to replace the dBm' was quite
a long winded process that the industry agonised over for some years. The dBV
was no good since it meant changing all the references and for a while the dBv
was promoted as a voltage based reference that wasn't 1 Volt but it was
considered confusing since it could easily be mis-typed as dBV which it wasn't.
Hence dBu which I understand as 'decibel unit' not 'decidel unloaded' as some
have suggested.

Graham

[Iain Churches[_2_]]

"Eeyore" wrote in message
...


Iain Churches wrote:

"Paul Babiak" wrote:

By definition, dBm is a power ratio, referenced to 1 mW into 600 ohms,
which is 0.775v across that same 600 ohm load.

When 600 ohm source and load impedances were no longer common in
professional audio, a new term was introduced, the dBu.

The dBu is a voltage ratio, referenced to 0.775v, and since it is a
voltage ratio, it is impedance-independant.


Hi Paul. Two points.

You seem to have fallen into the usual trap.
Unlike the dB, from which it is derived, dBm is *not* a
ratio but a unit, with a specific reference.

dBu was certainly not a *new* term.

Eh ?

Yes it was. It dates from the 1970s.

I am told it existed long before that. I will try to find some references
in earlier books.

'What to use to replace the dBm' was quite
a long winded process that the industry agonised over for some years. The
dBV
was no good since it meant changing all the references and for a while the
dBv
was promoted as a voltage based reference that wasn't 1 Volt but it was
considered confusing since it could easily be mis-typed as dBV which it
wasn't.
Hence dBu which I understand as 'decibel unit' not 'decidel unloaded' as
some
have suggested.

Many understand it as "decibel unspecified" (i.e.unspecified load)
This seems clearer.

Iain

[KeithR]

Paul Babiak wrote:

By definition, dBm is a power ratio, referenced to 1 mW into 600 ohms,
which is 0.775v across that same 600 ohm load.


The dBm is a power level by specifying a logarithmic ratio to 1mW. It is 1mW
no matter what the impedance is. We used dBm to specify the signal levels
received from spacecraft when I worked for NASA. Apollo signals were
about -80dBm, the deep space stuff went down to the -150s. Since the signal
was comming down a waveguide, I don't see 600 ohms being in the slightest
relevant.


And I'm not a ****wit.

Paul

Maybe, but you do have tunnel vision

Keith

[Phil Allison]

"keithr"


We used dBm to specify the signal levels
received from spacecraft when I worked for NASA. Apollo signals were
about -80dBm, the deep space stuff went down to the -150s. Since the
signal
was comming down a waveguide, I don't see 600 ohms being in the slightest
relevant.



** In the world of RF engineering, " dBm " has a different meaning.

There it refers to milliwatts in a 50 ohm load.


Context is everything.



........ Phil

[KeithR]

Phil Allison wrote:


"keithr"


We used dBm to specify the signal levels
received from spacecraft when I worked for NASA. Apollo signals were
about -80dBm, the deep space stuff went down to the -150s. Since the
signal
was comming down a waveguide, I don't see 600 ohms being in the slightest
relevant.



** In the world of RF engineering, " dBm " has a different meaning.

There it refers to milliwatts in a 50 ohm load.


Context is everything.



....... Phil

Unlike oils, milliwatts is milliwatts no matter what the impedance. You can
say that a driver will output 0dBm into a 10 ohm load and +3dBm into a 50
ohm load, just as you could say that it will output 1mW or 2mW in the same
case.

The fact that RF gear typically uses 50ohms and audio gear uses 600ohms is
irrelevant, 1mW is still the same amount of power.

Keith

[Eeyore]

Paul Babiak wrote:

By definition, dBm is a power ratio, referenced to 1 mW into 600 ohms,
which is 0.775v across that same 600 ohm load.

And in RF it's usually referenced to 50 ohms. The impedance reference should be stated.


When 600 ohm source and load impedances were no longer common in
professional audio, a new term was introduced, the dBu.

The dBu is a voltage ratio, referenced to 0.775v, and since it is a
voltage ratio, it is impedance-independant.

Yes.

Graham

[Eeyore]

Paul Babiak wrote:

The dBu is a voltage ratio

Like Phil, I need to correct you on this point.

The ***dB*** is a ratio.

The ***dBu*** is an unit of measure.

Graham

[John Byrns]

In article i,
"Iain Churches" wrote:

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

I have added a page to my website on the
subject mainly constructed from my dog-eared
student notes from the third quarter of the last century.
The page cannot yet be accessed
from the index page, but can be found at:

http://www.kolumbus.fi/iain.churches...heDecibel.html

Any comments or suggestions/additions would
be welcome. I am grateful for the help of my
pals Jim and Richard in the UK for their contributions.

A Bel or 10 dB seems like an awfully large loss for "a standard
telephone line over the distance of one mile, at 1kHz", is this really
correct? I wonder what a "standard" telephone line was back in the day?


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Eeyore]

John Byrns wrote:

A Bel or 10 dB seems like an awfully large loss for "a standard
telephone line over the distance of one mile, at 1kHz", is this really
correct?

I suggest you consider the DC resistance of a mile (and a mile back) of thin
wire.

It sounds perfectly likely to me.

Graham

[John Byrns]

In article ,
Eeyore wrote:

John Byrns wrote:

A Bel or 10 dB seems like an awfully large loss for "a standard
telephone line over the distance of one mile, at 1kHz", is this really
correct?

I suggest you consider the DC resistance of a mile (and a mile back) of thin
wire.

It sounds perfectly likely to me.

What does "thin" mean and how does it relate to the "standard telephone
line" mentioned on the web page?

If the loss of a "standard telephone line" were really 10 dB per mile as
stated on the web page, how did the telephone line serving my home,
approximately 5 miles from the CO, ever work before they upgraded to
"RTs"? That would imply a 50 dB loss between my home and the central
office, if I called the neighbor across the street, the total loss would
be 100 dB and I doubt conversation would be possible with that sort of
loss. The original statement on the web page just doesn't pass the
smell test.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Eeyore]

John Byrns wrote:

Eeyore wrote:
John Byrns wrote:

A Bel or 10 dB seems like an awfully large loss for "a standard
telephone line over the distance of one mile, at 1kHz", is this really
correct?

I suggest you consider the DC resistance of a mile (and a mile back) of thin
wire.

It sounds perfectly likely to me.

What does "thin" mean

Standard internal telephone wire here (CW1308) has 0.5mm^2 conductors that
measures 98ohms / km. Not sure what the overhead stuff is but most of the
underground stuff is similar AIUI.

So a mile there plus a mile back would be nearly 320 ohms.


and how does it relate to the "standard telephone
line" mentioned on the web page?

Feel free to google for info on US telephone wire sizes.


If the loss of a "standard telephone line" were really 10 dB per mile as
stated on the web page,

That's not what was stated.


how did the telephone line serving my home, approximately 5 miles from the CO,
ever work before they upgraded to
"RTs"?

What's an 'RT' ?


That would imply a 50 dB loss between my home and the central
office, if I called the neighbor across the street, the total loss would
be 100 dB and I doubt conversation would be possible with that sort of
loss. The original statement on the web page just doesn't pass the
smell test.

Your mistake is thinking that the dBs add.

5 miles would simply be 5 times the resistance or ~ 1500 ohms. That's NOT 50dB
attenuation. Probably about 20dB. Some loss could be ameliorated using 'loading
coils' AIUI.

Graham

[jim Gregory]

If 5 miles of wiring @ hypothetical loss of 10dB/mile are a continuous path,
in
cascade, that's got to be 50dB loss in my book. How else could it work out
according to your interpretation? Unit intrinsic loss per mile x distance =
Total loss.
And double that if connected to anothersub the same distance from Exchange!
But, mercifully, the voice-band end-to-end loss is probably 4-8dB in
reality.

Yes, pray, what's an RT? In UK we've never heard of that abbrev.
Do I dare guess you imply a repeater transformer which restores the source Z
at each insertion?
Telephony, essentially duplex simultaneous comms, is all about power /Z
matching, in both directions.
Jim

"Eeyore" wrote in message
...


John Byrns wrote:

Eeyore wrote:
John Byrns wrote:

A Bel or 10 dB seems like an awfully large loss for "a standard
telephone line over the distance of one mile, at 1kHz", is this
really
correct?

I suggest you consider the DC resistance of a mile (and a mile back) of
thin
wire.

It sounds perfectly likely to me.

What does "thin" mean

Standard internal telephone wire here (CW1308) has 0.5mm^2 conductors that
measures 98ohms / km. Not sure what the overhead stuff is but most of the
underground stuff is similar AIUI.

So a mile there plus a mile back would be nearly 320 ohms.


and how does it relate to the "standard telephone
line" mentioned on the web page?

Feel free to google for info on US telephone wire sizes.


If the loss of a "standard telephone line" were really 10 dB per mile as
stated on the web page,

That's not what was stated.


how did the telephone line serving my home, approximately 5 miles from
the CO,
ever work before they upgraded to
"RTs"?

What's an 'RT' ?


That would imply a 50 dB loss between my home and the central
office, if I called the neighbor across the street, the total loss would
be 100 dB and I doubt conversation would be possible with that sort of
loss. The original statement on the web page just doesn't pass the
smell test.

Your mistake is thinking that the dBs add.

5 miles would simply be 5 times the resistance or ~ 1500 ohms. That's NOT
50dB
attenuation. Probably about 20dB. Some loss could be ameliorated using
'loading
coils' AIUI.

Graham

[John Byrns]

In article ,
Eeyore wrote:

John Byrns wrote:

That would imply a 50 dB loss between my home and the central
office, if I called the neighbor across the street, the total loss would
be 100 dB and I doubt conversation would be possible with that sort of
loss. The original statement on the web page just doesn't pass the
smell test.

Your mistake is thinking that the dBs add.

Don't they, and if not why not?

5 miles would simply be 5 times the resistance or ~ 1500 ohms. That's NOT
50dB
attenuation. Probably about 20dB.

Telephone cable is not equivalent to a simple resistor as you would have
us believe, there is also a large amount of shunt capacitance, whose
effect you seem determined to ignore.

Note that I am not claiming that the local loop serving my telephone,
actually the one that did in an earlier day, has anywhere near 50 dB
attenuation, or even 20 dB, I was simply illustrating what the
nonsensical result would be applying the information on Iain's web page.

Some loss could be ameliorated using 'loading coils' AIUI.

Oops I guess I misspoke, you seem to recognize the shunt capacitance
after all, as loading coils are only of use because of the capacitance.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Iain Churches[_2_]]

"John Byrns" wrote in message
...
In article ,
Eeyore wrote:

John Byrns wrote:

A Bel or 10 dB seems like an awfully large loss for "a standard
telephone line over the distance of one mile, at 1kHz", is this really
correct?

I suggest you consider the DC resistance of a mile (and a mile back) of
thin
wire.

It sounds perfectly likely to me.

What does "thin" mean and how does it relate to the "standard telephone
line" mentioned on the web page?

If the loss of a "standard telephone line" were really 10 dB per mile as
stated on the web page, how did the telephone line serving my home,
approximately 5 miles from the CO, ever work before they upgraded to
"RTs"? That would imply a 50 dB loss between my home and the central
office, if I called the neighbor across the street, the total loss would
be 100 dB and I doubt conversation would be possible with that sort of
loss. The original statement on the web page just doesn't pass the
smell test.

John. I took this definition from notes written many years ago.
But I have no reason to doubt what I was taught. See:

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

Iain

[Arny Krueger]

"Eeyore" wrote in message
...


John Byrns wrote:

A Bel or 10 dB seems like an awfully large loss for "a standard
telephone line over the distance of one mile, at 1kHz", is this really
correct?

I suggest you consider the DC resistance of a mile (and a mile back) of
thin
wire.

Telephone wire in the US is usually 24 or 26 gauge. Taking the worst case:

26 gauge - 40 ohms/1000 feet = 211 ohms/mile, presuming the return path is
a low resistance ground.

Presuming the load is 600 ohms, loss is 0.739 or 2.6 dB.

[Eeyore]

Arny Krueger wrote:

"Eeyore" wrote:
John Byrns wrote:

A Bel or 10 dB seems like an awfully large loss for "a standard
telephone line over the distance of one mile, at 1kHz", is this really
correct?

I suggest you consider the DC resistance of a mile (and a mile back) of
thin wire.

Telephone wire in the US is usually 24 or 26 gauge. Taking the worst case:

26 gauge - 40 ohms/1000 feet = 211 ohms/mile, presuming the return path is
a low resistance ground.

The reurn is a length of the same wire.


Presuming the load is 600 ohms, loss is 0.739 or 2.6 dB.

The load is rarely 600 ohms AIUI.

Graham

[Arny Krueger]

"Eeyore" wrote in message
...


Arny Krueger wrote:

"Eeyore" wrote:
John Byrns wrote:

A Bel or 10 dB seems like an awfully large loss for "a standard
telephone line over the distance of one mile, at 1kHz", is this really
correct?

I suggest you consider the DC resistance of a mile (and a mile back) of
thin wire.

Telephone wire in the US is usually 24 or 26 gauge. Taking the worst
case:

26 gauge - 40 ohms/1000 feet = 211 ohms/mile, presuming the return path
is
a low resistance ground.

The reurn is a length of the same wire.

Yes, the line is usually balanced. So about double the loss to about 5
dB/mile.

Presuming the load is 600 ohms, loss is 0.739 or 2.6 dB.

The load is rarely 600 ohms AIUI.

Do you think it is higher or lower than 600 ohms?

[The Phantom]

On Wed, 15 Aug 2007 08:59:48 +0300, "Iain Churches"
wrote:

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

I have added a page to my website on the
subject mainly constructed from my dog-eared
student notes from the third quarter of the last century.
The page cannot yet be accessed
from the index page, but can be found at:

http://www.kolumbus.fi/iain.churches...heDecibel.html

Any comments or suggestions/additions would
be welcome. I am grateful for the help of my
pals Jim and Richard in the UK for their contributions.

Regards to all
Iain

The second paragraph is a little off. You say:

"The basic unit chosen was the power loss in a standard telephone line over
the distance of one mile, at 1kHz. This was called the Transmission Unit,
but was renamed, after the inventor of the telephone, and became known as
the bel..."

Let me quote from page 9 of the book, "Transmission Circuits for
Telephonic Communication", published by Bell Labs in 1925.

"Three different units have been used to express telephonic volume
efficiencies:

(1) The natural attenuation unit, also designated as a napier or as a hyp;

(2) The mile of standard cable or simply a mile;

(3) The so-called transmission unit or TU."

Regarding a mile of standard cable, they say:

"Standard cable is defined as a cable having uniformly distributed
resistances of 88 ohms per loop mile and uniformly distributed shunt
capacitance of .054 microfarad per mile. Its series inductance and shunt
leakance are assumed to be zero.*

* In England it has been customary to assume an inductance of .001 henry
per loop mile--which assumption reduces the attenuation constant of the
cable by approximately 3 per cent."

A cable with a resistance of 88 ohms per loop mile is made of standard 19
gauge wire. Later in the book they talk of long distance line composed of
10 or even 8 gauge wires spaced a foot apart.

"The mile of standard cable is based upon the constant percentage
decrease in current (or in power) per mile which takes place when an e.m.f.
of a frequency, such as f cycles (f = w/2pi = 5000/2pi), is impressed at
one end of an infinite length of such a cable."

The measurement frequency here is 796.4 Hz (w = 5000).

They also say:

"Due to the objection that the attenuation constant of standard cable
varies with frequency and also to the fact the use of a frequency of 796.4
Hz is not universally agreed upon, there has recently been adopted a new
transmission unit which (1) does not vary with frequency, (2) is of a
convenient size or magnitude--avoiding one of the chief objections to the
napier or natural attenuation unit--and (3) has a simple physical
significance so that its eventual universal adoption would appear probable.

This new transmission unit (referred to for the sake of convenience as a
TU) is defined by the relation

Ntu = 10*LOG10(W1/W2)

in which Ntu is the number of TU by which any two powers W1 and W2 are said
to differ."

They apparently had not yet decided to call it a decibel, and of course,
one Bel = 10 TU. This text was written in 1924 and published in 1925.

There is a table of conversions:

Relations between various types of units

Multiply by to obtain

miles .947 TU
miles .109 napiers
napiers 9.175 miles
napiers 8.686 TU
TU 1.056 miles
TU .115 napiers
---------------------------------------------------

So, we see that the attenuation of a mile of standard cable was not
called a transmission unit; it was just called a "mile". The transmission
unit (TU) was what would later be called a decibel; it was not a Bel, it
was a tenth of a Bel.

The Wikipedia article says, "Devised by engineers of the Bell Telephone
Laboratory to quantify the reduction in audio level over a 1 mile
(approximately 1.6 km) length of standard telephone cable, the bel was
originally called the transmission unit or TU." This is wrong, as the
quotations I've given show.

The TU was indeed devised to quantify the reduction in audio level, but
it wasn't intended to represent the attenuation in one mile of cable, as
you can see from the conversion table. The TU was defined as
10*LOG10(W1/W2), which wasn't the attenuation of a mile of cable; it was
close (1 mile = .947 TU), and that is why a TU was defined as
10*LOG10(W1/W2) and not just LOG10(W1/W2). They wanted a conveniently
sized unit.

From the conversion table we see that a mile was an attenuation of .947
decibels, to answer John Byrns' question.

[Phil Allison]

"The Phantom"

( snip lots of good stuff )


"Standard cable is defined as a cable having uniformly distributed
resistances of 88 ohms per loop mile and uniformly distributed shunt
capacitance of .054 microfarad per mile.


** That is not too dissimilar to modern twisted pair cables, like Cat 5.

The resistance is about half, due to thicker gauge capper wires and
similarly the capacitance per meter about half.

No doubt due to use of much thicker insulation than the modern, very thin,
PVC kind.

An overall, average voice band, loss of only 1dB mile represents darn good
engineering - for 1925 !!




........ Phil

[Iain Churches[_2_]]

"The Phantom" wrote in message
...

The second paragraph is a little off. You say:

"The basic unit chosen was the power loss in a standard telephone line
over
the distance of one mile, at 1kHz. This was called the Transmission Unit,
but was renamed, after the inventor of the telephone, and became known as
the bel..."

Let me quote from page 9 of the book, "Transmission Circuits for
Telephonic Communication", published by Bell Labs in 1925.

(huge snip)

Excellent info. many thanks indeed.


From the conversion table we see that a mile was an attenuation of .947
decibels, to answer John Byrns' question.

John seems to have assumed that the loss over 1 mile was one unit (i.e.
one Bel)

Regards
Iain

[John Byrns]

In article i,
"Iain Churches" wrote:

"The Phantom" wrote in message
...

From the conversion table we see that a mile was an attenuation of .947
decibels, to answer John Byrns' question.

John seems to have assumed that the loss over 1 mile was one unit (i.e.
one Bel)

Yes, that is exactly what I assumed after reading you web page, which
said; "In the early days of telephone engineering, a unit was needed to
measure the loss of signal over distance. The basic unit chosen was the
power loss in a standard telephone line over the distance of one mile,
at 1kHz. This was called the Transmission Unit, but was renamed, after
the inventor of the telephone, and became known as the bel circa 1924."

It is hard for me to see how anyone could come to a different conclusion
as a result of reading your web page? This quote from your web page
also implies that a "Transmission Unit" = a "bel", is that correct?

Also, the bel and decibel are simple logarithmic relations and it is
hard to believe that they would exactly match up as units with the loss
characteristics of a particular pre existing "standard line. I would
guess that the decibel was simply close to the loss of the "standard"
line, as illustrated by the 0.947 dB figure quoted above.


Regards,

John Byrns

--
Surf my web pages at, http://fmamradios.com/

[Iain Churches[_2_]]

"John Byrns" wrote in message
...
In article i,
"Iain Churches" wrote:

"The Phantom" wrote in message
...

From the conversion table we see that a mile was an attenuation of
.947
decibels, to answer John Byrns' question.

John seems to have assumed that the loss over 1 mile was one unit (i.e.
one Bel)

Yes, that is exactly what I assumed after reading you web page, which
said; "In the early days of telephone engineering, a unit was needed to
measure the loss of signal over distance. The basic unit chosen was the
power loss in a standard telephone line over the distance of one mile,
at 1kHz. This was called the Transmission Unit, but was renamed, after
the inventor of the telephone, and became known as the bel circa 1924."

Hi John.

I posted the link to this provisional page inviting comments.
In the case of their being errors, I thought that someone as
smart as yourself would be delighted to point them out :-)

It is hard for me to see how anyone could come to a different conclusion
as a result of reading your web page? This quote from your web page
also implies that a "Transmission Unit" = a "bel", is that correct?

Also, the bel and decibel are simple logarithmic relations and it is
hard to believe that they would exactly match up as units with the loss
characteristics of a particular pre existing "standard line. I would
guess that the decibel was simply close to the loss of the "standard"
line, as illustrated by the 0.947 dB figure quoted above.

I took what I wrote from some very old student notes. A search
under "dB" came up with Wiki, and the same information, which
I therefore took as being correct. It was not clear that the TU
was roughly equivalent to the dB not the bel.

I am grateful to The Phantom for taking the trouble to type the
large extract from the Bell Telephone publication.

Regards
Iain

[jim Gregory]

Does this mean, traditionally, telephone cable tx loss should be measured
with
a stimulus of centre freq @ 800Hz (near enough to 5000/2pi) in preference to
using 1kHz?
I seem to remember that in '80s-'90s, BT's Private Wires testers often
measured atten of lines at 800Hz.
Jim

"The Phantom" wrote in message
...
On Wed, 15 Aug 2007 08:59:48 +0300, "Iain Churches"
wrote:

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

I have added a page to my website on the
subject mainly constructed from my dog-eared
student notes from the third quarter of the last century.
The page cannot yet be accessed
from the index page, but can be found at:

http://www.kolumbus.fi/iain.churches...heDecibel.html

Any comments or suggestions/additions would
be welcome. I am grateful for the help of my
pals Jim and Richard in the UK for their contributions.

Regards to all
Iain

The second paragraph is a little off. You say:

"The basic unit chosen was the power loss in a standard telephone line
over
the distance of one mile, at 1kHz. This was called the Transmission Unit,
but was renamed, after the inventor of the telephone, and became known as
the bel..."

Let me quote from page 9 of the book, "Transmission Circuits for
Telephonic Communication", published by Bell Labs in 1925.

"Three different units have been used to express telephonic volume
efficiencies:

(1) The natural attenuation unit, also designated as a napier or as a hyp;

(2) The mile of standard cable or simply a mile;

(3) The so-called transmission unit or TU."

Regarding a mile of standard cable, they say:

"Standard cable is defined as a cable having uniformly distributed
resistances of 88 ohms per loop mile and uniformly distributed shunt
capacitance of .054 microfarad per mile. Its series inductance and shunt
leakance are assumed to be zero.*

* In England it has been customary to assume an inductance of .001 henry
per loop mile--which assumption reduces the attenuation constant of the
cable by approximately 3 per cent."

A cable with a resistance of 88 ohms per loop mile is made of standard 19
gauge wire. Later in the book they talk of long distance line composed of
10 or even 8 gauge wires spaced a foot apart.

"The mile of standard cable is based upon the constant percentage
decrease in current (or in power) per mile which takes place when an
e.m.f.
of a frequency, such as f cycles (f = w/2pi = 5000/2pi), is impressed at
one end of an infinite length of such a cable."

The measurement frequency here is 796.4 Hz (w = 5000).

They also say:

"Due to the objection that the attenuation constant of standard cable
varies with frequency and also to the fact the use of a frequency of 796.4
Hz is not universally agreed upon, there has recently been adopted a new
transmission unit which (1) does not vary with frequency, (2) is of a
convenient size or magnitude--avoiding one of the chief objections to the
napier or natural attenuation unit--and (3) has a simple physical
significance so that its eventual universal adoption would appear
probable.

This new transmission unit (referred to for the sake of convenience as a
TU) is defined by the relation

Ntu = 10*LOG10(W1/W2)

in which Ntu is the number of TU by which any two powers W1 and W2 are
said
to differ."

They apparently had not yet decided to call it a decibel, and of course,
one Bel = 10 TU. This text was written in 1924 and published in 1925.

There is a table of conversions:

Relations between various types of units

Multiply by to obtain

miles .947 TU
miles .109 napiers
napiers 9.175 miles
napiers 8.686 TU
TU 1.056 miles
TU .115 napiers
---------------------------------------------------

So, we see that the attenuation of a mile of standard cable was not
called a transmission unit; it was just called a "mile". The transmission
unit (TU) was what would later be called a decibel; it was not a Bel, it
was a tenth of a Bel.

The Wikipedia article says, "Devised by engineers of the Bell Telephone
Laboratory to quantify the reduction in audio level over a 1 mile
(approximately 1.6 km) length of standard telephone cable, the bel was
originally called the transmission unit or TU." This is wrong, as the
quotations I've given show.

The TU was indeed devised to quantify the reduction in audio level, but
it wasn't intended to represent the attenuation in one mile of cable, as
you can see from the conversion table. The TU was defined as
10*LOG10(W1/W2), which wasn't the attenuation of a mile of cable; it was
close (1 mile = .947 TU), and that is why a TU was defined as
10*LOG10(W1/W2) and not just LOG10(W1/W2). They wanted a conveniently
sized unit.

From the conversion table we see that a mile was an attenuation of .947
decibels, to answer John Byrns' question.

[bill ramsay]

Yes it does.

I used to be a transmission technical officer with British Telecom.
This is going back about 30 yeras. In this role I used to set up
private wires, or lines.

the spot frequencies we used for equalisation were

300, 500, 800, 1.2k, 1.6k, 2.0k, 2.2,.2.4, 2.6, 2.8,3.0,3.2

I am a bit vague on the exact values but they look right, the 1.2k
might be dodgy.

We used equalisation boxes with RLC ccts in them. Nearly always we
managed to equalise using the standard C with a variation of R for the
local conditions.

Bill




On Fri, 17 Aug 2007 16:30:46 GMT, "jim Gregory"
wrote:

Does this mean, traditionally, telephone cable tx loss should be measured
with
a stimulus of centre freq @ 800Hz (near enough to 5000/2pi) in preference to
using 1kHz?
I seem to remember that in '80s-'90s, BT's Private Wires testers often
measured atten of lines at 800Hz.
Jim

"The Phantom" wrote in message
.. .
On Wed, 15 Aug 2007 08:59:48 +0300, "Iain Churches"
wrote:

From e-mail I receive, it seems to me that
the decibel is a misunderstood and widely
abused term.

I have added a page to my website on the
subject mainly constructed from my dog-eared
student notes from the third quarter of the last century.
The page cannot yet be accessed
from the index page, but can be found at:

http://www.kolumbus.fi/iain.churches...heDecibel.html

Any comments or suggestions/additions would
be welcome. I am grateful for the help of my
pals Jim and Richard in the UK for their contributions.

Regards to all
Iain

The second paragraph is a little off. You say:

"The basic unit chosen was the power loss in a standard telephone line
over
the distance of one mile, at 1kHz. This was called the Transmission Unit,
but was renamed, after the inventor of the telephone, and became known as
the bel..."

Let me quote from page 9 of the book, "Transmission Circuits for
Telephonic Communication", published by Bell Labs in 1925.

"Three different units have been used to express telephonic volume
efficiencies:

(1) The natural attenuation unit, also designated as a napier or as a hyp;

(2) The mile of standard cable or simply a mile;

(3) The so-called transmission unit or TU."

Regarding a mile of standard cable, they say:

"Standard cable is defined as a cable having uniformly distributed
resistances of 88 ohms per loop mile and uniformly distributed shunt
capacitance of .054 microfarad per mile. Its series inductance and shunt
leakance are assumed to be zero.*

* In England it has been customary to assume an inductance of .001 henry
per loop mile--which assumption reduces the attenuation constant of the
cable by approximately 3 per cent."

A cable with a resistance of 88 ohms per loop mile is made of standard 19
gauge wire. Later in the book they talk of long distance line composed of
10 or even 8 gauge wires spaced a foot apart.

"The mile of standard cable is based upon the constant percentage
decrease in current (or in power) per mile which takes place when an
e.m.f.
of a frequency, such as f cycles (f = w/2pi = 5000/2pi), is impressed at
one end of an infinite length of such a cable."

The measurement frequency here is 796.4 Hz (w = 5000).

They also say:

"Due to the objection that the attenuation constant of standard cable
varies with frequency and also to the fact the use of a frequency of 796.4
Hz is not universally agreed upon, there has recently been adopted a new
transmission unit which (1) does not vary with frequency, (2) is of a
convenient size or magnitude--avoiding one of the chief objections to the
napier or natural attenuation unit--and (3) has a simple physical
significance so that its eventual universal adoption would appear
probable.

This new transmission unit (referred to for the sake of convenience as a
TU) is defined by the relation

Ntu = 10*LOG10(W1/W2)

in which Ntu is the number of TU by which any two powers W1 and W2 are
said
to differ."

They apparently had not yet decided to call it a decibel, and of course,
one Bel = 10 TU. This text was written in 1924 and published in 1925.

There is a table of conversions:

Relations between various types of units

Multiply by to obtain

miles .947 TU
miles .109 napiers
napiers 9.175 miles
napiers 8.686 TU
TU 1.056 miles
TU .115 napiers
---------------------------------------------------

So, we see that the attenuation of a mile of standard cable was not
called a transmission unit; it was just called a "mile". The transmission
unit (TU) was what would later be called a decibel; it was not a Bel, it
was a tenth of a Bel.

The Wikipedia article says, "Devised by engineers of the Bell Telephone
Laboratory to quantify the reduction in audio level over a 1 mile
(approximately 1.6 km) length of standard telephone cable, the bel was
originally called the transmission unit or TU." This is wrong, as the
quotations I've given show.

The TU was indeed devised to quantify the reduction in audio level, but
it wasn't intended to represent the attenuation in one mile of cable, as
you can see from the conversion table. The TU was defined as
10*LOG10(W1/W2), which wasn't the attenuation of a mile of cable; it was
close (1 mile = .947 TU), and that is why a TU was defined as
10*LOG10(W1/W2) and not just LOG10(W1/W2). They wanted a conveniently
sized unit.

From the conversion table we see that a mile was an attenuation of .947
decibels, to answer John Byrns' question.