[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

[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.

[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

[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.



Unlike oils, milliwatts is milliwatts no matter what the impedance.


** Irrelevant, asinine drivel.

The 50 impedance is a defined standard.

All manner of RF equipment uses the standard.


Maybe it does but that does not alter what 0dBm is. You can drive 1mW into
1ohm or 1megohm it is still 1 milliwatt and 0dBm, only the voltage and
current are changed to protect the innocent.

[KeithR]

Phil Allison wrote:


"keithr"

** Irrelevant, asinine drivel.

The 50 impedance is a defined standard.

All manner of RF equipment uses the standard.


Maybe it does but that does not alter what 0dBm is.


** It absolutely does alter what the term means.


**** off - you pathetic, pedantic, IMBECILE.



Ok then Rod Speed Lite carry on by yourself, you're probably pretty good at
that. I've better things to do than argue with someone so hell bent on
proving his own ignorance.

Bye

[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

[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]

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

[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

[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

[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

[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

[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 ,
"jim Gregory" wrote:

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

Keep in mind that I am not a Telephony professional, so I may not be
using all the terms correctly. In my lexicon RT means Remote Terminal.
Another term, which may now be obsolete, is pair gain system. There are
a number of different sorts of these systems. The first that I was
aware of were electromechanical relay based systems that essentially
moved the final switching stage from the Central Office out to a small
cabinet near a group of subscribers. This scheme allowed a relatively
small number of subscriber lines from the CO to serve a much larger
number of subscribers clustered in the area of the RT. The second
scheme I am familiar with uses a small number of lines from the CO as
digital T! lines and the RT includes codecs for a large number of
subscribers. The third type of system, which I believe my home
telephone currently is connected to, is similar to the second type, but
replaces the T1 lines with high capacity fiber optic lines. Just in
case anyone asks, T1 1.544 MBPS digital line.


Regards,

John Byrns

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

[Eeyore]

jim Gregory wrote:

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.

As I explained, no-one said it was like a transmission line with a dB/km loss. A
cable has to be much much longer to have transmission line behaviour at audio
frequencies. What really matters at these lengths is cable resistance and this
increases LINEARLY with distance.


How else could it work out according to your interpretation?

Why don't you do the sums yourself ? The cable resistance (and capacitance and
inductance) data is freely available.

Graham

[Eeyore]

jim Gregory wrote:

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?

News to me.

Graham

[John Byrns]

In article ,
Eeyore wrote:

jim Gregory wrote:

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.

As I explained, no-one said it was like a transmission line with a dB/km
loss. A
cable has to be much much longer to have transmission line behaviour at audio
frequencies. What really matters at these lengths is cable resistance and
this
increases LINEARLY with distance.

Actually the book I am reading on Telephony theory, which gives all the
"sums", says it is like a transmission line, which has always been
obvious to me in the case of loaded lines, but much less so in the case
of non loaded lines.

How else could it work out according to your interpretation?

Why don't you do the sums yourself ? The cable resistance (and capacitance
and
inductance) data is freely available.

If all there was to a telephone line was series resistance then you
would have a point, I think the main point you are missing is the effect
the capacitance has.


Regards,

John Byrns

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

[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

[Phil Allison]

"Andre Jute"
"Iain Cherchus"


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!



** Good practice dictates that when the only apprentice available is a MUG
like Cherchus - you make the damn tea yourself.




........ Phil

[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

[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

[Eeyore]

John Byrns wrote:

Eeyore wrote:
jim Gregory wrote:

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.

As I explained, no-one said it was like a transmission line with a dB/km
loss. A cable has to be much much longer to have transmission line behaviour at
audio
frequencies. What really matters at these lengths is cable resistance and
this increases LINEARLY with distance.

Actually the book I am reading on Telephony theory, which gives all the
"sums", says it is like a transmission line, which has always been
obvious to me in the case of loaded lines, but much less so in the case
of non loaded lines.

In which case I suggest you throw that book in the trash where it belongs.

It really said that about 5 km runs ?

Graham

[Eeyore]

John Byrns wrote:

If all there was to a telephone line was series resistance then you
would have a point, I think the main point you are missing is the effect
the capacitance has.

I'm not missing anything.

Equipment interconnects don't behave like transmission lines either.

Graham

[Eeyore]

Andre Jute wrote:

"Iain Churches" wrote:
"Eeyore" wrote

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, 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!

Been there done that.

You forgot the bit about polishing the jacks on the patch cords.

Graham

[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

[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.



Unlike oils, milliwatts is milliwatts no matter what the impedance.


** Irrelevant, asinine drivel.

The 50 impedance is a defined standard.

All manner of RF equipment uses the standard.





........ Phil

[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

[Iain Churches[_2_]]

"keithr" wrote in message
...
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?

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.

Hi Keith.

The link is now missing from this post. But did you see what I wrote?
http://www.kolumbus.fi/iain.churches...heDecibel.html



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.

Hmm. That's neatly expressed. Thanks.

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
...


Andre Jute wrote:

"Iain Churches" wrote:
"Eeyore" wrote

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, 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!

You forgot the bit about polishing the jacks on the patch cords.

If you were smart, you would use Tuchel connectors, not jacks.
They have spring mounted blades as connecting surfaces and are
self cleaning.

Now where's that tea, Graham? :-))

Cheers
Iain

[Phil Allison]

"keithr"

** Irrelevant, asinine drivel.

The 50 impedance is a defined standard.

All manner of RF equipment uses the standard.


Maybe it does but that does not alter what 0dBm is.


** It absolutely does alter what the term means.


**** off - you pathetic, pedantic, IMBECILE.



........ Phil