what's the difference?

i'm looking at specs for the new Emu 1820m.

below are the specs. the common mode rejection always seems to be
many db less than the crosstalk db. i don't think i understand
"common mode rejection".

to my slightly-trained eyes, the overall specs look nice. would an
experienced person agree?


Sample Rates: 44.1, 48, 96, and 192 kHz, from internal crystal or
externally supplied clock (no sample rate conversion).

Bit Depth: 16 or 24 bits

PCI Bus-Mastering DMA subsystem reduces CPU usage

Zero-latency direct hardware monitoring w/effects

Analogue line inputs-6

Type: servo-balanced, DC-coupled, low-noise input circuitry

Level (software selectable):

* Professional: +4dBu nominal, 20dBu maximum (balanced)
* Consumer: -10dBV nominal, 6dBV maximum (unbalanced)

Frequency Response: +/- .05dB, 20Hz – 20 kHz

THD+N (1 kHz at -1dBFS): -110dB (.0003%)

SNR (A-weighted): 120dB

Dynamic Range (1 kHz, A-weighted): 120dB

Stereo Crosstalk (1kHz at -1dBFS): -115dB

Common-Mode Rejection (60Hz): 40dB

Analogue line outputs-8

Type: balanced, low-noise, 3-pole low-pass differential filter

Level (software selectable):

* Professional: +4dBu nominal, 20dBu maximum (balanced)
* Consumer: -10dBV nominal, 6dBV maximum (unbalanced)

Frequency Response: 0.0/-.35dB, 20Hz – 20 kHz

THD+N (1 kHz at -1dBFS): -105dB (.0006%)

SNR (A-weighted): 120dB

Dynamic Range (1 kHz, A-weighted): 120dB

Stereo Crosstalk (1 kHz at -1dBFS) -115dB

Mic pre and line inputs-2

Type: TFPro™ combination microphone preamp and line input

Frequency Response: +0.8/-0.1dB, 20Hz – 20 kHz

Stereo Crosstalk (1 kHz min gain, -1dBFS): -120dB

Line Input:

* Gain Range: -12 to +28dB
* Max Level: +17dBV (19.2dBu)
* THD+N (1 kHz at -1dBFS, min gain): -94dB (.002%)
* Dynamic Range (A-weighted, 1kHz min gain): 100dB
* SNR (A-weighted, min gain): 100dB

Microphone Preamplifier

* Gain Range: +10 to +50dB
* Max Level: -12dBV (-9.8dBu)
* THD+N (1 kHz at -1dBFS, min gain): -95dB (.0018%)
* SNR (A-weighted, min gain): 100dB
* Input Impedance: 330 ohms
* Common-mode Rejection Ratio (60Hz): 80dB

Phone input stereo

Type: RIAA equalized phono input

Frequency Response: +/-0.5dB, 50Hz – 20 kHz

THD+N (1 kHz, 10mV RMS unbalanced input): -76dB (.015%)

SNR (10mV RMS unbalanced input, A-weighted): 90dB

Stereo Crosstalk (1kHz at -1dBFS) : -80dB

Maximum level:

* Professional: 80mV RMS
* Consumer: 20mV RMS

Input capacitance: 220 pF

Input impedance: 47K ohm

Digital I/O

S/PDIF:

* 2 in/2 out coaxial (transformer coupled)
* 2 in/3 out optical (software switched with ADAT)
* AES/EBU or S/PDIF format, switchable under software control

ADAT:

* 8 channels, 24-bit @ 44.1/48kHz
* 4 channels, 24-bit @ 96kHz (S-MUX compatible
* 2 channels, 24-bit @ 192kHz

Firewi

* 400 Mbps 1394a port (6-pin)
* Compatible with DV cameras, storage peripherals, etc

Midi:

2 in, 2 out

xy wrote:
what's the difference?

i'm looking at specs for the new Emu 1820m.

below are the specs. the common mode rejection always seems to be
many db less than the crosstalk db. i don't think i understand
"common mode rejection".

CMRR is a measure of how well balanced a balanced connection is.
It is a measure of how good the rejection of common mode noise is.

to my slightly-trained eyes, the overall specs look nice. would an
experienced person agree?

Anybody can get these sorts of numbers today. How does it sound?
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 2 Jul 2004 13:02:02 -0400, in rec.audio.pro you wrote:

xy wrote:
what's the difference?

i'm looking at specs for the new Emu 1820m.

below are the specs. the common mode rejection always seems to be
many db less than the crosstalk db. i don't think i understand
"common mode rejection".

CMRR is a measure of how well balanced a balanced connection is.
It is a measure of how good the rejection of common mode noise is.

to my slightly-trained eyes, the overall specs look nice. would an
experienced person agree?

Anybody can get these sorts of numbers today. How does it sound?
--scott
yep, and I notice that they only seemed to spec CMRR at line
frequency, Really they should give some idea of what its like at 20K.
Normaly its easy to get high CMRR at line freq, but it tends to rise
at about 6dB/octave

Common-Mode Rejection (60Hz): 40dB
Generally means that they have cheated the cmrr specification. ie
anyone can get 40dB, but in production it could actually be say 60dB.

But they give such a loose spec, that you never can complain. One
device could be 41dB, and another 75dB, and still be in spec!



martin

Serious error.
All shortcuts have disappeared.
Screen. Mind. Both are blank.

In article writes:

what's the difference?

They're two different things.

below are the specs. the common mode rejection always seems to be
many db less than the crosstalk db. i don't think i understand
"common mode rejection".

Common mode rejection is the ability to reject noise that's common to
the two wires of a balanced input. An example is RF interference
that's picked up by both wires equally. Since a balanced input works
on the voltage difference between the two wires, if the same stray
signal is picked up equally by both its difference will be zero and it
will be rejected.

Crosstalk is how much signal from one channel leaks into another
channel. Having all balanced wiring between stages is one way to use
common mode rejection to reduce crosstalk but hardly anyone builds
gear like that any more. Today crosstalk is usually a specification of
a chip designed to handle a two-channel signal, or it's a function of
the circuit board layout (particularly the grounding scheme) of a
piece of hardware.

to my slightly-trained eyes, the overall specs look nice. would an
experienced person agree?

All specs look good these days. That's why the only important ones are
how big it is, how much it weighs, and whether it has the right number
and kind of inputs and outputs.

"How it sounds" isn't a specification, but it's a fact of life.

--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

xy at wrote on 7/2/04 12:47 PM:

Analogue line inputs-6

....
Stereo Crosstalk (1kHz at -1dBFS): -115dB
THis means that a 1kHz tone blasting at essentially full level on ONE
channel (with NOTHING suppsedly on the other channel) will leak across to
the other channel at -80dB
This spec is limited as defined (only at 1kHz... you want to see this rated
at full bandwidth 20-20kHz or at least in the upper end where it's really
noticed.


Common-Mode Rejection (60Hz): 40dB
this tells how well-matched each leg of a ballanced line is.
the LARGER the figure-- the GREATER the common noise rejection (induced hum
on a mic cable or such, hence the rating at 60Hz)
Theoretically ANY signal coming down EACH of the signal legs (pin 2 and 3)
with the EXACT SAME LEVEL and polarity will completely cancel out at the
signal input. Transformers do this inherently. electronically balanced
inputs are less precise, so Real Inputs on Real Gear include a trimmer on
one or both legs so this can be precision matched... haven;t seen THAT on
anything in the affordable vein recently.

--
John I-22
(that's 'I' for Initial...)
Recognising what's NOT worth your time, THAT'S the key.
--

xy wrote:

what's the difference?

i'm looking at specs for the new Emu 1820m.

below are the specs. the common mode rejection always seems to be
many db less than the crosstalk db. i don't think i understand
"common mode rejection".

Common mode rejection is the ability to purposefully combine signals
from two adjacent wires (on a balanced input) to avoid picking up the
noise that they both experience equally along the way.

Crosstalk is when you *accidentally* combine the signals from two
nearby wires (through some kind of coupling) and so each wire's signal
becomes noise on the other wire. Channel separation is the extent
to which you manage to avoid crosstalk.

So basically crosstalk is "hey, you got your peanut butter in my
chocolate" and common mode rejection is "hey, your peanut butter
and my peanut butter were opposite polarities, and they both
got equal amounts of chocolate in them, and now by subtracting
your peanut butter from mine, we have eliminated the unwanted
chocolate."

- Logan

"Mike Rivers".

Common mode rejection is the ability to reject noise that's common to
the two wires of a balanced input. An example is RF interference
that's picked up by both wires equally.


** That not a correct example. RF interference is defeated firstly by
the use of shielded cable and secondly by the use of filters to reduce such
signals at the inputs of the balanced pre-amp.

Common mode rejection operates across the audio band and maybe a little
beyond but is usually most effective at the lower frequencies since the main
aim is to eliminate ground hum from audio systems. A ground hum voltage will
appear equally on the two wires and so be rejected.


Since a balanced input works
on the voltage difference between the two wires, if the same stray
signal is picked up equally by both its difference will be zero and it
will be rejected.


** A voltage injected into a balanced audio line by external magnetic
fields ( like nearby high AC current cables and transformers) creates a hum
signal in differential mode that the pre-amp *will* amplify - its CMRR has
no effect.

This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal picked
up every inch or so along the line and hence cancels it out. Where multiple
twisted pairs are used in the same cable the twisting reduces crosstalk in
the same way as above.

"Star Quad" cable uses four twisted wires instead of two to enhance the
effect of the twisting and virtually eliminates induced hum problems even
when used near to high current AC cabling.




............ Phil

On Sat, 3 Jul 2004 15:07:57 +1000, "Phil Allison"
wrote:

This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal picked
up every inch or so along the line and hence cancels it out. Where multiple
twisted pairs are used in the same cable the twisting reduces crosstalk in
the same way as above.

Don't try this at home kids.

Chris Hornbeck

On Sat, 3 Jul 2004 15:07:57 +1000, "Phil Allison"
wrote:

** A voltage injected into a balanced audio line by external magnetic
fields ( like nearby high AC current cables and transformers) creates a hum
signal in differential mode that the pre-amp *will* amplify - its CMRR has
no effect.

And, by the way, before you go "correcting" someone like Mike
Rivers, who's a real engineer as evidenced by the depth and
clarity of his thinking, you might get the most basic stuff
straight.

For a beginning to a clue, think about the wavelength of a
hum field.

A man apologizes when he's wrong. Deal.

Chris Hornbeck

"Chris Hornbeck"...
"Phil Allison"


Don't try this at home kids.

Chris Hornbeck


** Imbecile.



............ Phil

"Chris Hornbeck"
"Phil Allison"


And, by the way, before you go "correcting" someone like Mike
Rivers, who's a real engineer as evidenced by the depth and
clarity of his thinking, you might get the most basic stuff
straight.


** Mr Rivers is no "real engineer".



For a beginning to a clue, think about the wavelength of a
hum field.


** Nor are you.


A man apologizes when he's wrong. Deal.

Chris Hornbeck


** Start now if you like.



........... Phil

Phil Allison wrote:

"Mike Rivers".

Common mode rejection is the ability to reject noise that's common to
the two wires of a balanced input. An example is RF interference
that's picked up by both wires equally.

** That not a correct example. RF interference is defeated firstly by
the use of shielded cable and secondly by the use of filters to reduce such
signals at the inputs of the balanced pre-amp.

Common mode rejection operates across the audio band and maybe a little
beyond but is usually most effective at the lower frequencies since the main
aim is to eliminate ground hum from audio systems. A ground hum voltage will
appear equally on the two wires and so be rejected.

So why is it you think that RF can't exist at audible frequencies?
Even if it's caused by A/C current and at 50 or 60 Hz, it's still
radio noise picked up by the wires because they're acting as antennas,
right?

- Logan

thanks for all the replies everyone. i remembered later that "common
mode" often has to do with hum on an ac line, but these descriptions
were very clarifying.

i haven't heard the box yet. i'm doing lots of organizing in july.
and then starting in august, i'm going into listen/buy mode.

there are two things that "concern" me about this new emu box. one:
the clock..how good could it be at this price point? that can be
solved since it has word clock input.

but the second concern: the power on the breakout box comes from the
ethernet cable (there is no ac mains cord on the breakout box). how
much juice can an ethernet cable deliver, and isn't it a bad idea to
have power and digital signal going down the same line?

i guess a third concern would be the quality of the analog circuits
surrounding the converters.

but i think it's cool that emu is using the top of the line converters
that the expensive digi boxes are using. and the layout of the box is
very appealing.

thanks for all the replies everyone. i remembered later that "common
mode" often has to do with hum on an ac line, but these descriptions
were very clarifying.

i haven't heard the box yet. i'm doing lots of organizing in july.
and then starting in august, i'm going into listen/buy mode.

there are two things that "concern" me about this new emu box. one:
the clock..how good could it be at this price point? that can be
solved since it has word clock input.

but the second concern: the power on the breakout box comes from the
ethernet cable (there is no ac mains cord on the breakout box). how
much juice can an ethernet cable deliver, and isn't it a bad idea to
have power and digital signal going down the same line?

i guess a third concern would be the quality of the analog circuits
surrounding the converters.

but i think it's cool that emu is using the top of the line converters
that the expensive digi boxes are using. and the layout of the box is
very appealing.

"Logan Shaw"
"Mike Rivers".

Common mode rejection is the ability to reject noise that's common to
the two wires of a balanced input. An example is RF interference
that's picked up by both wires equally.

** That not a correct example. RF interference is defeated firstly by
the use of shielded cable and secondly by the use of filters to reduce
such
signals at the inputs of the balanced pre-amp.

Common mode rejection operates across the audio band and maybe a little
beyond but is usually most effective at the lower frequencies since the
main
aim is to eliminate ground hum from audio systems. A ground hum voltage
will
appear equally on the two wires and so be rejected.


So why is it you think that RF can't exist at audible frequencies?


** RF = "radio frequencies". Radio frequencies are those used for radio
communication - ie much higher than audio frequencies. The term Mike used
was " RF interference" - which refers to unwanted injection of RF energy
into a circuit.


Even if it's caused by A/C current and at 50 or 60 Hz, it's still
radio noise picked up by the wires because they're acting as antennas,
right?


** No. The wires inside the cable are acting as an induction loop in
magnetic field while frequencies concerned are in the audio range.



.............. Phil

On Sat, 3 Jul 2004 16:15:50 +1000, "Phil Allison"
wrote:

** Imbecile.

Oh, OK, cool. So you're doing what in America is called
guerilla theater. I've seen a few great ones, even done
one myself. Here's the setup:

My beautiful young friend Rebekah in belly dancing drag
enters from house rear, dances through the house and onto
stage. I'm crouched at stage lip with disposable flash
camera. Dance, flash, dance, flash.

Argument ensues; gets personal, very personal. Nobody, esp.
including sound man has been notified. Rebekah has to wave
repeatedly to get dance music to stop.

I step up onto the stage; Rebekah grabs the camera; I grab
it back and throw it against the wall. Audiences ****s.

We alternate (yeah, I know, but it worked) reading:

"Life is short,
Art is long
Opportunity fleeting,
Experiment treacherous
Judgement difficult.
Hippocrates"

We called it "I Dream of Rebe" and later did a cell phone
piece (didn't work at all) called "My Dinner with Rebe".

So how do you envision yours?

Chris Hornbeck

In article writes:

** That not a correct example.

A bad penny always returns. Welcome back, Phil.

RF interference is defeated firstly by
the use of shielded cable and secondly by the use of filters to reduce such
signals at the inputs of the balanced pre-amp.

However, what doesn't get caught by those crude mechanical methods
will be reduced by common mode rejection. That was a bad rebuttal. Try
harder next time you want to get theoretical in a practical world.

Common mode rejection operates across the audio band and maybe a little
beyond but is usually most effective at the lower frequencies since the main
aim is to eliminate ground hum from audio systems. A ground hum voltage will
appear equally on the two wires and so be rejected.

In theory it can operate in any frequency range, even DC. Did anyone
say anything about audio here?

** A voltage injected into a balanced audio line by external magnetic
fields ( like nearby high AC current cables and transformers) creates a hum
signal in differential mode that the pre-amp *will* amplify - its CMRR has
no effect.

Whoa! This is EXACTLY where common mode rejection is useful in audio
circuits. It's what lets us connect microphones with zip cord. The
reason why it doesn't work as well as we'd like it to is that it's
rare that the noise voltage at both inputs is rarely exactly the same,
so there will always be some difference, which will be amplified. The
difference is most often due to imbalance in the source, or different
loop area of the two wires in the cable. We work hard (sometimes) to
make these as good as we can, but it's only perfect in theory.

This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal picked
up every inch or so along the line and hence cancels it out. Where multiple
twisted pairs are used in the same cable the twisting reduces crosstalk in
the same way as above.

A very twisted explanation. I suggest that anyone really interested in
the theory behind this statement read the book about cable written by
Steve Lampen of Belden. It's pretty easy to understand.

"Star Quad" cable uses four twisted wires instead of two to enhance the
effect of the twisting and virtually eliminates induced hum problems even
when used near to high current AC cabling.

This is correct.

Knowing who I'm talking to, I feel compelled to make this statement.
I've offered the correct answer, explained to other readers why your
response isn't quite correct, and tried really hard not to make you
look like a jerk this time around. That's all I have to say on the
subject until someone changes it.

--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

Logan Shaw wrote:
Phil Allison wrote:

"Mike Rivers".

Common mode rejection is the ability to reject noise that's common to
the two wires of a balanced input. An example is RF interference
that's picked up by both wires equally.

** That not a correct example. RF interference is defeated firstly by
the use of shielded cable and secondly by the use of filters to reduce such
signals at the inputs of the balanced pre-amp.

Common mode rejection operates across the audio band and maybe a little
beyond but is usually most effective at the lower frequencies since the main
aim is to eliminate ground hum from audio systems. A ground hum voltage will
appear equally on the two wires and so be rejected.

So why is it you think that RF can't exist at audible frequencies?
Even if it's caused by A/C current and at 50 or 60 Hz, it's still
radio noise picked up by the wires because they're acting as antennas,
right?

RF doesn't exist at audible frequencies... and 60 Hz isn't RF. RF is stuff
at hundreds of KHz or higher.

You can't hear RF directly, you can only hear RF when it gets rectified and
turned into audio frequencies by electronics.

The two strategies for dealing with RF are to prevent it from getting picked
up in the first place, and prevent it from being rectified.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article writes:

The two strategies for dealing with RF are to prevent it from getting picked
up in the first place, and prevent it from being rectified.

Does that mean that Phil actually got the goods on me this time? I
used RF as an example because everyone knows what that is, and it's
possible, if you're sloppy, for both leads going to a differential
input to act as similar antennas. OK, I'll admit that I was right, but
didn't use the best example.

We have been known to have RFI problems around the studio, too.

My usual example when explaining how a differential input works to
reject common mode noise is hum induced from a magnetic field, but that
involves too long an explanation to answer a simplistic question. But I
see that eventually someone did give that explanation.

Next time I'll just shut up and let the incomplete answers flow in so
the poster can sort 'em out.



--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

"Mike Rivers"
Phil Allison:

** That not a correct example.

A bad penny always returns. Welcome back, Phil.


** Typical ****head reply from the NG's parrot.


RF interference is defeated firstly by
the use of shielded cable and secondly by the use of filters to reduce
such
signals at the inputs of the balanced pre-amp.

However, what doesn't get caught by those crude mechanical methods
will be reduced by common mode rejection.


** What pig ignorant drivel !! Shielding and filtering are not "crude"
and CMR has no effect on FRO interference.


That was a bad rebuttal.


** What Rivers posted was as worthless crap.



Common mode rejection operates across the audio band and maybe a little
beyond but is usually most effective at the lower frequencies since the
main
aim is to eliminate ground hum from audio systems. A ground hum voltage
will
appear equally on the two wires and so be rejected.

In theory it can operate in any frequency range, even DC.


** Irrelevant reply - as usual for a parrot.


Did anyone say anything about audio here?


** Not Mike Rivers anyhow.



** A voltage injected into a balanced audio line by external magnetic
fields ( like nearby high AC current cables and transformers) creates a
hum
signal in differential mode that the pre-amp *will* amplify - its CMRR
has
no effect.

Whoa! This is EXACTLY where common mode rejection is useful in audio
circuits.


** More pig ignorant drivel. The two wires in a balanced line form a loop -
loops pick up induced hum just perfectly.


It's what lets us connect microphones with zip cord. The
reason why it doesn't work as well as we'd like it to is that it's
rare that the noise voltage at both inputs is rarely exactly the same,
so there will always be some difference, which will be amplified.


** The voltage induced in a loop is a differential signal - same as the
wanted signal on an audio line.


This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal
picked
up every inch or so along the line and hence cancels it out. Where
multiple
twisted pairs are used in the same cable the twisting reduces crosstalk
in
the same way as above.

A very twisted explanation.


** More pig ignorant drivel.


I suggest that anyone really interested in
the theory behind this statement read the book about cable written by
Steve Lampen of Belden. It's pretty easy to understand.


** Polly want another cracker ???


"Star Quad" cable uses four twisted wires instead of two to enhance
the
effect of the twisting and virtually eliminates induced hum problems
even
when used near to high current AC cabling.

This is correct.


** And does kinda prove that twisting of the cable is responsible for the
rejection of induced hum from external fields.


Knowing who I'm talking to,


** Mike - you have NO idea who you are talking to.


I feel compelled to make this statement.


** What a posturing ass you are Mike.


I've offered the correct answer,


** Not one thing about that post was correct.


explained to other readers why your
response isn't quite correct, and tried really hard not to make you
look like a jerk this time around. That's all I have to say on the
subject until someone changes it.



** LOL - how pathetic.




............ Phil

Mike Rivers wrote:
Does that mean that Phil actually got the goods on me this time? I
used RF as an example because everyone knows what that is, and it's
possible, if you're sloppy, for both leads going to a differential
input to act as similar antennas. OK, I'll admit that I was right, but
didn't use the best example.

We have been known to have RFI problems around the studio, too.

My usual example when explaining how a differential input works to
reject common mode noise is hum induced from a magnetic field, but that
involves too long an explanation to answer a simplistic question. But I
see that eventually someone did give that explanation.

I'm no electronics expert, so perhaps I'm missing something, but I
thought radio waves were simply (changing) magnetic fields that
induce (changing) electric fields that in turn cause magnetic
fields, and the process repreats until the energy is absorbed
and/or an antenna picks up the signal by having one of those
generations of magnetic field turn into electricity within the
conductor.

The point being, I don't see how there can be a distinction between
hum caused by changing magnetic fields and hum caused by radio.
If the two wires are really close together and the frequency is,
say, 100 Hz, then wavelength is really, really long, and in fact
the distance between the transmitting and receiving antenna is
waaay less than one wavelength. But does that mean it's not
the same thing as radio?

By the way, from what I can dig up, it seems that the US Navy
uses radio at 76 Hz to communicate with submarines. Apparently
the antennas are fairly long, like way over 10 miles long.

- Logan

"Logan Shaw"

I'm no electronics expert,


** The time to get reading on what the term " RF " means.


http://www.google.com.au/search?hl=e...q=define:Radio
+Frequency





............ Phil

On Sun, 04 Jul 2004 03:09:21 GMT, Logan Shaw
wrote:

I'm no electronics expert, so perhaps I'm missing something, but I
thought radio waves were simply

Thou and I will never have any remotely useful intuitive
understanding of any of the following topics:

(changing) magnetic fields
(changing) electric fields
magnetic fields,
energy
absorbed
an antenna
electricity
conductor.

And, if we come to think that we do, we can fall into one
of the many conceptual traps exhibited in this thread.

The point being, I don't see how there can be a distinction between
hum caused by changing magnetic fields and hum caused by radio.
If the two wires are really close together and the frequency is,
say, 100 Hz, then wavelength is really, really long, and in fact
the distance between the transmitting and receiving antenna is
waaay less than one wavelength. But does that mean it's not
the same thing as radio?

You have an incisive viewpoint. But a complete description of
electromagnetic fields won't help your understanding, IMO.

But neither will some of the misinformation abounding.

Chris Hornbeck

Newsgroups are kind of like a bar, where folks of all stripe sit around
talking about everything they care to. With one exception: no one gets
their lights punched out, no matter how obnoxious.

On Sat, 03 Jul 2004 06:33:26 GMT, Logan Shaw
wrote:

So why is it you think that RF can't exist at audible frequencies?

What's the definition of "RF" on your planet? :-)

CubaseFAQ www.laurencepayne.co.uk/CubaseFAQ.htm
"Possibly the world's least impressive web site": George Perfect

Logan Shaw wrote:

I'm no electronics expert, so perhaps I'm missing something, but I
thought radio waves were simply (changing) magnetic fields that
induce (changing) electric fields that in turn cause magnetic
fields, and the process repreats until the energy is absorbed
and/or an antenna picks up the signal by having one of those
generations of magnetic field turn into electricity within the
conductor.

Right, but the end result of this is a self-propagating field that
behaves _very_ differently than just a single E or B field.

The point being, I don't see how there can be a distinction between
hum caused by changing magnetic fields and hum caused by radio.

The effect is very different. An alternating B field doesn't propagate
very far. You need a lot of turns on a coil in order to pick it up because
what you are doing is picking it up through direct magnetic induction. The
field is very limited and you have to be in the path between the pole
pieces to pick it up.

But, if you have real RF, you get stuff that can be picked up just from a
short length of cable (because you are basically picking up the E field)
with no magnetic induction required.

If the two wires are really close together and the frequency is,
say, 100 Hz, then wavelength is really, really long, and in fact
the distance between the transmitting and receiving antenna is
waaay less than one wavelength. But does that mean it's not
the same thing as radio?

It's not the same thing as radio, no. There is a discussion of this in
the ARRL Handbook, I think.

And it was really a stroke of genius on the part of Maxwell, by the way,
to explain how two related but differently-behaved phenomena can combine
to make a third even more differently-behaved phenomenon.

By the way, from what I can dig up, it seems that the US Navy
uses radio at 76 Hz to communicate with submarines. Apparently
the antennas are fairly long, like way over 10 miles long.

Yup. Because they need to generate an E field at that frequency.
It's not easy to make RF at very low frequencies because of the size
of everything required... capacitive and magnetic coupling are much
more likely at low frequencies.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

In article writes:

** The time to get reading on what the term " RF " means.
http://www.google.com.au/search? . . . . .

You can learn a lot on the Internet, but you have to be sufficiently perceptive
to look in enough places, and sufficiently knowledgable about the topic so
that you can find the whole picture and not just snippets of related (often
only by the dictionary meaning) information.. That's what schools can (or
at least should) do that web surfing can't.

Any boob can search Google for "Radio Frequency" and post a link. A
truly helpful person would post his (valid) interpretation of the inofrmation
to be found there and explain the answer to the question (which may
not have been asked but is important to a full understanding of the issue).
That's what I attempt to do when I feel it's called for. That way either the
question will be answered to the satisfaction of the one who asked it, or
he'll be guided to ask a relevant follow-on question.

Share knowledge, not insults.

Or keep your blowhard ass outa here.

Thank you.

--
I'm really Mike Rivers )
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me he double-m-eleven-double-zero at yahoo

Logan Shaw at wrote on 7/3/04 11:09 PM:

The point being, I don't see how there can be a distinction between
hum caused by changing magnetic fields and hum caused by radio.
If the two wires are really close together and the frequency is,
say, 100 Hz, then wavelength is really, really long, and in fact
the distance between the transmitting and receiving antenna is
waaay less than one wavelength. But does that mean it's not
the same thing as radio?

while they are essentially the same in what they ARE, they are HUGELY
different in how they ACT. I general the higher the frequency, the more
intrusive and seemingly creative it is in showing up as newer
harder-to-diagnose problems. I think this is due to both extremely (well
above audio) Hf material's ability to travel across all sorts of electrical
'barriers' to audio as well as the fact that we can;t hear it unless/until
it gets rectified/demodulated into audible range, and that happens in MANY
'intereting')annoying and varied and unpredictable) ways in all sorts of
different circuit topologies... Scott can undoubtedly shed a a better light
on the WHY's of this but simple practical fact is, RF is DAMNED good at
creating all SORTS of seemingly IMPOSSIBLE symptoms.. I've been chasing one
gremlin in this flavor for YEARS here, while PRACTICALLY it's been solved by
simply NOT USING THAT SET OF LINES for anythign other than headphone returns
and other line-level feeds, it's turned into a personal thing as to wanting
to know just WHY this one in-house snake is PLAGUED with RFI, differently on
different lines, while other single lines and snakes running along the same
route are CLEAN... and according to my electrical and shop inspections and
tests There's (apparently) Nothing Wrong with any of the lines, the box or
the connectors...



By the way, from what I can dig up, it seems that the US Navy
uses radio at 76 Hz to communicate with submarines. Apparently
the antennas are fairly long, like way over 10 miles long.

and getting information across a SLOOWWWWWWW system like that is like
pouring marbles through molasses.


--
John I-22
(that's 'I' for Initial...)
Recognising what's NOT worth your time, THAT'S the key.
--

Mike Rivers at wrote on 7/4/04 11:12 AM:


Share knowledge, not insults.

Or keep your blowhard ass outa here.

Thank you.

(here is heard the sound of Much Gloved Applause)

"Mike Rivers"
" Phil Allison"


** The time to get reading on what the term " RF " means.
http://www.google.com.au/search? . . . . .

( snip more posturing Mike River's drivel)


Any boob can search Google for "Radio Frequency" and post a link.


** Any parrot can do that too.


A truly helpful person would post his (valid) interpretation of the
inofrmation
to be found there and explain the answer to the question (which may
not have been asked but is important to a full understanding of the
issue).


** There is no issue regarding RF - just the meaning of the term.


That's what I attempt to do when I feel it's called for.


** Shame Mike River's "explanations" are riddled with gross errors.


Share knowledge, not insults.



Or keep your blowhard ass outa here.


** Did I just see another gratuitous insult ??????

What stinking hypocrite.





................. Phil

"JoVee"
Mike Rivers


Share knowledge, not insults.

Or keep your blowhard ass outa here.

Thank you.

(here is heard the sound of Much Gloved Applause)



** Sycophants are really sickening people.





.............. Phil

In article ,
"Phil Allison" wrote:
** A voltage injected into a balanced audio line by external magnetic
fields ( like nearby high AC current cables and transformers) creates a hum
signal in differential mode that the pre-amp *will* amplify - its CMRR has
no effect.

Actually, it's current that gets induced by a changing magnetic field,
not voltage. This current acts upon whatever impedances are present to
then create a voltage.

Yeah, it seems like a subtle point, but it completely determines if and
what sort of interference will be present. If the balanced line has
balanced impedances on both sides, then all the interfering voltage
created by the interference current will be common mode and could be
completely cancelled out by an ideal receiver. If there is an impedance
imbalance, then some of the interference will result in a difference
mode signal, one that can never be removed by any balanced receiver.

This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal picked
up every inch or so along the line and hence cancels it out. Where multiple
twisted pairs are used in the same cable the twisting reduces crosstalk in
the same way as above.

Yes, twisting makes the loop area effectively zero, so there's no mutual
coupling and thus no induced current and thus no induced voltage.



Regards,

Monte McGuire

"Monte McGuire"...
"Phil Allison"

** A voltage injected into a balanced audio line by external magnetic
fields ( like nearby high AC current cables and transformers) creates a
hum
signal in differential mode that the pre-amp *will* amplify - its CMRR
has
no effect.

Actually, it's current that gets induced by a changing magnetic field,
not voltage. This current acts upon whatever impedances are present to
then create a voltage.


** Wrong - a voltage is induced. The current that flows depends on the
impedances, conductor cross section etc. The lower they are the more the
current - but this is not relevant to the problem since hum voltages are
what get amplified and heard.


Yeah, it seems like a subtle point, but it completely determines if and
what sort of interference will be present.


** That just compounds the first error.


If the balanced line has
balanced impedances on both sides, then all the interfering voltage
created by the interference current will be common mode and could be
completely cancelled out by an ideal receiver. If there is an impedance
imbalance, then some of the interference will result in a difference
mode signal, one that can never be removed by any balanced receiver.



** Wrong - the loop formed by the two signal carrying lines is *the
circuit* in which the hum voltage is induced. This is in differential
ode - same as the wanted signal.


This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal
picked
up every inch or so along the line and hence cancels it out. Where
multiple
twisted pairs are used in the same cable the twisting reduces crosstalk
in
the same way as above.

Yes, twisting makes the loop area effectively zero, so there's no mutual
coupling and thus no induced current and thus no induced voltage.


** A balanced audio line that is **NOT** twisted is just an induction loop.



............... Phil

On Mon, 05 Jul 2004 03:46:09 GMT, Monte McGuire
wrote:

In article ,
"Phil Allison" wrote:

This sort of interference is reduced by the fact the two wires are
*twisted* inside the cable which reverses the phase of any hum signal picked
up every inch or so along the line and hence cancels it out. Where multiple
twisted pairs are used in the same cable the twisting reduces crosstalk in
the same way as above.

Yes, twisting makes the loop area effectively zero, so there's no mutual
coupling and thus no induced current and thus no induced voltage.

Imagine the simpler case of a balanced signal on an untwisted pair
line. What is now different in the induced current? First consider
the case of the two conductors having *no* spacing. Then the case of
spacing significant compared to a wavelength.

Next consider the case of the same lines with a single twist centered
on a symmetrical hum source. Then with infinite twisting.

All this talk about "reversing the phase" etc. is fundamentally
flawed, IMO.

Chris Hornbeck

On Mon, 5 Jul 2004 14:02:57 +1000, "Phil Allison"
wrote:

** A balanced audio line that is **NOT** twisted is just an induction loop.

This, I think, is the source of your misconception, IMO.

Just for fun, pretend that you don't believe this, then rework
the same steps.

Good fortune,

Chris Hornbeck

"Chris Hornbeck"
"Phil Allison"


** A balanced audio line that is **NOT** twisted is just an induction
loop.

This, I think, is the source of your misconception, IMO.

Just for fun, pretend that you don't believe this, then rework
the same steps.



** Dear Chris - you are a complete ****wit.

Have a nice day.



.............. Phil

On Mon, 5 Jul 2004 14:02:57 +1000, "Phil Allison"
wrote:

** Wrong - a voltage is induced. The current that flows depends on the
impedances, conductor cross section etc. The lower they are the more the
current - but this is not relevant to the problem since hum voltages are
what get amplified and heard.

I retract my earlier post; this is the more fundamental misconception.
Maybe thinking in terms of a conventional power electrical generator
would help clear things up.

All the same rules apply.

Good fortune,

Chris Hornbeck

"Chris Hornbeck"

Imagine the simpler case of a balanced signal on an untwisted pair
line. What is now different in the induced current? First consider
the case of the two conductors having *no* spacing. Then the case of
spacing significant compared to a wavelength.

Next consider the case of the same lines with a single twist centered
on a symmetrical hum source. Then with infinite twisting.

All this talk about "reversing the phase" etc. is fundamentally
flawed, IMO.





** For Christ's sake Chris - do a *real test * instead of posting
ASININE thought experiments with wrong outcomes.

Get a length of insulated wire, connect the ends to pins 2 and 3 of an
XLR, plug it into a mic pre and try the effect of having an open loop,
closed loop and then twisted tightly all along its length when held close
proximity to an AC power transformer.




............ Phil

On Mon, 5 Jul 2004 15:28:33 +1000, "Phil Allison"
wrote:

** Dear Chris - you are a complete ****wit.

No doubt. Tell me something new.

Have a nice day.

Thanks. What should I name my new cat? Neighborhood kids
apparently named her Padme after some Starwars character
I've missed, but it doesn't mean much to me.

She only weighs seven pounds but she raised seven beautiful
kittens to weaning all on her own. We've only caught four
of them so far, but we're very determined. If I'd done a
tenth as well as she did in my life I'd be content.

And I've had an extra fifty years or so headstart.

Chris Hornbeck

On Mon, 5 Jul 2004 15:35:20 +1000, "Phil Allison"
wrote:

** For Christ's sake Chris - do a *real test * instead of posting
ASININE thought experiments with wrong outcomes.

Get a length of insulated wire, connect the ends to pins 2 and 3 of an
XLR, plug it into a mic pre and try the effect of having an open loop,
closed loop and then twisted tightly all along its length when held close
proximity to an AC power transformer.

Exactly right. And if you vary the spacing between the conductors
you'll discover an anomaly in your model.

More fundamentally, magnetic fields induce currents. To discuss
the topic with ordinary mortals, you'll just have to bend your
phrasing to the conventional.

Or as the antique saying goes "Watch your phraseology."

Chris Hornbeck