If all frequencies are equally delayed, the phase response is a
straight line (on a linear frequency scale). If I plot my delay it
is flat down to about 4k, then reduces in a curve.

What does it do to the sound? What should a design aim for?

Is this why pedal steel guitars swim about? Are they meant to do
that?

Can someone point me in the direction of enlightenment please.

cheers, Ian

Hi RATs!

Charting data points is interesting. It allows insight into phenomena.

Listening is fun. It allows the experience of music.

Trying to correlate charts and music is an interesting challenge. Some people
would rather think than listen.

Not me

Listen and think, that's the ticket!

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

"TubeGarden" wrote

Charting data points is interesting. It allows insight into
phenomena.

Listening is fun. It allows the experience of music.

Trying to correlate charts and music is an interesting challenge.
Some people
would rather think than listen.

Not me

Listen and think, that's the ticket!

Music is a social enterprise. Fidelity is a moral imperative.

Thinking needs talking in order to make sense.

Music does its own talking.

cheers, Ian

Hi Ian . . .

I think that uniform phase response is important . . . but of course people
do have different opinions . . .

. . . but it's definately a given that the phase response and group delay
is extremely important when applying feedback. There are a number of
volumes on the subject, both with regard to vacuum tubes and solid-state
circuits. In this instance, it is necessary to understand both
frequency-dependant and frequency-independant delay to predict and optimize
the amplifier's operation when feedback is applied.

When feedback is not being used, then the frequency-independant delay is
generally regarded as inconsequential . . . what is important is
frequency-dependant delay. If I read your post correctly, you have a
difference in delay, and consequently phase, of mid-band frequencies and
high-band frequencies . . . this would definately be something I would want
to correct . . . especially given the high sensitivity of human hearing in
the 3-4KC area.

Regards,

Kirk Patton






"Ian Iveson" wrote in message
...
If all frequencies are equally delayed, the phase response is a
straight line (on a linear frequency scale). If I plot my delay it
is flat down to about 4k, then reduces in a curve.

What does it do to the sound? What should a design aim for?

Is this why pedal steel guitars swim about? Are they meant to do
that?

Can someone point me in the direction of enlightenment please.

cheers, Ian

Music is a social enterprise. Fidelity is a moral imperative.

Thinking needs talking in order to make sense.

Music does its own talking.

cheers, Ian


Hi RATs!

Sales are a social enterprise. Some bits of music can be bought and sold.
Fidelity is just a cheap sticker to put on the bits of music we have for sale.

The greater glory of music, which cannot be bought, nor sold, no matter how
many lawyers we hire, does not require any silly sticker. People buy music,
with and without stickers. Stickers are not the big deal. Music is the big
deal. The only people who think the stickers are the important thing are the
sticker salesmen, and their feeble customers

Happy Ears!
Al



Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

Ian Iveson wrote:

If all frequencies are equally delayed, the phase response is a
straight line (on a linear frequency scale). If I plot my delay it
is flat down to about 4k, then reduces in a curve.

What does it do to the sound? What should a design aim for?

Is this why pedal steel guitars swim about? Are they meant to do
that?

Can someone point me in the direction of enlightenment please.

cheers, Ian

By differential phase error, do you mean
having a different phase angle in the signal applied to the
output stage?
Say one signal was at +90 degrees, and the other at -90 degrees,
then since the two sigs would normally be 180 degrees apart, the signa;
applied to
the output stage would be 0 degrees, ie a common mode sig is applied,
and
there is almost zero output.

So if the applied signal to the output stage drifts from 180 degrees
difference,
the signal is attentuated at the output, but the amp works just as hard
dissipating heat withing itself, and the imd of any
signal passing through is increased.

Patrick Turner.

"Patrick Turner" wrote

[see below]

No. Sorry, I should be more clear.

I'm looking at an ordinary bode plot, and homing in on the phase
error.

It has a log frequency scale, and shows phase angle on the Y axis.
This view is useful for determining phase margin for purpose of
stability (see kirk's post, but you know that anyway of course).

It may not very useful for wondering how it might sound, or at least
how the mono sound sounds...let's leave out stereo effects for the
moment.

If all frequencies are delayed by the same *time*, then the sound
should be the same...the concert just starts a little sooner. The
same delay applied to all frequencies would result in a shift in
phase angle that would be proportional to frequency. So if you
plotted phase angle v frequency on a linear scale, you would get a
straight line, sloping down to the right.

As an example, if the delay is 1us, then at 1k the phase error would
be 360/1000 degrees, and at 2k it would be 720/1000 degrees, which
is twice the angle.

Now, being a straight line, its differential will be a constant, so
if you plot da/df, where a is phase angle and f is frequency, you
should get a horizontal line. (it follows that da/df is proportional
to delay, which makes sense if you think about it, I hope).

Well, if you examine the bode plot of a typical valve amplifier,
especially with no global negative feedback, you will see the phase
plot shows phase lead at low frequencies, curving down to zero error
at around 1k perhaps, and then an increasing delay, sloping down
to -180 degrees somewhere beyond the point where gain is zero,
hopefully, around 1MHz?

If I translate that into delay, with my amps and probably with most,
it is constant from around 5k to perhaps 100k. Below 5k it falls to
zero and then accumulates an increasing negative value, curving down
sharply below 100Hz. Negative delay is lead.

Lead, or negative delay, is tricky to get my head round. It has two
important features. Firstly, there is the matter of how the sound
gets ahead of itself in the first place. In the transient domain you
can follow the response to the beginning of a steady, low-frequency
tone, and see that the wave marks time for a cycle before coming in
ahead of the next. So in the frequency domain it begins with a
slightly higher frequency until the shift has taken place.

Secondly, you must see all this as relative. But for the first few
cycles, the situation is as if the bass had happened just before the
mid frequencies, which amounts to the same thing as mid frequency
delay.

At the moment I can't see why that should make the sound different.
Not if the orchestra is playing a sustained chord, anyway. It should
only make a difference to the attack of each note, and I dunno under
what circumstances that would be audible.

But what if the same instrument plays a run of notes spanning
several octaves? Or, in the extreme, does what a pedal steel guitar
does, and slide whole chords?

Does shifting delay applied to a sliding tone sound like movement?

What do SS amps do? What do op-amps do? Does all that feedback
eliminate the phase error?

cheers, Ian

"Ian Iveson"


( snip load of confused puppy talk)


Does shifting delay applied to a sliding tone sound like movement?

What do SS amps do? What do op-amps do? Does all that feedback
eliminate the phase error?



** Ian - do try to get your facts straight.

The phase shifts you see on a Bode plot are ACCOMPANIED by GROSS
response errors !!!!

Once NFB is applied the response gets nice and flat and the phase shifts
disappear too !!!!!

Phase shift of the output signal compared to the input signal in hi-fi
power amps is there as a direct result of sub-sonic and super-sonic roll
offs in the overall response. The shift follows the order of the filters
used - typically they are first order so you get 45 degress shift at the 3
dB down points.

If you really want something to fret about - go measure all the phase
shifts in a 3 way loudspeaker !!!!!





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

Ian Iveson wrote:

"Patrick Turner" wrote

[see below]

No. Sorry, I should be more clear.

I'm looking at an ordinary bode plot, and homing in on the phase
error.

It has a log frequency scale, and shows phase angle on the Y axis.
This view is useful for determining phase margin for purpose of
stability (see kirk's post, but you know that anyway of course).

It may not very useful for wondering how it might sound, or at least
how the mono sound sounds...let's leave out stereo effects for the
moment.

If all frequencies are delayed by the same *time*, then the sound
should be the same...the concert just starts a little sooner. The
same delay applied to all frequencies would result in a shift in
phase angle that would be proportional to frequency. So if you
plotted phase angle v frequency on a linear scale, you would get a
straight line, sloping down to the right.

As an example, if the delay is 1us, then at 1k the phase error would
be 360/1000 degrees, and at 2k it would be 720/1000 degrees, which
is twice the angle.

Now, being a straight line, its differential will be a constant, so
if you plot da/df, where a is phase angle and f is frequency, you
should get a horizontal line. (it follows that da/df is proportional
to delay, which makes sense if you think about it, I hope).

All that makes sense.



Well, if you examine the bode plot of a typical valve amplifier,
especially with no global negative feedback, you will see the phase
plot shows phase lead at low frequencies, curving down to zero error
at around 1k perhaps, and then an increasing delay, sloping down
to -180 degrees somewhere beyond the point where gain is zero,
hopefully, around 1MHz?

If I translate that into delay, with my amps and probably with most,
it is constant from around 5k to perhaps 100k. Below 5k it falls to
zero and then accumulates an increasing negative value, curving down
sharply below 100Hz. Negative delay is lead.

Lead, or negative delay, is tricky to get my head round. It has two
important features. Firstly, there is the matter of how the sound
gets ahead of itself in the first place.

Ah, the wonders of sound arriving at the ear, well before it set out on
the journey.
Everyone can handle phase lag, or sine waves "slipping back"
a few degrees, but arriving ahead?
Its confusing until you realise there has to be a start time to the
stream of sine waves, and the energy is sped up a bit..

In the transient domain you
can follow the response to the beginning of a steady, low-frequency
tone, and see that the wave marks time for a cycle before coming in
ahead of the next. So in the frequency domain it begins with a
slightly higher frequency until the shift has taken place.

Secondly, you must see all this as relative. But for the first few
cycles, the situation is as if the bass had happened just before the
mid frequencies, which amounts to the same thing as mid frequency
delay.

At the moment I can't see why that should make the sound different.
Not if the orchestra is playing a sustained chord, anyway. It should
only make a difference to the attack of each note, and I dunno under
what circumstances that would be audible.

The less phase shift, the better.



But what if the same instrument plays a run of notes spanning
several octaves? Or, in the extreme, does what a pedal steel guitar
does, and slide whole chords?

Does shifting delay applied to a sliding tone sound like movement?

You could play several oscillators all simultaneously,
and all producing tones of related harmonics, and providing their F
remain
related to each other, slowly altering the phases of each, which will
have random
phase relationships, somewhat different to a string on an instrument,
you theoretically shouldn't hear very much.



What do SS amps do? What do op-amps do? Does all that feedback
eliminate the phase error?

Yes, and the phase relationships that exist in the original signal
are quite accurately reproduced in a wide BW high NFB amp,
with only a tiny error, and the open loop phase errors
are corrected just like the other open loop distortions.


Patrick Turner.



cheers, Ian

"Ian Iveson" wrote in message news:HSvAb.957

As an example, if the delay is 1us, then at 1k the phase error would
be 360/1000 degrees, and at 2k it would be 720/1000 degrees, which
is twice the angle.

But this is measuring the frequency-independant phase response of the
network, which (without NFB) is inconsequential, unless you care that you
are listening to your music 1uS later than it is coming into the amp. Now
if there is a 1uS delay at 1KC, and a 2uS delay at 2KC, then you have a
frequency-dependant phase error, and this is what we want to avoid.

Well, if you examine the bode plot of a typical valve amplifier,
especially with no global negative feedback, you will see the phase
plot shows phase lead at low frequencies, curving down to zero error
at around 1k perhaps, and then an increasing delay, sloping down
to -180 degrees somewhere beyond the point where gain is zero,
hopefully, around 1MHz?

You didn't mention whether you are measuring frequency-dependant or
frequency-independant phase error, or the sum of the two. There is a very
good and very understandable AES paper on phase measurements with regard to
transformers that was done by Deane Jensen, I think it makes these
distinctions very clear . . . Jensen Transformers doesn't post it online but
their website says that they'll mail you a copy if you ask for it.

http://www.jensentransformers.com/apps_wp.html

But what if the same instrument plays a run of notes spanning
several octaves? Or, in the extreme, does what a pedal steel guitar
does, and slide whole chords?

Hmmm. This is a tough question . . . trying to describe the sound of a
phase shift. One thing to keep in mind is that most all of the pitch
fundamentals of instruments are in the midrange and below, it is their
overtones that are in the higher frequencies. And even in a sustained note,
what we perceive as "timbre" is very closely related to the amplitude and
phase of the overtones with relation to the fundamental.

Does shifting delay applied to a sliding tone sound like movement?

I'm not sure what you mean by "movement". The closest effects device to this
phenomenon might be a guitar "flanger". The term "flanging" (not related
to, but also not exclusive of, "flogging", esp. among guitarists) originated
from the practice of taking two tape recorders that are SMPTE'd together,
with the same signal being recorded on both (and maybe being played back
simultaneously, I forget) and sticking one's hand on the flange of the
supply reel of one machine. This introduces a delay in one signal, and a
phase difference that increases with frequency. Whether or not a given
phase problem in an amplifier will sound exactly like a flanger, though,
would be pure speculation on my part . . . I do think that it would be safe
to say that it would have a significant effect on the perception of stereo
image, probably room reverb, and in significant doses, on timbre.

What do SS amps do? What do op-amps do? Does all that feedback
eliminate the phase error?

Well, I'm definately not RA!-RA! for solid state in general, but most SS
stuff doesn't really have exactly these same problems in the audio band.
The phase response of most SS amps and opamps are defined by a single-pole
Miller capacitor across the voltage amp stage for high frequencies, and at
low frequencies by the capacitors that couple in the signal or ground the
feedback loop. Just as in a tube amp, "improvements" in measured
closed-loop phase response via application of feedback doesn't really fix
the problem, it simply trades one thing for another . . . i.e., closed loop
stability for bandwidth, etc. And this can be a complex problem with SS
topologies because bandwidth above unity gain is usually very high, and
open-loop gain is also very high but varies significantly with frequency.

The complicating factor in tube amps with phase response is, er, the output
transformer. While these things IMHO have many benefits that outweigh their
shortcommings, more complex time-domain behaviour is simply part of the
package. It's just a question of how much and where (frequency-wise).

Anyway, an interesting and thought-provoking subject.

Best regards,

Kirk Patton

Hi RATs!

Interesting!

Important? That's up to you

Happy Ears!
Al


Alan J. Marcy
Phoenix, AZ

PWC/mystic/Earhead

When an amp has different phase shift for different freqs it can definately
effect the sound. Some may view this effect as a positive thing others not.
I guarantee some strange phase shifts in a guitar amp but whether or not
this is causing some specific problem I have no idea.

"Ian Iveson" wrote in message
...
If all frequencies are equally delayed, the phase response is a
straight line (on a linear frequency scale). If I plot my delay it
is flat down to about 4k, then reduces in a curve.

What does it do to the sound? What should a design aim for?

Is this why pedal steel guitars swim about? Are they meant to do
that?

Can someone point me in the direction of enlightenment please.

cheers, Ian

"Jimmy"

When an amp has different phase shift for different freqs it can
definately
effect the sound.


** Seeing as phase shifts are produced by response changes that is true -
but one hears the response changes.


I guarantee some strange phase shifts in a guitar amp but whether or not
this is causing some specific problem I have no idea.


** Those last four words are the key here.




........ Phil

Thanks for your contributions, Kirk. I have been thinking, slowly.

Trying to cover points you made in both posts not otherwise
addressed.

Yes I lumped all the phase response together...as I said originally,
I was looking at the bode plot of an amplifier.

My thought has been about where frequency-independent phase error
(hardly response, since it is independent...) fits in. Originally I
said that constant delay would appear as a straight line, sloping
down to the right, and that any deviation from straightness would be
frequency-dependent delay.

Since delay is proportional to distance from source, I suggested
"movement" because, if the delay varies with frequency, then the
source should sound like it is moving forward or backward: into or
out of the speakers. For this to be true relies on the assumption
that, although we cannot sense distance directly, we can sense
*change* in distance.

Further, if you were listening to a group of sources playing in time
(to themselves) from a distance, perhaps frequency-dependent delay
places the notes, rather than the instruments, at different
distances. Combined with the frequency-independent delay, the
soundstage would be dynamically warped. Perhaps a touch of this
movement allows us to locate the centre of motion of each
instrument, and so enhances the sense of space. Too much feedback
would kill the space, too little would jumble it.

I blame all this on Jim.

Right, erm, now to the bit I missed out. For a linear phase response
to signify *only* constant delay, it must pass through the origin.
Hence delay at DC would be indeterminate (0/0) rather than infinite.

So a straight line not passing through the origin must have a
component of delay which is in linear proportion to frequency.
Trouble is, if you have a curved line, the difference between the
two forms of non-linear delay is difficult to guess. Let's see...if
I extent the straight section of my bode plot (on a linear frequency
axis, remember) leftwards, should the extension pass through the
origin if there is none of this second form of non-linear delay
present?

Without reading heaps of stuff, it looks like my phase response
could be summarised by:

theta = a/f + b +cf

So if you take the differential as indicative of delay, you
eliminate "b", highlight the effect of "a" by squaring its "f", and
show that the "c" is insignificant because constant delay doesn't
matter.

That is what Menno van der Veen implies in his evaluation of Plitron
transformers.

BUT it is a mistake to eliminate "b"...constant phase error is a
form of frequency-dependent delay too. Wonder why he did that.
Perhaps I should ask him.

I should also find out the speed of sound and work out what kind of
order of magnitude these distances would be. If they are millimetres
then I have been wasting my time.

Lexicon have made a fortune from sound processing equipment, because
someone there knows this stuff. Much more is known than is in the
public domain, unfortunately.

cheers, Ian

"Kirk Patton" wrote in message
m...

"Ian Iveson" wrote in message
news:HSvAb.957

As an example, if the delay is 1us, then at 1k the phase error
would
be 360/1000 degrees, and at 2k it would be 720/1000 degrees,
which
is twice the angle.

But this is measuring the frequency-independant phase response of
the
network, which (without NFB) is inconsequential, unless you care
that you
are listening to your music 1uS later than it is coming into the
amp. Now
if there is a 1uS delay at 1KC, and a 2uS delay at 2KC, then you
have a
frequency-dependant phase error, and this is what we want to
avoid.

Well, if you examine the bode plot of a typical valve amplifier,
especially with no global negative feedback, you will see the
phase
plot shows phase lead at low frequencies, curving down to zero
error
at around 1k perhaps, and then an increasing delay, sloping down
to -180 degrees somewhere beyond the point where gain is zero,
hopefully, around 1MHz?

You didn't mention whether you are measuring frequency-dependant
or
frequency-independant phase error, or the sum of the two. There
is a very
good and very understandable AES paper on phase measurements with
regard to
transformers that was done by Deane Jensen, I think it makes these
distinctions very clear . . . Jensen Transformers doesn't post it
online but
their website says that they'll mail you a copy if you ask for it.

http://www.jensentransformers.com/apps_wp.html

But what if the same instrument plays a run of notes spanning
several octaves? Or, in the extreme, does what a pedal steel
guitar
does, and slide whole chords?

Hmmm. This is a tough question . . . trying to describe the sound
of a
phase shift. One thing to keep in mind is that most all of the
pitch
fundamentals of instruments are in the midrange and below, it is
their
overtones that are in the higher frequencies. And even in a
sustained note,
what we perceive as "timbre" is very closely related to the
amplitude and
phase of the overtones with relation to the fundamental.

Does shifting delay applied to a sliding tone sound like
movement?

I'm not sure what you mean by "movement". The closest effects
device to this
phenomenon might be a guitar "flanger". The term "flanging" (not
related
to, but also not exclusive of, "flogging", esp. among guitarists)
originated
from the practice of taking two tape recorders that are SMPTE'd
together,
with the same signal being recorded on both (and maybe being
played back
simultaneously, I forget) and sticking one's hand on the flange of
the
supply reel of one machine. This introduces a delay in one
signal, and a
phase difference that increases with frequency. Whether or not a
given
phase problem in an amplifier will sound exactly like a flanger,
though,
would be pure speculation on my part . . . I do think that it
would be safe
to say that it would have a significant effect on the perception
of stereo
image, probably room reverb, and in significant doses, on timbre.

What do SS amps do? What do op-amps do? Does all that feedback
eliminate the phase error?

Well, I'm definately not RA!-RA! for solid state in general, but
most SS
stuff doesn't really have exactly these same problems in the audio
band.
The phase response of most SS amps and opamps are defined by a
single-pole
Miller capacitor across the voltage amp stage for high
frequencies, and at
low frequencies by the capacitors that couple in the signal or
ground the
feedback loop. Just as in a tube amp, "improvements" in measured
closed-loop phase response via application of feedback doesn't
really fix
the problem, it simply trades one thing for another . . . i.e.,
closed loop
stability for bandwidth, etc. And this can be a complex problem
with SS
topologies because bandwidth above unity gain is usually very
high, and
open-loop gain is also very high but varies significantly with
frequency.

The complicating factor in tube amps with phase response is, er,
the output
transformer. While these things IMHO have many benefits that
outweigh their
shortcommings, more complex time-domain behaviour is simply part
of the
package. It's just a question of how much and where
(frequency-wise).

Anyway, an interesting and thought-provoking subject.

Best regards,

Kirk Patton