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Old 22nd April 2004, 03:45 PM   #1
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Default First cycle distortion - Graham, what is that?

First cycle distortion - Graham, what is that?

I have never heard it before.

Quote:
In another thread I mentioned distortion of the leading edges of first cycles, and called it 'FCD' = first cycle distortion.
http://www.diyaudio.com/forums/showt...559#post373559

Can you explain? Has music really signals that can be regarded as startup transients? Maybe for slow amp?

Can you give me any reference material?
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Old 22nd April 2004, 04:03 PM   #2
sreten is offline sreten  United Kingdom
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From the accounts I've read so far it appears to be a rather
disingenuous way of referring to what is already known.

It is not as far as I can tell any sort of "new" distortion
mechanism, it appears to be the effect of closed loop
bandwidth and stability on a sine wave that abruptly
starts at t=0.

As far as I can tell reproduction of any discontinuity
emphasises wide bandwidth design over high feedback
levels in the audioband with the necessarily restricted
bandwidth required for stability.

And by definition such an approach would imply that
the JLH class A type circuit favoured by GM would
come out well under such analysis.

sreten.
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Old 22nd April 2004, 04:12 PM   #3
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Quote:
Originally posted by sreten
on a sine wave that abruptly
starts at t=0.
But what has this to do with music?
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Old 22nd April 2004, 04:15 PM   #4
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by peranders

But what has this to do with music?
I'm afraid I can't help you here, but IMO the
same as any waveform with discontinuities.

sreten.
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Old 22nd April 2004, 04:33 PM   #5
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Isn't this the same thing as step response?
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Old 22nd April 2004, 04:52 PM   #6
boholm is offline boholm  Denmark
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It has do with music is this way (in my world ;-)):

Music is comprised of transients. There is almost - exept when a tone is prolonged on purpose - no steady-state in music. Hit a string on a guitar and it will start swinging as soon as the plectrum or your finger lets go of the string. But the amplitude of the swinging wil not be constant, it will decrease untill it stops. Concluding; No steady state. As you may or may not see/know, the first swing from start point to start point is the one with the biggest amplitude, as it will fade gradually as already sayd.

Here comes the question: What can cause for a signal to be dampened in it first swing - or cycle (as Graham appropiately calls it)?

Here comes another question: How important is this first swing? Would you be able to hear it, if it wasn't there or if it was unintentionally dampened? And since music is - almost - pure pulses (first swings) will you be able to hear this somehow?

If you have looked at the graphics Graham presented you could see that the first period was dampened before reaching a steady state, and I do not know what could cause this - I haven't seen it before. But what intriques me, is that the very sudden start of this first period. There is no "gentle" start of it. But maybe these two factors are not connected?

"Must figure this out . . . " he said leaving the house to visit some other friends . . .
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Old 22nd April 2004, 04:58 PM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by peranders
Isn't this the same thing as step response?
Step, pulse, square, triangular doesn't make a lot of difference.

A discontinuity is a discontinuity, requiring theoretically infinite bandwidth.

And reproduction depends on closed loop bandwidth and stability.

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Old 22nd April 2004, 07:19 PM   #8
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I was rather under the impression (and I could be wrong) that
Graham was making reference to distortion created by the
active character of the loudspeaker, not intrinsic distortion of
an amplifier using feedback.

Feedback in power amplifiers is far too fast to create first
cycle distortion due to feedback delay, at least in the context
of audio frequencies. The notion that it takes a cycle or two
for the amp to "get it" is erroneous.

The loudspeaker on the other hand may not exhibit the same
impedance on the first cycle as subsequently due to the back
emf generated by a moving voice coil, and so can draw more
current on the first cycle. It seemed to me that this or
something similar was the basis of Graham's argument.
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Old 22nd April 2004, 07:29 PM   #9
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Quote:
Originally posted by Nelson Pass
The loudspeaker on the other hand may not exhibit the same
impedance on the first cycle as subsequently due to the back
emf generated by a moving voice coil, and so can draw more
current on the first cycle. It seemed to me that this or
something similar was the basis of Graham's argument.
But I'm left wondering the same thing I was left wondering in the previous thread; what's the fundamental difference between the back EMF of the loudspeaker and the back EMF of any other RLC resonant circiut?

se
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Old 22nd April 2004, 07:40 PM   #10
andy_c is offline andy_c  United States
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Quote:
Originally posted by Nelson Pass
I was rather under the impression (and I could be wrong) that
Graham was making reference to distortion created by the
active character of the loudspeaker, not intrinsic distortion of
an amplifier using feedback.(...)
Actually, I believe Graham was talking about the amplifier itself, possibly including loading effects. He simulated the transient response to a sine wave in SPICE, then did an FFT on the first cycle to determine the harmonic distortion of just the first cycle.

I believe jcx summed it up best in this post http://www.diyaudio.com/forums/showt...915#post369915 when he pointed out that a single pole low pass filter has considerable "first cycle distortion" unless the bandwidth is quite large. It's clear from that post that "first cycle distortion" has little to do with non-linearity per se and is mostly due to the transient response of the circuit.
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