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Author Topic: APL's Time Domain Analysis - Update?  (Read 2884 times)

Jack Regula

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Non-linear Distortion Analysis
« Reply #20 on: February 18, 2017, 12:31:19 pm »

TDA can show harmonic distortion relative to the fundamental or as a percentage.  Examples are shown below.
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Jack Regula

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End of review (almost)
« Reply #21 on: February 18, 2017, 12:38:07 pm »

APL_TDA has become my favorite tool for evaluating my DIY speaker efforts in room, the listening position itself, and the effectiveness of various room treatments Iíve been experimenting with.  It has helped me see issues that I missed or misinterpreted with conventional tools. I still use those programs for simulation and crossover development but TDA is where I turn to see (as opposed to hear) how well Iíve done in speaker and system design and implementation and integrating the speaker into my room.

Well, that was the end of my review but I want to continue to probe the limits of TDA's ability to discriminate between the direct response and reflections.  That is what its utility depends on and we need to understand its limits.

To explore this topic, we will look at a speaker both alone and in the presence of a strong reflection, delayed by 5 ms. and down only 3 db.  For most small rooms, a 5 ms. delayed reflection would have traveled more than twice as far as the direct sound and would be down by more than 6 db.   Thus, it is a worse than worst case situation and has been created artificially rather than by measurement. 

The speaker has 4 ways with crossovers at 200 Hz, 1.6 kHz, and 5 kHz that use 4th order LR filters.   Measured close up and free from room effects, TDA shows a clean pl graph, with group delay increasing towards low frequencies and an impressively flat frequency response.

The AFR and GDR below were taken with FFT-Q = 8.  The GDR below is plotted with minimum phase subtracted, leaving the crossover group delay. The slight group delay ripple at 50 Hz correlates with the European AC mains power and is an equipment artifact, not part of the speakerís response.
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Jack Regula

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Re: End of review (almost)
« Reply #22 on: February 18, 2017, 12:43:30 pm »

A couple more graphs of the reflection free response before I show what happens when the strong borderline early reflection is added.

The first attachment is the GDR obtained by subtracting minimum phase.  This is the crossover group delay.  If the tool can show us the same group delay in the presence of a strong reflection, we should be impressed.

The second attachment is the PFR or phase frequency response.  We can see the crossovers in this phase response.
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Jack Regula

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Effect of a strong early reflection
« Reply #23 on: February 18, 2017, 12:47:06 pm »

Next, we will look at the same waveforms in the presence of a reflection delayed by 5 ms. and down only 3 dB.   The TDA 3D display shows twin blades at high frequencies, with the direct response blade bending towards the reflection as the mid-bass is approached. 

The effect of the reflection is seen more clearly in the pl graph in the 2nd attachment below.

TDA separates out the reflection with no difficulty evident above 1.5 KHz but with increasing difficulty as frequency lowers.  The direct response line broadens as frequency lowers and ripples of increasing magnitude appear in the direct response.

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Jack Regula

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Re: Effect of a strong early reflection
« Reply #24 on: February 18, 2017, 12:51:58 pm »

FFT-Q windowed AFR, GDR, and PFR graphs help us deal with the reflection.  By tightening the window, we limit the effect of the reflection.

Looking at the FFT-Q AFR with Q=8 in the first attachment, we see the expected comb filtering.  The windowing excludes the reflection induced comb filtering above 3 KHz.

With the FFT-Q lowered to 2 in the second attachment, more of the comb filtering is excluded:
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Jack Regula

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Re: Effect of a strong early reflection
« Reply #25 on: February 18, 2017, 12:54:08 pm »

Similar combing can be seen in the GDR with FFT-Q = 8 in the first attachment and reduced combing with FFT-Q=2 in the second attachment.
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Jack Regula

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Re: Effect of a strong early reflection
« Reply #26 on: February 18, 2017, 12:58:53 pm »

The reflection cause similar distortion in the phase response, shown in the first attachment.

However when we subtract minimum phase the undistorted GDR  and phase responses appear, second and third attachments. 

The GDR and PFR obtained by subtracting minimum phase in the presence of the strong reflection match those obtained in the absence of a reflection.  Thus, with TDA, one can evaluate crossover timing using indoor measurements in a reverberant environment.
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Raimonds.Skuruls

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Re: APL's Time Domain Analysis - Update?
« Reply #27 on: April 21, 2017, 07:55:42 am »

Dear Colleagues,

The new TDA IM intermodulation distortion measurement software is just added to TDA software family.
http://aplaudio.com/conc2/products/tda-im/
Now we can evaluate the IM distortion`s dependence  of frequency in form of frequency responses.

Beta testers are welcome to take participation in final testing!

BR,
Raimonds
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