Jon_Risch's Web Site

Jon Risch's Web Site

New Test Signal, Cont'd
What I wanted was a spacing ratio that would make as many of the products spaced away from one another and from the original tones as much as possible. Eventually, I found that the ratio known as Phi, or the Golden Ratio, did the trick. I started out with simple multitones, and gradually developed several different multitones based on various permutations of the number Phi, including submultiples of useful ratios added to integer numbers.
Besides the use of same spacing intervals, and integer spacing ratios, one of the original multitone test signals, the one that coined the term spectral contamination, used either entirely or primarily out of the audio band ultrasonic signals. The idea here was to stress the component under test and see how it reacted to this stress in terms of in band audio distortions. It also used same frequency spacing with the attendant problems. -
Out of all this came a core of useful test signals that all have their place in terms of sensitivity to various effects and distortions.
There are two basic types: evenly spaced tones using the same Phi multiplier on each subsequent tone, and split band types which concentrate a set of tones at various portions of the spectrum, leaving large areas of the audio spectrum free from stimulus tones. In both cases, the IM distortion products, the cross modulation products, and most all of the harmonic products are free from cover up by the original tones, and free from stepping on one another.
For the distortion products at higher orders that do come within a closer spacing that can easily be distinguished on a spectrum analyzer, there is a simple technique that allows positive determination of the source of the distortion. Again, this is something that was not only impossible with some of the other multitone methods, but was made practical for the first time due to the avoidance of multiple and close stack ups.

Final Test Signal Versions
In addition to working out this spacing relationship, I also came up with a spreadsheet that allows easy determination of the source of a particular distortion, which when used in conjunction with the technique mentioned above, allows you to exactly determine which two (or more) frequencies were the source for that particular distortion product. Final Test Signal Versions Here are the the final versions of the test signals I developed:
Phi 6 Spectral: 100, 261.8, 685.4, 1794.4, 4697.9, 12299 Hz. Individual level of each tone is at -15.6 dB.
Phi 12 Revised Spectral: 100, 122.0, 261.8, 348.2, 685.4, 987.0, 1794.0, 2870.4, 4697.9, 6765.0, 12299, and 16358 Hz. Individual level of each tone is at -21.6 dB.
Phi Low-High Split Band Spectral: 100, 116.18, 134.98, 156.80, 182.19 and 4697.9, 5927.8, 7479.7, 9437.9, 11909 Hz. Individual level of each tone is at -20 dB.
Phi Low-Mid Split Band Spectral: 100, 116.18, 134.98, 156.80, 182.19 and 986.99, 1245.4, 1571.4, 1982.8, 2502.0 Hz. Individual level of each tone is at -20 dB.
Phi Tri-Band Spectral: 100, 116.18, 134.98, 156.80, and 986.99, 1245.4, 1571.4, 1982.8, and 6764.9, 7618.5, 8579.8, 9662.5 Hz. Individual level of each tone is at -21.6 dB.
For my tests, I generated the tones in the digital domain, burned them to a CD-R, and used a squeaky clean CD player as the test signal source. Analysis was performed on a 16 bit FFT based system, using 8K point FFT's. This results in 4096 real data points within the audio band, resulting in an approximately 5 Hz bandwidth for each FFT frequency bin. Other instrumentation for FFT spectrum analysis is available that is capable of more than 16 bits, with resolution up to 64k FFT's, which makes the FFT bin width about 0.6Hz wide. Some of these cost a fortune, and others as little as a current PC with a studio grade sound card and the appropriate software.
During the course of testing, it was found that CD players had measurable amounts of IM distortion, and all measured different using these test signals. So much for all CDP's are alike. I used a CDP that had near source signal levels of output distortion so as not to compromise the dynamic range of my measurements. To my knowledge, no one has published anything about these IM distortion differences, probably because they do not tend to show up with traditional two-tone IM tests as well. One player showed lower SMPTE IM and harmonic distortion levels, yet had significantly higher levels of Phi Spectral IM distortion than another player. Many other measurements were made as well, a very high quality cassette deck was compared to a mini-disc deck; digital electronics were tested and compared, raw loudspeakers and loudspeaker systems were tested and compared. Each one of these audio components reacted in their own specific fashion to the test signal, and in most cases, levels of the traditional IM and harmonic distortions were inconclusively low between the various component test pairings. Sometimes one unit would look significantly better using traditional measurements, yet end up measuring much worse using the multitone. The correlation between which unit sounded good, and which didn't followed the Phi Spectral measurements almost exactly.
Of course, there was one other audio component that was measured: speaker cables.

Check out other Spectrals
A quick note: one of the current spectral contamination measurements is currently being used at Mix magazine for studio monitor reviews.
Check it out at : http://www.mixmag.com/ and click on the "Tannoy Reveal 2-Way Reference Monitors" link, and go down to Fig. 4. Note the number of tones at even frequency intervals.
One other thing about my version of Phi Spectral signals. They have a very high crest factor, on the same order as the amount of individual tone reduction factor, or a little less. That is about 15 to 21 dB of crest factor. This is also more like real music, instead of a single sine wave with it's 3 dB crest factor.

Testing Speaker Cables with the PHI Spectral
Now that I have covered a little bit about the test signal itself, I will go into the actual cable tests. Explanation of Measurements In measuring cables, I hypothesized that for bi-wired speaker cables, the big difference would be in the current flow, as the impedance’s of the crossover would create a preferential situation with regard to the flow of current in the woofer cable vs. the tweeter cable. To check this, I used a Pearson Electronics Model 411 current sensor (http://www.pearsonelectronics.com/) to measure and analyze the current flow in the speaker cables. No other method of measurement will yield the same information, as a series resistor to convert the current to a voltage will introduce it's own resistance, tending to swamp any speaker cable differences out. Voltage measurements will not provide the same information, unless you are using a resistor load, without crossovers and actual speakers involved. I initially tested a single 12 gauge zip cord against my cross-connected 89259 in bi-wire mode, that is what was published in my AES preprint. The deadline for that paper was June 15th, since then, I have tested various combinations of gauges, various retail audiophile cables, all of which have been tested in bi-wire and single cable modes. While I have found differences between single speaker cables of 3 and 4 dB between the various cables in single wire mode, which I believe to be repeatable and consistent, but these small changes would no doubt be questioned endlessly by certain parties, so I make no definite claims for these results, other than to relate them for future study. Perhaps a 20 or 24 bit analyzer with the appropriate resolution can quantify these cable differences more clearly. Gauge did not strongly influence the distortion levels, the difference due to smaller and larger gauges were well within the measurement error limits, while the differences between audiophile cables and zip cord were generally higher, although not above a level beyond any doubt, as I have indicated above.

Analysis
When I obtained the distortion plots for the bi-wire vs. single wire conditions, I was careful to analyze the results, so as to fully understand what was going on. At first, I felt that the reductions might be due to the simple aspect of the crossover impedance’s, and that perhaps the distortion reductions followed the crossover curves. Close examination revealed that not only does the distortion plot NOT exactly follow the curve of the crossover current flow (this can be examined by looking at the web site, page7 vs. page 8, and comparing specific frequencies. I posted a set of example frequencies and discrepancies earlier in response to concerns about the crossover roll off being the only valid effect here.) but for the Phi Spectral signal, the crossover roll off DOES NOT DIRECTLY AFFECT THE DISTORTION PRODUCTS AT THAT PARTICULAR FREQUENCY because they are not generated by any frequencies at that location in the frequency domain, but by the bundle of tones at the low end and at the high end! The distortion products would not be affected by the current flow (or voltage) frequency response at that point on the graph!!!! In order to make this perfectly clear, let me give a concrete example. For instance, the distortion spike at just above 1 kHz in Fig. Z on page8 at the web site. It is about 25 to 27 dB higher for the single cable than it is for the bi-wired tweeter cable. Now even if we completely ignore the fact that this distortion product derives from a pair of original tones that are either in the 100-200 Hz band, or in the 4.7 to 11 kHz band, and pretend that it is tied precisely to that point in the frequency domain, there is something inconsistent with the accusations made: the current flow for just the tweeter cable is only down 15 dB from the peak current flow, not 25 dB If we look at the current flow for the single wire cable, it is at a level that is only 4 dB higher, not 25 dB. I will post this graph, to be called Fig. W, on my web site at page9.

CONT'D

Link to next page, analysis continued.

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