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Cables orientation, RMAA results
post Sep 13 2004, 01:21
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There is a french audiophile forum where someone foolishly asked if cables had an orientation.
You know, do they sound different if you switch the source side with the
destination side. http://www.homecinema-fr.com/forum/viewtopic.php?t=29744347

21 pages and 3 monthes later, the discussion went on possible measurments.
So I did some RMAA measurments between the two possible ways of plugging a
line in-line out coaxial cable. I wouldn't talk about it here if I hadn't got some
significantly different results, according to the orientation of the cable.

The cable is an RG179bu, silver plated copper, 75 Ohms. Hand soldered with 2%
silver alloy, on basic teflon / golden plugs.

Picture of the opened plug.

The cable is covered with white tape, so as to preserve silver from light, since the
cable is a bit transparent. The audiophile myth about cable orientaton says that a
cable has normally no orientation, but slowly gets one as and when it is used. It
would be a burn-in phenomenon. This one was plugged in one direction for one
year, and reversed three days ago. It worked for several hours during these three
days. This orientation is the one called "Bon" (=good).

With RMAA 5.4, ( http://audio.rightmark.org/index_new.shtml ), I generated a 44100 Hz 16
bits test file. I burned it on CD, together with the calibration signal.
The CD player is a Yamaha CDX-860 from 1991. It plays CDR bit perfect. The
CDR was scanned by KProbe with a JLMS XJ-HD165H drive at full speed. The
maximum C1 error rate was 14.
The CD Players plays the CD. The cable is plugged between the fixed line output
of the CD player and the line input of the Sony DTC55ES DAT deck, that works in
48000 Hz 16 bits. The analog input level is set so that the -1 db RMAA calibration
signal is at -1 db. The digital output of the DAT deck is recorded bit-exact by
SoundForge into a 48 kHz 16 bits wav with a Marian Marc 2 souncard.

In order to evaluate the accuracy of the measurments, I repeated 10 times each recording.

We can see that there is one measurment that gives different results according to
the orientation of the cable : intermodulation distortion.

Here are the complete results for measurments 1 and 2 (I added the swept
frequencies with FrontPage).


We can see on all graphs a difference between the "bon" and the "mauvais"
orientation (Good / Bad) : in the good direction, odd harmonics of 50 Hz (150,
250, 350 etc until 950 Hz) are higher.
It means that in the good orientaton, the cable gets a 50 Hz signal, quite squared
(fundamental + odd harmonics), in a stronger way than in the bad orientation.
Here are zooms on the 1 and 2 intermodulations. The Intermodulation Distortion
(IMD) are the 6040 Hz and 7060 Hz peaks. The cursor displays the 7060 Hz values.

I don't know how the total is calculated, but the difference between the two
measurments is of the order of magnitude of 1 db. That here stands, if we convert
in %, for 1 significant digit. Thus the differences in the tables, that affect the
second significant digit, are too small to appear on these graphs. But we can zoom
for example the graph one :

And there is something weird here : the IMD is clearly inferior in the good
orientation than in the bad. Both at 6040 Hz and 7060 Hz. I don't see why RMAA
says it is the opposite ! It is probable that the IMD values in the table don't come
from the real IMD peaks, but from all the neighbourous noise that is visible
around. I don't know where these 20 Hz spaced peaks come from.

Whatever, there is a measured difference between the two pluggings.
Three factors have changed between them :

-The cable position (can be closer or farther to power supplies, or AC cords)
-The contact quality (possible additional fingerprints on the plugs, plugs more or less well plugged)
-The cable orientation.

The 20 recording were made with the cables in quite similar positions. Let's test
the influence of the cable position, trying different ones :
Position 1 : same as previously
Position 2 : The cables are inserted between the CD player and the tape deck.
Position 3 : the cables are pulled away from the machines (but there is still an AC
cord near them).

We can see that the position has no effect on the IMD value.

I have made two last recordings after that plugs are cleaned with 90° alcohol.
Still the bad orientation, position 1, then good orientation again.

This time the good orientation value is very close to the bad one. It doesn't match
the results previously got in the right orientation.

It seems thus that the main factor is the contact quality of the plugs.
In order to test this hypothesis, I'll have to make 20 other recordings, but
alterning the orientation, so as to see if the result varies randomly (because of the
quality of the contacts), or alternately (because of the orientation of the cable).
Now that I forgot which was the good and the bad orientations, I'm going to let the
cable burn in a little in a given orientation wink.gif
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