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Using a coathanger as a digital cable


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#1 of 45 OFFLINE   Chu Gai

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Posted September 01 2003 - 04:17 PM

The link where this was first done, http://www.magnani.n...ireLabTest.html no longer exists. Thanks to http://web.archive.org it still lives on...at least for a few more years. What follows are the details found on the last link for that page.

Quote:
*** "A delicate digital AC-3 signal originating from my $4500.00 Theta DaViD transport THROUGH A WIRE HANGER?!?" ***

Calm down Mrs. Crawford, yes, through a wire hanger... It'll work just fine...

Ok,

So if any of you followed the digital wire wars a while back you may recall that some people maintained that you absolutely, positively, NEEDED a 75 ohm digital cable to connect your DVD player to your pre/pro... Nothing else will do... Anything else, and you risk errors in the bitstream so bad, that they are not only uncorrectable but you will also lose that "smooth airiness on the highs; open and more believeable soundstage; (insert your favorite audiophile tripe here)" and that basically the more money you spend on a digital cable, the more likely you are to achieve a sonic nirvana.

And you may or may not remember that after some rumblings and an e-mail from Jon Wenger, I built "The Finest Digital Transfer Wire In-the-WORLD!"® by taking two blue painted wire hangers and cutting an old, cheap, ugly green rca patch cord in two, soldering the ends onto the wire hangers.

You may remember my initial listening tests between that, a cheap, ugly yellow patch cord, a proper RG-6, 75 ohm cable with gold rca's, and the optical tos-link for comparison, yielded no discernable results, BUT there were too many other factors, like my ears may not be as golden or magical as someone else's, or that the toslink was shoddy anyway, etc.

Well, with Jon's help, I have now finished lab testing the cable.

For the dvd player, we used a professional version of the Sony 7000 reference player. It's actually model number DVP-S7000TP, serial # 2023. This is a pretty cool player... it has a nice gray matte, professional looking face, with a really cool rotary region selection switch near the headphone jack! For the processor we used a Dolby Labs model number DP562 multichannel ac-3 decoder, serial # 500280. The very cool feature, which is very necessary for our scientific experiment (since my pedestrian ears can't be trusted to be refined enough for the audiophiles whose heads travel in extra rarified air,) of this Dolby produced decoder is that it will do a bit error rate count... Yup, it will count each and every error it sees... Which is crc (cyclic redundancy check) protected which means the odds of having multiple errors such that the crc check passes an error in the data stream, is almost impossible. The output of the dvd player is an rca coax connector, and the input to the decoder is an XLR balanced connector. Jon normally has a Canare XLR to rca wire connecting the two.

I brought all my wires in case the number of errors that the wire hanger wire rolled was so great, that we would want to try the others and tabulate results... If you would like to see a picture of the wires (including the Sky-Blue/Lime-Green model of "The Finest Digital Transfer Wire In-the-WORLD!"®) go to: See a photo of the wire on my main Home Theater page.


Now we initially had a little problem with the hookup because we planned on using an rca female to female adapter and putting the test wire right between the Sony 7000 and the Canare cable, BUT we could not find the female to female... Not wanting to give up without giving it the old college try, Jon found two wires with alligator clips on them, so we used those.

So just to recap this thing to death, we had: The professional Sony S7000TP reference dvd player, going to a 20 year old, ugly green, rca patch cord which was cut in two. On one side of the green rca I soldered a blue painted wire hanger to the shield and another to the center conductor. I soldered the other ends of the wire hangers to the other half of the ugly green rca patch cord. We then clipped the alligator clips with thin wire to the centers and shields of the rca connectors of my cable and of the Canare cable, and then plugged the other XLR-balanced side of the Canare into the Dolby Labs decoder. I honestly did not know how badly we would be rolling errors on this one... and with open, scientific minds, we played a dvd...

Are you all sitting down? Good. We played the King Crimson Deja VROOM dvd for over fifteen minutes with this configuration and not only did it sound good, but the Dolby Decoder reported ZERO errors... Did you all get that ok? ZERO, nadda, nihil, zippo, nothing, none... error-free. Given that, there was not much point in trying the higher quality cables I had brought with me.

Conclusion: IT DOES NOT MATTER WHAT YOU USE FOR YOUR DIGITAL CABLE CONNECTION BETWEEN YOUR DVD PLAYER AND YOUR PROCESSOR... YOU CAN BASICALLY USE ANYTHING THAT LOOKS ELECTRICALLY LIKE METAL. IT WILL WORK JUST FINE. (Between the coax connectors that is... I have not successfully used the wire hangers on the tos-link optical connectors... yet...) If you like you can hook the shield on one side to one of your Rodan bronze statues, and the other shield to somewhere else on the statue, and then hook the center conductors to another Rodan bronze, and it will work JUST FINE. The sonic clarity will be stunning, as well as visually pleasing.

So you can use the rca cables they throw in for free into your component boxes that you've been meaning to throw away for months because they offend your sensibilities, or you can spend hundreds of thousands of dollars on Rodan Bronzes and use those, or anything in between... but please do not tell me that it sounds "warmer, cleaner, more open" or anything like that BECAUSE of the WIRE... It may sound different to you or to others... There are lots of reasons why it MAY sound different, not ALL of which are in your head... If it in fact IS different, then this difference is NOT due to the $800 wire you just swapped in. It may be that your player or decoder are somehow defective, or not designed very robustly (i.e. - if there is a difference that is NOT your imagination, then something is broken and you should not be wondering about the wire... you need to look elsewhere for the answer...)

Lastly, if you remember, my original preliminary offer was to sell this cable - "The Finest Digital Transfer Wire In-the-WORLD!"® to whomever wanted one, for a check or money order for $750.00 and two soiled shirts and I would send you the cable, in your choice of designer sky blue with lime green ends, or designer bone shell white with banana yellow ends, and your shirts laundered... However, in light of the new lab-tested/hand-made nature of the cable, I am going to have to bump up the price. Yes, you guys had your chance, I am going to have to raise the price to $1249.00 and two soiled shirts. Keep in mind that this is for an error-free, lab-tested on official Dolby lab's equipment, cable. (Don't worry Jon, I'll split the profits with you for an occasional, over the weekend testing session!)

I've pretty much completed this experiment to my satisfaction, and believe I have beaten the proverbial dead horse to a bloody pulp... If someone thinks I may have overlooked something however, my mind is not closed on this topic, and I would be happy to address any serious scientific hypothesis regarding this or other results... Thanks for paying attention... It's been real... Remember, do have fun with your hobby... you are doing this for fun aren't you?



#2 of 45 OFFLINE   Kevin C Brown

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Posted September 01 2003 - 07:15 PM

I also heard about a similar experiment where someone actually used chicken wire. Same results. But...

In both cases there was no measurement of (uh oh...) jitter.

But if it's true that there was no audible difference (A/B/X blind? Maybe not...), then the (any) jitter introduced might not matter.

In this case, they only checked for errors, no mention of sound quality itself? Plus doesn't look like they looked at PCM either, which is usually a higher bit rate than DD.
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#3 of 45 OFFLINE   John Royster

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Posted September 02 2003 - 02:55 AM

I call BS. Especially the "testing" methodology.

When testing a digital cable you use an injector on one end and a receiver on the other. Then you run patterns and sweeps across the entire frequency range and determine if what-goes-into = what-comes-outta. You never rely on actualy end user equipment to test digital transmission. Its just not reliable.

Do we know the sensitivity of the receiver? Was PCM tested? What was the bit-rate?

#4 of 45 OFFLINE   Doug_H

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Posted September 02 2003 - 03:27 AM

I have always understood the need for good cables when sending an analog signal but I have a hard time grasping why they are needed for a digital audio signal such as AC3.

I don't know the answer so I am not being a smart ass.

To me you are sending 1's and 0's so I can't imagine how a cable could affect the sound. You either get the data or you don't. Any change in the sound would have to take place in the processing of the digital data once it is received.

So where is my logic failing me?
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#5 of 45 OFFLINE   John Royster

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Posted September 02 2003 - 03:37 AM

Doug,

The problem stems from the fact that you aren't sending 1s and 0s on the wire. You're sending an analog signal that represents digital data.

Sure, sending digital signals over a wire is no big deal. We do it everyday we pick up a telephone, use the internet or cell phone, etc. But from the prospect of sending digital data over a cable at speeds greater than 1 megabit/sec, the cable is everything.

Now I'm not saying one needs some super duper cable. Heck, any RG59 or RG6 coax will work. Twisted pair will work. Its how well does it work?

There are specifications on just how much impedance can vary, how much crosstalk is allowed, how much return loss is allowed. If a coat hanger meets these, then great.

However I did not see it tested. Only a "hey our processor didn't show any errors". Which from a datacommunication standpoint is totally unreliable. There is more to data communication than a bit error. Bit errors are real easy to detect - if the checksum on the packet doesn't match the receivers calculated checksum then the packet is corrupt. This is the simplest test there is. But it isn't the only error.

Jitter, clock slips, bit error in the encoding but not the payload come to mind.

Just wanted to point out the common misconception of "its just 1s and 0s". Because it is not 1s and 0s. Its ones and zeros encoded into a bit stream that is put on a analog wire (hence an analog waveform).

#6 of 45 OFFLINE   Matt_Doug

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Posted September 02 2003 - 05:08 AM

I'm confused Posted Image
according to the quick start guide for the DP562 on Dolby lab's website the cyclic redundancy checksum (CRC) is used to check audio & video timing errors in the mpeg decoding process not errors within the data blocks. Please correct me if I'm wrong?

shielded twisted pair (stp) is the standard transmission line of the computer industry. Ridiculous pricing aside, I find it curious that when A/v cable manufacturers implement stp it its considered voodoo & market speak by some including said EE's

#7 of 45 OFFLINE   Neil Joseph

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Posted September 02 2003 - 07:25 AM

Ah yes, the ol' coat hanger.

I am willing to bet that it would work but there would be intermittant dropouts.
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#8 of 45 OFFLINE   John Royster

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Posted September 02 2003 - 08:51 AM

As an after thought maybe if I get time or bored I could make up some kind of twisted pair-coat hangar-twisted pair cable and scan it.

Generally even a foot of no-twist causes the cable to fail above 1 Mhz.

#9 of 45 OFFLINE   Chu Gai

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Posted September 02 2003 - 08:57 AM

Well maybe Neil. They didn't have any dropouts though. Might not be my first choice in a connector, but it's pretty interesting.

John, if you've read some of the postings I've made over the past several months, where I cited some of the work done by the late Julian Dunn, a partial summary is that so long as the cable can transmit the information without any errors, the jitter is a non-issue. And really we're not talking about 100's or 1000's of feet where the effects become cumulative. We're talking a couple of meters. The reason why the impedance is overblown here as an issue is related to the length of the signal compared to the length of the wire. I mean, you may think HDTV is something fantastic, but let's get real now. It doesn't compare to a hi rez calibrated monitor now.

Got a coat hanger and some spare RCA's laying around along with a soldering iron? Knock yourselves out gentlemen Posted Image

#10 of 45 OFFLINE   Matt_Doug

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Posted September 02 2003 - 09:53 AM

Well maybe Neil. They didn't have any dropouts though.
As I read it a valid checksum only means certain parameters like the number of audio blocks in the frame are valid. It does not mean all the the bits that make up the frame are valid. So you can't say that there weren't any bit dropouts but you can say there were no invalid frames

#11 of 45 OFFLINE   Cees Alons

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Posted September 02 2003 - 10:35 AM

Yes, a valid checksum and a few more ECD bits do not guarantee a 100% that everything is OK. But on the other hand, that certainly doesn't mean it isn't.

Millions of bytes are sent over all sorts of transmission channels each day, and when was the last time you received an e-mail in which a letter was changed during transmission? Or in a post like this?

I'm sorry, but jitter is not a problem of the transmission cable between player and receiver. Once the bitstream is established and baked onto the CD or DVD, all that the equipment has to do (including the cables) is: make sure that same pattern is transferred to the place where it's transformed to analogue again. And as long as it does exactly that, without adding or deleting bits, there will be no discernible differences.

Of course there's a real-time aspect: if the stream is interrupted waaaay too long, it will be impossible to recreate the proper sound in time, but now we're talking about quite different time-lags than caused by the cables we're discussing.

Just suppose I would claim that the CDs I brought to my home by foot (from the shop where I bought them) sound so much better "to my ears" than the ones I carried in a car or a bus or train. And that those latter only started to sound more clear and transparent, with much better defined bass regions and more punch, after I took them back to the shop and this time walked them to my home. Would anyone believe that? Why not?

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#12 of 45 OFFLINE   Chu Gai

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Posted September 02 2003 - 10:53 AM

I believe you Cees. You da administrator!

#13 of 45 OFFLINE   Cees Alons

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Posted September 02 2003 - 10:57 AM

And you knew, of course, Chu, that I believed you, even though you're not an administrator here Posted Image.

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#14 of 45 OFFLINE   Chu Gai

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Posted September 02 2003 - 11:11 AM

Sheesh, italics for emphasis too!

I hope everyone understands that I'm not advocating coathangers, or statues as interconnects, just looking to put a little perspective on one of those engineering axioms, 'engineering is 2/3 knowing what you can get away with'. But you don't have to be a soldering pro to try the idea out.

#15 of 45 OFFLINE   Cees Alons

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Posted September 02 2003 - 11:55 AM

Chu,

I happen to know Al Magnani personally (even met him in SF). He's an electro-technical engineer and a very intelligent and critically thinking person.
He did the coat-hanger experiment partly for fun, partly because he's a born scientist and partly to make a practical point that could be checked in real life by his HT-friends.

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#16 of 45 OFFLINE   John Royster

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Posted September 02 2003 - 11:58 AM

I really do wish you guys would brush up on your EE, packets and digital clocking before you make broad strokes of "if its digital it will get there".

The analogy of E-mail and what not has no bearing in a real-time stream. That works on the concept of if there is a problem (and there very frequently is) the packet will be retransmitted by the sender. The e-mail application is actually totally unaware the there was a problem.

In our audio systems there is no retransmission, no way for the receiver to say to the sender "hey, that last piece wasn't right...can you resend it so I can add it to the total message and not be left hangin?"

This is dealt with everyday in digital transmission, from voice to video where you deal with real-time streams. And believe it or not every single receiver I've ever worked with actually measures jitter, signal strength, CRC errors, etc. Tap into the DA converter in your cell phone and the software - I can guarntee jitter is measured and stored. If jitter gets too high it will slow down the transmission. Ever hear an echo on your cell phone? That's jitter.

So, yes, jitter matters. Is it audible? Now that is golden question.

#17 of 45 OFFLINE   Cees Alons

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Posted September 02 2003 - 12:45 PM

John,

You're talking about cell phones. That's a different environment: both ends are RT, the generating and the receiving ends. Ether transmissions and relays in between.

DVDs and CDs are recorded. Buffers can be added, "read ahead" is possible. Retransmission too, if the designer chooses so.

But apart from that: jitter isn't a topic in a 5-10' (or even 45') long cable.

Cees

#18 of 45 OFFLINE   John Royster

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Posted September 02 2003 - 01:20 PM

Jitter and the other parameters I've described for sending a digital signal apply no matter what the media.

#19 of 45 OFFLINE   Chu Gai

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Posted September 02 2003 - 02:03 PM

Don't forget the coathanger now!

#20 of 45 OFFLINE   Cees Alons

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Posted September 02 2003 - 06:54 PM

John,

But heavily dependent in magnitude by the physical length and complexity of the travel-path. And the effect of tiny jitter is effectively annihilated by buffering. It's the ultimate result that counts, not the intermediate problems.

Cees