TDRing Digital Cables - MIT AVt

[House de Kris]

In another thread, comments were made about 'better' digital interconnects being closer to the ideal 75ohm characteristic impedance and that some 'audiophile' cables are not necessarily all that close. I kinda offered to measure cables for people, then measured a video cable that came packed in with a VCR. It measured 50ohms and that got me real curious. So, at lunch today, I ran over to the local audio shop and picked up a couple "real" digital cables to see how close to idea they are.

The MIT AVt (1 meter) packaging doesn't offer much information, or claims, about their cable. All I know is that it is Level 1 (their best), it has the typical MIT network at the destination end, it has a solid silver center conductor, and triple shielded. It does have some nifty RCA connectors that you can tighten down to your equipment. My price was $150, I don't know if this is typical street price or not.

TDR picture is below. The cable starts out right around 75ohms, but rises to a max value of 83 ohms closer to the other end, most likely due to skin effect. The propogation through this line implies air imprenated teflon dielectric. Also, the fast rise to infinity at the end of the line makes it look like the MIT network really works. A TDT would provide more information.

I'll leave it at that, since this thread is intended to only deal with the characteristic impedance of interconnects rather than sound.

 

[Toka]

What is that network box supposed to do, anyway? I read somewhere (damned if I can remember where) that someone took one apart and it was essentially a low-pass filter. This was on a pair of speaker cables mind you, but I guess the question still applies.

EDIT: On yea...big thanks for taking the time and (especially) the financial hit to do this...much appreciated!

 

[House de Kris]

In the digital interface cable, the network MIT provides is there, presumably, to compensate for known losses in the cable. In my comments in the first posting in this thread, I mentioned the rising measured impedance was probably due to skin effect, or ACR (AC Resistance), of the cable. This really seems like a huge amount of loss for such a short length. MIT claims this to be a triple shielded cable although the outside diameter is nothing out of the ordinary. To me, this implies the center conductor is an extremely small gauge, thus the skin effect even though the center is reported to be solid silver. The network then helps recover some of the edge speed which should be degraded with so much ACR.

I've attached another measurement picture I took, with TDT data included. The rising edge that comes out of this cable is indeed screaming fast. A testamate to how good their selection of network components are (which, I believe, is really only a single inductor). I'll also note that the characteristic impedance of the cable does not really rise as the TDR makes it look like it does. Rather, due to the ACR it is very difficult for the TDR to accurately see the true impedance of this cable. TDR pictures show Zo+ACR, and in most cables ACR is low enough to not influence the TDR picture.

All cables are different. For whatever reason, MIT chose to use this cable and then to compensate for the known losses. Their philosophy certainly seems to work, and work well. I'm not trying to be an MIT commercial here. Other methods, like the beautiful Belden or Canare cables, get similar output edge rates by using very good cables with minimal ACR and no networks. I'm not here to pronounce which method is superior.

I can't make any sort of comment on the MIT speaker cables since I've never played with them. Frankly, I have no idea what they are attempting there.

I enjoyed making these measurements, and I learned a lot more than I was expecting to. The majority of the cables I tested were a loan from another AK member. Thus, my total outlay of cash for this education was slightly less than $300. Hey, all in the name of science and AK!

 

[Jon_Logan]

Thank you for this scientific approach. I've been waiting for this. Obviously you've got access to an hp DCA with a TDR plug in and a 75 ohm cal kit. Awesome. It looks like, from your shot, the far end was open. Tha impedance is very flat at 75 ohms. I am sure I could get a similar TDR with Canare crimp BNC/RCA connectors wityh Belden 1550A (RG-59) for about $12 per cable. Actually, I think the Canare connector would have a better impedance signature. The left side of the scope shot (lauanch end) shows some impedance discontinuity. I think they probably open the braid and hand solder. The Canare's I think would do better here as the conductors geometry is maintained with the crimped connector.

But thank you for a real measurement approach. Loooooong overdue!!!

 

[Face]

Great thread, thank you!

 

[House de Kris]

The left side of the scope shot (lauanch end) shows some impedance discontinuity. I think they probably open the braid and hand solder.

There is a huge discontinuity at the launch end. I believe this comes from the RCA style connector itself. I don't believe it was designed for a specific characteristic impedance, or wide bandwidth. Here's a link to another cable I measured that started me down the path of suspecting the RCA connector.

Also, there a number of cables tested, not just the two in the stickies. Search on "TDRing Digital Cables" and "House de Kris" in this forum and you should find a nice collection.

 

[Jon_Logan]

Again, thank you for this type of measurement. I am fully into this type of approach regarding cabling. I think you are absolutely right about the discontinuity coming from the RCA connector. We've (Agilent) seen this with SMA connectors as well. I haven't had a chance to see the signature of the Canare crimpable RCA's or BNC's, but the cables geometry seems well maintained through the connector/cable interface.

If you ever get bored, I'd be happy to send you a home brewed Canare/Belden to measure I'm not worried, though. I think they are good enough for video (no ghosts) and my digital apps. Signal integrity (IMO) is a good way for validating high speed cables. Next would be BER. I think with those TDR/TDT results, the eye would be well open at Digital audio frequencies. You probably can tell that I'm not into the cable sound business.

 

[House de Kris]

We've (Agilent) seen this with SMA connectors as well.

Agilent huh.... I was at Agilent when I wrote the "TDRing Digital Cables" series. I worked in Santa Clara in building 54L. We have since spun-off from Agilent, and still do the same things we did under the Agilent name, and HP name before that. If I may ask, where abouts are you?

 

[Jon_Logan]

Agilent huh.... I was at Agilent when I wrote the "TDRing Digital Cables" series. I worked in Santa Clara in building 54L. We have since spun-off from Agilent, and still do the same things we did under the Agilent name, and HP name before that. If I may ask, where abouts are you?

I was building 52 Lower, HSIO>NSD>laid off>Avago SJ now. 54 was TMO yes? We had a 93k and tons of rack and stack. It's nice when you get to buy at cost! Nice to bump into ex hp. Some friends still work there in life science area. They said the building's all tricked out now. They knocked out the ceiling of Lobby 50 and it's all granite and marble. Corporate HQ ya know. Thanks for the good measurements.:thmbsp:

 

[wifihifi]

Great thread!!! I enjoyed reading it. Thanks for sharing.

 

[ke4mcl]

the boxes in those cables i believe are called zobel networks. a telephone company idea to keep impedance consistent on long runs that were of guaranteed frequency response.

 

[Wazz]

How short does a cable have to be before none of this makes any difference? 2 feet, one, 3 inches? What is a troublesome cable discontinuity at 4 GHz is maybe not a problem at 1 MHz? With all the gear someone should be able to show a difference in a data cable or not.

 

[House de Kris]

If you could define the point at which all human's hearing threshold poops out, we could work at getting an answer to your question. But, all people hear differently, therein lies the rub. Plus, all DACs behave differently (such as sensitivity to such things), compounding the difficulty in defining what is audible and what isn't.

The judgement of good and bad of cables was not the point of this series of threads on digital cable measurements. Rather that, ALL cables measure differently, thus it should not be a surprise that some people can hear differences between different digital cables. And that, if all cables are different, perhaps experimentation to find a cable that works best for you is indeed worth your time.

You are on your own to determine if you can hear a difference, and if so which is superior. No one, especially faceless people on the Internet (like me), should tell you what's the best cable, or set arbitrary rules about when cables don't make a difference.

 

[whoaru99]

So another question is would a Canare DIY cable work for these digital outs, if not, can you make a DIY cable that would work or is it best to buy a cable that is made for this?

I'm like a sponge trying soak up an ocean!

The Canare twisted quad cables are technically incorrect material for consumer coaxial digital audio interconnects. Coaxial digital interconnects, by spec, should have 75-ohms characteristic impedance. Will the Canare cable work, yeah, almost certainly will over any reasonable distance. Is it "right"? No.

What's wrong with the Monster interconnects you have?

 

[whoaru99]

Probably asking the wrong guy. I like a decent quality cable, something nicer than the freebies, but beyond that into the "sound" thing, well...I'll just stop there.

http://www.bluejeanscable.com/store/digital-audio/index.htm

 

[Patachu]

watching people spending over 10bucks on a 75ohm cable, for a digital signal transmission purpose reminds me at which point people doesn't know jack shit about the laws of electronics and physics. thanks for this thread, guess it's gonna serve as a good demonstration on how the only thing to keep in mind is the impedance of the cable itself.

still, we're on signals that are below or equal to the 10MHz range, in some contexts i bet even an RG/58 CU cable on a short distance wouldn't bring a single loss.