| Bas Horneman |
I can't find many diy designs on the net (many reference a certain brand/model of cable)...so last night I just took 1...tp from cat5 cable..and some 3m shielding tape around it soldered it to bnc on dac side and rca on transport side.
Good enough? Any suggestions for improvement?
I will listen to it soon! (Don't have coax dig out on my cd player)
Also ......brace yourself..this may sound very stupid.....how do you measure that it is 75 ohm?
Regards,
Bas |
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| Elso Kwak |
| quote: | Originally posted by Bas Horneman
I can't find my diy designs on the net (many reference a certain brand/model of cable)...so last night I just took 1...tp from cat5 cable..and some 3m shielding tape around it soldered it to bnc on dac side and rca on transport side.
Good enough? Any suggestions for improvement?
I will listen to it soon! (Don't have coax dig out on my cd player)
Also ......brace yourself..this may sound very stupid.....how do you measure that it is 75 ohm?
Regards,
Bas |
Hi Bas,
You can simply use double shielded TV/video/satellite 75 Ohm coax. But in order to avoid reflections you have to use 75 Ohm connectors and chassis parts. F-type connectors for satelliet TV would work but are totally hopeless in construction. The most readily available is 75 Ohm BNC.
Just ask for 75 Ohm coax, double shielded in the shop.
:cool: |
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| Bas Horneman |
| quote: | | Just ask for 75 Ohm coax, double shielded in the shop. |
Thanks Elso...I will...
Don't you think it is funny they use RCA though for co-ax out....when it is not a 75ohm part?
Still that leaves me with the question...how the hell do you measure it to be 75ohm? Either the cable or a RCA plug? (Or both)
Cheers,
Bas |
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| Elso Kwak |
| quote: | Originally posted by Bas Horneman
Thanks Elso...I will...
Don't you think it is funny they use RCA though for co-ax out....when it is not a 75ohm part?
Still that leaves me with the question...how the hell do you measure it to be 75ohm? Either the cable or a RCA plug? (Or both)
Cheers,
Bas |
Hi Bas,
A 75 Ohm RCA plug does not excist to my knowledge. And even if the plug is claimed to be 75 Ohm the chassis part is not.
For the cable you have to trust the vendor, but 75 Ohm TV coax is fairly common.
It is extremely curious that a RCA phono connector is the standard as described in table IV of IEC 268-11 for the consumer interface.
Ask Jocko or Fred if you are suspicious of my answer. They are the expert on this subject.
:angel: |
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| Giorgio |
AFAIK, the previous suggestions and considerations about RCA are absolutely correct.
I would just like to add a piece to the picture. The problem is that apparently it is the wrong piece.... it does not fit in the frame!!!!
Just to summarize: at tnt-audio we are testing a bunch of digital interconnects. We tested them under different aspects including audio jitter at the output of a DAC outout, smoothness of the transition in time ("eye" in the oscilloscope without neither too much rounding nor spikes), correct impedence, etc. and obviously sound.
What apparently results at present time is that the best cables (which are MUCH better then all the others) from the sound point of view are the ones with the most regular eye pattern, as expected.
The problem is that they are also the ones causing more jitter and with the impedence more different from 75ohm (one far above and one far below)... And this AFAIK is both in contrast with theory (and eye pattern results too).
The impedence test was performed with a MFJ-259, an antenna analyzer which is able to give the impedence module and phase of any complex system, and the measure sounds correct (the measures perfectly fit technical data for all the cables for which these are available...).
I could think there might be a problem in jitter measures, but even them have been performed so many times and sound so stables that I am not so sure about this explanation. The difefrences in jitter are anyway rather reduced.
Has anyone any idea ?
:xeye:
Next steps could be to measure the wire isolation from external disturbs, but the previous tests leavme me rather upset. Has anyone seen anything similar??
Thank you in advance
Giorgio |
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| Jocko Homo |
How many times do I have to say that???
I don't want to have to 'splain this again.
So Bas.........what you wanted to know.....
2 ways:
Measure it on a TDR,
Or better yet.........
Pay someone like me who has one to do it for you.
Hint, hint....have TDR, will travel.
Jocko |
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| DragonMaster |
| Why do you want 75Ohm?!? connection between 2 SPDIF components could be anything(until voltage remains in min-max range). A 75Ohm cable is not. VHF antennas are told 300Ohm but are infinite ohms, one wire goes to an antenna and an other one to an other. |
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| Bas Horneman |
| quote: | | Pay someone like me who has one to do it for you. |
What is your hourly rate? ;) |
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| Jocko Homo |
Why 75 ohms..........?
Because that is the spec for SPDIF..........0.5 Vp-p, into a 75 ohm load.
Yes....if you are making your own.........you can make it anything that you want. As long as the TX, RXC, and cable impedances are all the same.
As for antennas.............don't get me off on that tangent.
Antennas are not designed to "look" like a specifed impedance over a few decades, as transmission lines are.
Jocko |
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| DragonMaster |
| quote: |
Yes....if you are making your own.........you can make it anything that you want. As long as the TX, RXC, and cable impedances are all the same. |
:( I though you could use any piece of ****(like I did, and there was no problem) to make one. |
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| Giorgio |
Jocko,
You are right about antennas, I think that almost everyone knows that they are efficient only on a reduced band.
The MFJ-259, however, has a variable test oscillator and is able to measure complex impedence and SWR (and a lot of other related things) on a 1.7-170MHz range. Should be enough, in your view?
What I am not sure of, by the way, is if the SWR sensitivity is high enough. It is clear that 1 would be perfect, but what would be in your view an acceptable SWR, and on which range?
Many years ago, when I worked in telecom on baseband modems with biphase code the required bandwidth was considered twice the bitrate, but in my view this is ridiculously low, if we need precise edges. The eye pattern in fact was very rounded.
By the way, what TDR model have you? HP, isn't it?
Kind regards
Giorgio |
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| Jocko Homo |
As we say in Texas.
Yes, you would use an impeance bridge when measuring an antenna. And yes......I am old enough to remember the first MJF product..........along with the Omeag-T noise bridge. Almost went to work there after I moved to Texas.
However.........transmission lines are measured with TDRs, which are little more than a glorified step generator built into a very high speed 'scope. If you were stuck using an impedance bridge.......I would look to at least the seventh harmonic to see how low the rho (reflection coefficient) is.
On my fancy-schmancy SPDIF TX and RX boards.......the rho is around 0.02. Return loss greater than 30 dB, in other terms.
My TDR is an H-P......made in '63.....still working........bought military surplus......where it spent the first 30 years of its life at "Dickey Goober AFB", in Missouri.
Jocko |
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| lucpes |
| Short question: anything wrong with F-type connectors/RG59 or 'sattelite' 75 ohm coax besides the not-so-reliable appearance if it's not going to be much fiddling around with it? |
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| Elso Kwak |
| quote: | Originally posted by lucpes
Short question: anything wrong with F-type connectors/RG59 or 'sattelite' 75 ohm coax besides the not-so-reliable appearance if it's not going to be much fiddling around with it? |
Hi lucpes,
Yes they will work as my satelliete receiver is proving everyday.
The construction is sloppy as the center pin is the core of the cable and a pull on the cable will slip it out of the connector.
:bawling: |
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| Jocko Homo |
Not designed for repeated insertion/removal cycles.
Jocko |
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| CheffDeGaar |
Bas,
Elso's suggestion on TV/SAT cable works for me. But as our dear Jocko said, impedances must be matched all along the way, for start to end, including connectors. Making one's own SPDIF cable to achieve good performance doesn't make sense if you still use RCA connectors. IMHO, just begin to use 75 Ohms BNC connectors at both ends, and connect a 75 Ohms SAT cable between (Jocko prefers lengths > 1m, if I remember...) And make sure your TX and RX are 75 Ohms matched too (check the components on the PCBs). This includes replacing the existing input/output connectors on the equipments by 75 R BNC sockets. Once you have the BNC connectors, feel free to test any make of cable, provided its impedance is 75R... And keep the one you like the most.
If you want to be anal, get your hand on a 75R wideband reference load, calibrate your BNC-cable-BNC setup with the load, and TDR it...
But IMHO, your ears are your best measurement devices, and just try different existing cables. Of course you might want to diy one, but just keep in mind that you'll have to achieve a 75R impedance on a wide frequency range. You can find formulas on the web for various configurations of transmission lines, so take the configuration you find the most suitable for DIYing, and compute parametres to get the right impedance.
Just let your ears speak ;) |
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| jewilson |
CheffDeGaar,
My ears don't say anything but if they start I’ am going I'll have to see the doc about some better drugs.... :D
Well, I do listen to Jocko when he talking about the SPDIF interface since he know what he's talking about. I know that difference his attention to detail has made in the sound of my systems. And round two is coming up....Right
Man, you don't have to be anal to test your designs with TDR or a spectrum analyzer. He know how to get the best results because he not guessing. Too much of this golden ear **** leads to poorly design equipment. Don't get me wrong, listening is the final proof. Having said that, if you don't know where to start, then you don’t know where you’re going, then your lost.
:) |
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| Jocko Homo |
If all your impedances are amtched, then every cable should sound alike.
Ah....but we don't live in a perfect world.
As for the ears.....they tend to tell the truth, but we don't always know how to understand what it is telling us.
I think that I told this one before.....so I'll ty to be brief.
(Dragon Master ought to find this especially amusing.)
A big name audio guru from the West Coast made one of the silliest SPDIF cables that I have ever seen.
He could not understand how to make a proper build out resistor on th TX end, so he found out......by using his ears...that a 93 ohm cable sounded best. Lo and behold.......his ears then told him that it sounded better still with some goofy ham radio connector......also close to 93 ohms.
Because he was off by about 20 ohms too high on his TX impedance.
Too bad when you used in any other piece of gear that it sounded like dreck.
Only he wouldn't accept that, because:
It worked so well in his system.
True story.
Jocko |
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| Giorgio |
In theory I pefectly agre, I have been up to few days ago a strong supporter of perfect 75 ohm alignment... Now I doubt of anything...
As per previois e-mail in same thread, at tnt-audio we have been testing a bunch (8) of digital cables, and the acustic tests results were perfectly aligned on various different systems and with different reviewers; by the way, I am told they are aligned with world consideration (I have stopped reading reviews.... unless I am very interested in the unit ;-))). The only problem is that if we assume that MFJ impedance measures are vaguely correct, the best wires from the sound point of view are at FAR, FAR away from 75ohm...
And I have definitely not only no explanation for this, but looking to the live SPDIF signal on my scope, the best cables seem to have the best signa form, which again maks sense but does not match with theory.
One explanation, in my view, could be in fact be that the terminations are not correct, but I have not checked yet...
br
Giorgio |
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| gmarsh |
If you want to make a good "low jitter" SPDIF cable, here's my recommendations:
- keep the impedances matched... USE 75 OHM CABLE! CAT5 is 100 ohms, which means you'll certainly get reflections on the cable.
- Connectors usually don't have impedance specs, unless they're physically large connectors that are designed to move high frequency signals. If you're using largeish connectors (eg the right-angle PCB-mount BNC's from AMP that are about 1.5" long) then make sure you use the 75 ohm ones. If the connectors are physically small and don't have a spec, then don't worry about it.
And F connectors are good! The ones on the back of chinese TV sets aren't, but on most head end equipment at cable companies you'll find super-high-quality, sometimes even gold plated F connectors.
- For long cables especially, make sure the cable you're using has good shielding and little attentuation. You want the magnitude of the signal at the end of the cable to have as high a SNR as possible; after all, introduced noise can push around the detected edge of a clock. |
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| jewilson |
| quote: | | - Connectors usually don't have impedance specs, unless they're physically large connectors that are designed to move high frequency signals. If you're using largeish connectors (eg the right-angle PCB-mount BNC's from AMP that are about 1.5" long) then make sure you use the 75 ohm ones. If the connectors are physically small and don't have a spec, then don't worry about it. |
Hello............ you need to check out the parts catalog for BNC's |
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| gmarsh |
| quote: | Originally posted by jewilson
Hello............ you need to check out the parts catalog for BNC's |
Find me a RCA jack with an impedance spec ;)
BNC connectors almost always have impedance specs, because they're used well into the GHz range where a mismatched connector impedance can become measurable.
What I meant to imply was that at ~6-7MHz SPDIF frequencies, the connector impedance doesn't mean a whole lot unless the connector is large (eg the AMP connector I mentioned). Of course re-reading my post, this isn't all that obvious. I shouldn't post before my morning coffee... :D |
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| jewilson |
gmarsh,
Looks like were saying the same thing.
There are companies that claim to have 75ohm RCA's but really. |
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| Jocko Homo |
You can hear the effects of 50 ohm connectors in a 75 ohm SPDIF setup.
Type F connectors can look good.......since the centre conductor is used for the pin in the male connector. A technique used for a lot microwave stuff.
RCAs measure in the 25-30 ohm range.
Jocko |
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| gmarsh |
| quote: | Originally posted by Jocko Homo
You can hear the effects of 50 ohm connectors in a 75 ohm SPDIF setup.
Type F connectors can look good.......since the centre conductor is used for the pin in the male connector. A technique used for a lot microwave stuff.
RCAs measure in the 25-30 ohm range.
Jocko |
really... I haven't done any calculations on this (maybe i should) but the impedance mismatch caused by a 1 inch/50 ohm connector at 6-7MHz will be *very* small. I'd expect a much greater audible effect from the tolerance of the 75 ohm terminating resistor in the receiver than the impedance of the connector.
Which brings up an interesting point; would replacing the resistor on the input of a DAC create an audible improvement? And has anyone ever hooked up a vector impedance meter to the SPDIF input on their DAC? (which someone really should!) |
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| Jocko Homo |
Yes, I know......
My training as an engieer says that at frequencies below 300 MHz or so, it should not matter that much.
But......my ears.....those pesky things that can confuse us, tell me that you can hear it.
Vector meter?.......yeah, you could. But that is what TDRs do best.
And, yes......I have.......countless times over the last 13 years.
Most inputs look nowhere near 75 ohms....even the ones with 75 ohm terminations.
Jocko |
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| MalichiConstant |
You don't need to do any very esoteric measurements of calculations to see the reflections from 50 ohm BNC or ever worse a "75 ohm RCA" (there is no such thing but the new WBT RCA connector for video and digital coax look like a pretty good shot at it).
I have often used a method I call "real life TDR." Make you 75 coax about 10 or more and measure the reflection coming back from the load to source. The longer cable gives enough delay for the reflection to have enough delay to clearly separate it from the incident waveform. Drive this set up with a circuit that has a similar logic family and rise times. You can easily see enough detail with a 100 MHz to manipulate termination connectors, impedance mismatch and wiring effects. There is lot to be said testing with the waveforms the circuit will see in actual use rather than interpolating from a test signal that is much faster the one it will actually see. I have seen Jocko do TDR but lack his familiarity with the test set up and resultant waveform and how the it correlates to logic risetime seen in the actual interface is not something I posses. In this light I often feel like I am trying to tell what type of tree I am looking at by placing my eyeball about an inch from the bark without the benefit of being a tree surgeon.
Call me McGuyver but I like to make measurements with stuff pretty easy get a hold of and or build, a real handy method for
the typical DIYer, has a good sound card and Spectrum analyzer software from the web, and a scope, and has never even seen an Audio Precision test set up in real life. With much the test equipment out there today I can build a test set up from stuff around the lab faster than I can figure out the user interface software for the test equipment. I still have a theory that HP test equipment was so difficult to use because they felt you couldn't figure the user interface out that you were not worthy to use their test equipment. The knuckle heads just can't get it through their heads that you might only use the equipment every few months not like to spend four times as long as reading the manual as making the measurement. |
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| DragonMaster |
| quote: | | The SPDIF spec is 75Ohm load |
I've looked at some schematics having SPDIF(75), Video(75) or MIDI(220 Ohm) outputs and the 75 ohm load(220 for midi) is before the connector.
You can see it at:
http://elm-chan.org/works/yuv2rgb/report.html(Scheme download at the end, before the RGB out)
http://www.maxmidi.com/diy/serial/ (MID Interface, pin4 of the DIN5 is output, see the 200 ohm resistor right before?) |
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| SupraGuy |
I always believed that our ears lie to us all the time, and as such are not to be trusted as a "final authority" on much.
Case in point: I have a friend who fancies himself a real audiophile. Just to see, I gave him a speile about some cables, spewing pop Quantum Theory, and other such nonsense. I then did a "blind test" (He couldn't see) where I played a CD track using 3 different sets of cables. The first set was a regular, reasonably high quality set which were used from the CD to the DAC, and a nice set of RCAs from the DAC to the pre-amp. He was told that this was his "reference cable" -- ie: the one that he used normally.
We did not tell him which the other two sets were, but the next set we allowed "slip" a comment about cheap connectors while installing a $3 set of cables from Radio Shack. The comments were about the lack of detail and harshness of the sound. Okay, maybe these cables WERE inferior...
The third set was YET ANOTHER $3 SET. Here I muttered something about "careful with that one" while connecting it. My friend couldn't gush enough about the "openness" of the sound, and how superior in detail this was, and how these cables brought his system to life. Was he EVER ****ed at us when we removed the blindfold and he saw those cheap things attatched to his DAC. My partner in crime videotaped the entire "listening test"
Just an illustration of how our ears can lie to us. The placebo effect is alive and well in the audio world. Many a testimonial can be found insisting that some **** which has absolutely zero measurable effect can make a dramatic improvement in sound quality. Many more can be easily found stating that some product which has a distinct and measurable negative effect makes huge improvements. Subjective improvements seem to be directly proportional to the cost of the product.
For the subject at hand...
Cables should have as low an impedance as possible. I used to deal with coax network cable all the time, and the "good" cable always had very low impedance. Lower impedance cable could be used to make longer runs, and had lower instances of packet errors. Connectors had as close to zero impedance as possible, regardless, and for the on-board stuff, that wasn't something that could necessarily be "matched." But for the record, I've disassembled a number of old network cards, and there was no kind of matching on the BNC mounts, it just got the wire to the PCB, where the signal was routed to a decoder.
They were called "50 Ohm" cabling because there were 50 Ohm terminating resistors on either end of the chain to prevent reflections from causing more packet errors. I was so glad to rip out my last client's coax network years ago, and replace it all with cat5. :D |
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| Jocko Homo |
How can a connector have "as close to zero impedance as possible"? There is no such thing.
And why does cable TV use 75 ohms, instead of 50 ohms??? More loss, according to your logic......................
But you are right about the ears. Back when I built stuff full-time, I needed to find testers who actually could hear, but knew nothing about what I was doing to prevent bias.
The number of candidates who thought that they could hear a difference when there wasn't one was annoying.
Just because they heard me throw a switch didn't mean that I was actually swithcing the signal.
(I wasn't. It was on an idle piece of equipmnet. Yeah, I am a PITA!)
Jocko |
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| pburke |
maybe a dumb but related question: if I have a DAC that is small enough to fit into a transport chassis, what would happen if I hardwired SPDIF out to SPDIF in - no connectors, just a 6" short Coax cable maybe?
I'm sure there are better ways to connect DAC to transport if its in the same box, but if I have no idea how to tap into signals and how to mod the DAC to take the direct IS2 or whatever it's called, I am really limited to use the SPDIF.
Somewhere I read that 1.5 meters is the ideal caox cable length for SPDIF due to reflection decay, etc. How would doing the short direct link without connectors do in that respect, or is it really the connectors that are the issue and can I bypass the whole problem by hardwiring the DAC to the transport, short or long cable?
Peter |
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| Jocko Homo |
Who came up with 1.5 m??? The connectors are not the problem, and the worst sounding D/A box that I made had a 6" coax into it........
If they are in the same box...........there are ways around using a PLL to screw up the clock. That is the problem with SPDIF.
Jocko |
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| pburke |
| quote: | Originally posted by Jocko Homo
Who came up with 1.5 m??? The connectors are not the problem, and the worst sounding D/A box that I made had a 6" coax into it........
If they are in the same box...........there are ways around using a PLL to screw up the clock. That is the problem with SPDIF.
Jocko |
so if 6" was bad, maybe there's something about the 1.5 meters?
this is where I saw it :
http://www.positive-feedback.com/Issue14/spdif.htm |
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| Jocko Homo |
His reasoning is ok, but I prefer longer.
Jocko |
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| ChuckT |
| A pro-audio guy told me that any solder point between the 75ohm cable and 75ohm connector and you'll screw up the total impedance. That's why the Canare 75ohm connectors are design without soldering required, but what about inside the equipment? |
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| Ouroboros |
TV uses 75R cable for historical reasons. Antenna co-ax cable is 75R simply because that's a good match to the impedance at the centre of a resonant half-wave dipole. (similarly for 300R twin-feeder for conection to a resonant folded half-wave dipole). They simply carried on using antenna cable even for the baseband video links.
You can detect the difference between 50R and 75R co-ax, even over short lengths using a TDR, but it is hardly possible to see the impedance mismatch between 75R and 50R BNC connectors.
Like many other people, I hate the idea of using RCA (phono) connectors for digital connections but they're here to stay for a while by the look of it. My choice woiuld be SMA gold-plated screw-lock, but who would be willing to pay the price?
Chris Morriss. |
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| fmak |
[QUOTE]Originally posted by Jocko Homo
[B]How can a connector have "as close to zero impedance as possible"? There is no such thing.
And why does cable TV use 75 ohms, instead of 50 ohms??? More loss, according to your logic......................
----------------------------------------------------------------------------------
All TV and video in Europe use 75 Ohm. Pity they use **** ultra cheap connectors though.
Koko, thought you were abandoning this forum?
:bigeyes: :bigeyes: |
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| ergo |
I'm also currently messing with digital interconnect problem.
At the moment my setup is following
Sony CDP-XB930 spdif tapped from transport PCB -> divided to proper amlitud by resistor network -> LM6171 video opamp as unity gain buffer with 75 ohm resitor in series feeding the 75 ohm BNC connector -> 75 ohm coax cable -> straight to Behringer DCX2496 DSP board -> Schott transformer -> 75 ohm resistor between secondary windings -> Crystal CS8420 receiver IC.
First I tested the setup so that the digital cable was not soldered to DAC, but instead to BNC connector with 75ohm resistor to ground. This connector went straight to oscilloscope input. When proper syncronisation was found the signal looked very good with minimal overshoot and no visible reflections.
I tried the setup with different transport also (cheap end Technics player). The BNC->RCA plug was used and the spdif output on Technics was in it's original form. The sound of the two systems were very clearly different. My own setup had much more air and deeper soundstage - so I must be on the right track.
Thank you goes to Jocko also. I was bombing his e-mail box a while ago picking his brain :) for possible implementations.
I quess there would be room for improvement especially in RX end and I intend to do it as soon as I get the analog stage on bar with the current level of digital end.
Regards,
Ergo |
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| ergo |
For reference this is the cable I used as 75 ohm coax
ELFA
Part code: 55-943-04
Ergo
PS. The long link straight to product does not work :( |
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| Jocko Homo |
Where do I start..........
OK........75 ohm cable has less loss than similar 50 ohm cable. The guy kvetching about replacing coax with CAT 5 is raising the impedance. Twisted pairs start at around 100 ohms, and go higher.
OK.....we aren't talking aboout 1/4 wave dipoles.......75 ohms is used because it has less loss in a DISTRUBUTED cable system, where loss is important.
So......you wanna show me how I can't measure AND hear the difference between 50 ohm and 75 ohm connectors. Bring your $$$$$.........I need it. [joke]
And just what did this Pro Audio guy say???? I think he mispoke.
Type F connectors......like many other connectors designed for high frequency use........do not have a centre pin on the male connector: it uses the centre conductor of the cable. At very high frequencies, it makes a difference.
And don't feed me that **** that Canare has 75 ohm RCAs, 'cuz the Laws of Physics say it ain't so. So there.
Jocko |
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| Ouroboros |
| I don't even believe that you can hear the difference between 75 and 50R co-ax on a short (less than 1 metre) length of interconnect, never mind the BNC connector, but if it keeps you happy to think you can, then it's OK by me. |
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| Ouroboros |
Cable loss is not simply related to the characteristic impedance of a cable.
To quote from a cable website:
How to minimize the attenuation in coax ?
For a line with fixed outer conductor diameter, and whose outer and inner conductors have the same resistivity, and assuming you use a dielectric with negligible loss (such as polyethylene or Teflon in the high-frequency range at least), then you get minimum loss in coax if you minimize the expression:
(1/d + 1)/ln(1/d)
where d is the ratio of inner conductor diameter to outer conductor ID. A spreadsheet or calculator gets you close pretty quickly: D/d = 3.5911 is close. Thr formula was claimed to be derived from the formula for coax impedance versus D/d and a formula for loss that you'll find in "Reference Data for Engineers" published by Howard Sams, on pg. 29-13 in the seventh edition.
The interesting thing to notice is that this minimum loss does not directly yield a line impedance: the line impedance depends on the dielectric constant of the dielectric. For air insulated line, the corresponding impedance is about 76.71 ohms, but if the line is insulated with solid polyethylene, then minimum attenuation is at about 50.6 ohms. So, however it came to be, all the RG-58 we use for antenna feeds and test equipment connections is pretty close to minimum attenuation given the above conditions, and that the dielectric is polyethylene.
But if the line uses foam dielectric with a velocity factor of 0.8, then the impedance of minimum atten would be about 61 ohms. However, that minimum is a pretty broad one, and you don't start loosing a lot till you get more than perhaps 50% away from the optimal impedance.
Note that foam-dielectric line with the same impedance and outer diameter as solid-dielectric line will have lower loss. That's because, to get the same impedance, the foam line will have a larger inner conductor, and that larger conductor will have lower RF resistance, and therefore lower loss. |
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| MikeB |
Hi !
I made my spdif-cable with normal 75ohm-antennacable, soldered
rca-plugs to both sides (nice golded) and ready ! It is 7meters longs,
and never showed up any problems. I don't hear any difference in
comparison with a 50cm toslink optical one...
DTS for example uses checksums to detect defect data (DTS-CD) and
simply drops frames if defect. I had never a single drop with this
7meter cable.
Mike |
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| Jocko Homo |
It isn't that simple. But someone invented 93 ohm coax........because they needed a lower loss one.
(Capacitance goes down as Z goes up. That part is that simple.....)
The point is some guy complained about coax, and bragged how muchj better twisted pair was, without having any knowledge of transmission lines.
I do......and yes, I can measure the difference. And I have demonstrated that people can hear the difference. If you want to believe that all cables sound the same on your SPDIF setup, then that is fine by me, 'cuz you surely won't be buying any cables from me anytime soon.
Right?
Jocko |
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| MikeB |
hmm, maybe i am interested in your cables if my amp is finished ? :D
But one thing i never really understood, spdif is a digital connection,
transfering 0s and 1s, as long data is not corrupted how can different
cables change the sound ? Even if they measure different, as long
as no data gets lost, there shouldn't be a difference. But, the only
possibility for affecting the sound would be if the carrier is used as
samplerate, without any correction. But wouldn't this be nuts ?
Just for comparison, i am downloading music from the net, nobody
would have the idea that the quality of the modemcable affects the
quality of this music ?
Mike |
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| fmak |
I do......and yes, I can measure the difference. And I have demonstrated that people can hear the difference. If you want to believe that all cables sound the same on your SPDIF setup, then that is fine by me, 'cuz you surely won't be buying any cables from me anytime soon.
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There are some guys over at audio asylum going ga ga about the Benchmark DAC being totally cable independent and jitter free! In my experience, those who claim that cables don't sound different have some weakness in their system. The more sophisticated the setup, the more it exposes cable differences. And direction too.
:smash: :smash: :smash:
ps: Jocko; thought you were gone? |
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| Jocko Homo |
Just busy trying to keep the doors open. Fixin' to get busy again in a few days. More money going out, but hopefully some will come back this time.
Anyway.........right......1's and 0's....that is all it is.
Until you factor in that you have to recover the clock, with a PLL, from a serial data stream.
Which means jitter......
And jitter has a value, and more importantly, a spectrum.
Once you understand that, it is obvious why cables and interface problems affect sound.
How exactly........well, no one can really explain that part. But the better the interface, the "less digital" the system will sound.
Ideally, if there were zero reflections, and no jitter from the PLL, all cables would sound the same.
But we are not in an ideal world.
Jocko |
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| Jocko Homo |
Here is a plot from my 40 year old TDR. I know that most of you won't understand it, but you can see how much a 50 ohm part on a 75 ohm cable mucks up stuff.
BTW......most input stages don't have a rho (reflection coefficient) as low as 0.027. Multiply that number by about 10............
Signoro TDR, aka Jocko |
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| tbla |
T heo D o R.......:xeye: |
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| DragonMaster |
| quote: |
Once you understand that, it is obvious why cables and interface problems affect sound. |
And PCB traces, and the cables IN the player, huh? ;)
..........:smash:
My Sony home theater came with a standard cable with orange plugs for SPDIF. And it's replaced with a Acoustic Research cable, that I haven't decided to buy since it's my cousin who got it + receiver+DVD for my mother. There's NO difference between them, but it remains true that it's
...:smash:
Sony
:D |
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| rcavictim |
| Does SPDIF stand for Spiffy Diffy? Sure, you can make Spiffy Diffy cables yourself! ;) |
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| rcavictim |
You guys are a tough crowd. OK, I'll take another guess. Is it "Sound (something starting with P) Digital Inter Face"?
This IS a serious question. I don't have all of the latest toys, so I'm not up on the latest trendy acronyms.. |
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| studiotech |
| S/PDIF = Sony Philips Digital Interface Format |
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| rcavictim |
| Thanx Studiotech. I NEVER would'a guessed that:) |
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| Jocko Homo |
I don't think that they ever expected that anyone would use it, and if they did............TOSLINK was good enough!
If they had their way in the first place, CDs wold have been limited to 15 kHz response, and 32 Khz sampling rate.
(One of the few things I recieved from all the money I wasted on AES dues.)
Speaking of AES........
The only thing worse than SPDIF was the original version of AES/EBU. I tried to tell them, but no.........................
Eventually, they fixed the obvious problems. Except for XLR connectors..........
Jocko |
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| fmak |
| quote: | Originally posted by Jocko Homo
Here is a plot from my 40 year old TDR. I know that most of you won't understand it, but you can see how much a 50 ohm part on a 75 ohm cable mucks up stuff.
BTW......most input stages don't have a rho (reflection coefficient) as low as 0.027. Multiply that number by about 10............
Signoro TDR, aka Jocko |
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:hot:
Jocko
WHY does your GIF leave a quick launch logo on my desktop; 83 bytes??
Fred Mak
:smash: :smash: :smash: |
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| Jocko Homo |
HUH???? No idea what that is, let alone why.
I'll e-mail you the larger version. Tell us if it does the same.
Hey........look at the bright side......it will be easier to read.
Jocko |
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| MikeB |
Okay, but this means, that if the DAC fed by the spdif uses the
samplerate delivered by the spdif-carrier directly, jitter gets audible.
If it uses the carrier only for syncronizing, recreating its own samplerate
with its own quartz, it becomes completely cableindependent, unless
the cable does not destroy bits, right ?
I ask this, because typically these signals are fed into a dsp, (at least
for HT-amplifiers) automatically rebuilding the samplerate...
This would explain why i heard no difference with different digitalcables.
Mike |
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| fmak |
| quote: | Originally posted by MikeB
Okay, but this means, that if the DAC fed by the spdif uses the
samplerate delivered by the spdif-carrier directly, jitter gets audible.
If it uses the carrier only for syncronizing, recreating its own samplerate
with its own quartz, it becomes completely cableindependent, unless
the cable does not destroy bits, right ?
I ask this, because typically these signals are fed into a dsp, (at least
for HT-amplifiers) automatically rebuilding the samplerate...
This would explain why i heard no difference with different digitalcables.
Mike |
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Not necessarilly; even a word clock signal cable on it's own affects the sound. I have a 3 wire sdif (not spdif!) 75Ohm connection to my dCS 954 and not only a5re sonics cable dependent but cable direction dependent.
Measuring the signals correctly terminated give almost perefect wavefroms and better than any dac I have seen!!
I think Jocko is probably right.
:smash: :smash: |
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| DragonMaster |
| quote: | | cable direction dependent. |
There's diodes, or what? (Let's make you remember that wires are made out of a big stack of purified cooper, and there's no direction in that thing) |
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| fmak |
| quote: | Originally posted by DragonMaster
There's diodes, or what? (Let's make you remember that wires are made out of a big stack of purified cooper, and there's no direction in that thing) |
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I have stopped postulating. Cable direction can be as important
and cable type. Purity of Cu does eem to come into it. The Japanese 8N stuff can sound ordinary.
Yes, the earth is flat. |
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| DragonMaster |
| Try them with an Auto AB comparating switch. And what if you changed the direction of your mains wires to see if it sounds different? |
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| Jocko Homo |
Yes, I have heard it, and it annoys me. I should be able to measure it, but all cables look like a bunch of squiggles on a TDR. Which squiggle dos what......I have no idea, but that must be it.
It better be it........!
Jocko |
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| pburke |
| quote: | Originally posted by Jocko Homo
Who came up with 1.5 m??? The connectors are not the problem, and the worst sounding D/A box that I made had a 6" coax into it........
If they are in the same box...........there are ways around using a PLL to screw up the clock. That is the problem with SPDIF.
Jocko |
Jocko - a followup on this because I just decided to add a USB interface to my DAC... and everything I can come up with is a USB to SPDIF chip or dongle type unit. Based on your comments it sounds like you wouldn't want to run SPDIF from one component to another without a nice long cable in between? But what if I kept the the lead length under something like 1 inch?
I'm talking using a PCM2902 or simlar chip to turn USB into SPDIF, then run a "cable" to the CS8412 in my DAC from that daughter card.
Am I looking at big problems here or is there a different logic between these chips than with real "cables"?
Peter |
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| Spoonfed |
Interesting thread.
I have also DIY my own Spiffy Diffy sorry SPDIF :)
Used RG6 quad shelded coax with full 'metal case' gold plated RCA's (the type with the sping bits... only sort that would fit RG6 that i could find :) )
The coax core has a very good contact in with the centre pin on the RCA..... solder was added only on the exposed side to keep in place.
I also have one of these http://www.eichmanncables.com/digital_cable/index.html
"proper" SPDIF cables i am trialing.
I "was" in the bits is bits camp with SPDIF..... i guess my previous receiver (as with seemgly many receivers) really do struggle in ablity to resolve such differences.
My new Panasonic XR45 on the other hand really does open ones eyes :)
The "home brew" vs "proper" SPDIF's do sound different. The thing is im not sure which is "right".... if that is even possible :)
The "proper"cable brings vocals a little bit more forward.... a little more full.... though perhaps a bit more "dry".
The "home brew" seems to have a bit more "air" with vocals staged a touch further back... and a little more "open" i think.
The "fall off" at the edge of vocals/etc seems to be a touch sharper with the "proper" cable.
i am really still quite suprised at the difference.... did not expect to hear such... esp as noticable as it can be on some tracks.
The thing is which is "right"????
The "proper" cable while having RCA's are "sort of" not RCA's with the outer "sheath" earth replaced by a "pin" extention of the cable..... this setup.. along with the "core" of the cable running full length inside the centre pin (even visable at the tip) perhaps goes as far as reducing the RCA's effects as discussed in this thread?
see http://www.eichmanncables.com/bullet_plug/index.html
But more importantly........ someone tell me if what im hearing from my "home brew" is better than the $120 AUD cable im near buying haha |
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| pburke |
those bullets are great for analog signal cables (I use them exclusively in those positions) but comparing them with a standard Canare BNC connection on my DAC was a no-contest win for the BNC. The advantages of the low mass (unshielded) plugs/pings just don't seem to work very well in the 75-ohm digital cable position.
I think a nice long 5 foot+ Teflon Beldon coax cable (1694A) with BNCs on both ends, connecting to high-grade bnc jacks on both components is the reference to beat.
I still don't know what to do with short runs - coiling up a long wire inside the component doesn't seem to be a proper approach when you need a 1" spdif connection between two chips, but I can't find much of an answer on this. I suppose I just have to build it and try it...
Peter |
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| Dr.H |
Tried 1695a as well as some other belden cable and must say that for SPDIF, 1695a is rellay good.
Comparisons were made against the following:
Monster M1000: Monster sounded bright, edgy, digital
Mark Levinson MDC 1: This was compared with 1695a on the following system: ML Reference DAC, Reference Transport, Reference pre-amp and some big ML monoblocks (not reference) with the B&W Nautilus Signature.
The MDC-1 was marginally better in that system than the FREE (got samples) 1695a. Even the hifi sales guy had to admit to this.
Worth trying. |
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