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Old 23rd July 2004, 01:15 PM   #21
gmarsh is offline gmarsh  Canada
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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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Old 23rd July 2004, 03:19 PM   #22
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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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Old 23rd July 2004, 03:50 PM   #23
gmarsh is offline gmarsh  Canada
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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...
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Old 23rd July 2004, 04:07 PM   #24
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gmarsh,

Looks like were saying the same thing.

There are companies that claim to have 75ohm RCA's but really.
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Old 23rd July 2004, 05:19 PM   #25
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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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Old 23rd July 2004, 05:34 PM   #26
gmarsh is offline gmarsh  Canada
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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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Old 23rd July 2004, 06:09 PM   #27
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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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Old 23rd July 2004, 07:37 PM   #28
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Default Keep it simple

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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Old 13th August 2004, 10:21 PM   #29
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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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Old 21st September 2004, 05:55 PM   #30
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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 crap 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.
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