Red: I've heard very similar sonic traits when using certain analogue cables as digital cables. Since the impedances between transport to DAC would normally be very different from DAC to preamp, another line level source to preamp or preamp to amp, i don't know why this is. Having said that, some folks that i've talked to that have cables built to their spec insist that every manufacturer introduces their own sonic signature into what they make. That is, the grade of copper or silver, how the wire is formed or drawn, etc... all plays a key in the sonic signature. As such, some folks have had cabling made to the same spec by several different manufacturers and they all sound different. This is the kind of stuff that boggles the mind yet would probably reveal quite a bit if someone were willing to devote the time to research the subject. Then again, maybe that's why "audiophile" cables cost so much i.e. we have to pay for all of the R&D that's taking place : ) Sean
> |
I have a question for the engineers here. When playing around with cables I found something odd. Let me give you an example. If I used a particular belden cable design, albeit in different guages, using all teflon insulation and silver coated copper, I found a consistent flavour to these cables regardless of their application. You will have to trust me that I verified this with some blind testing, but once I recognised the flavour I could always hear when one was inserted into the system. This happened whether a fat one was used for a power cable, or a speaker cable, or a thinner one was used as an audio interconnect or a digital interconnect. I am not saying I didn't hear other things about these cables, but I am saying that there was a consistent additive quality, perhaps some form of ringing, regardless of the application of the particular type of cable.
I recently added such a cable to power a CD player that was connected to the system I was listening to but was not the source I was listening to and immediately heard its sonic signature. I was not expecting to hear it, and was stunned to hear it, so I don't think the issue was psychologically induced.
So other than me being mad (which is a whole 'nother question), is it possible that any cable connected to a system is an antenna of some sort and introduces audible effects even when used as a digital cable, provided there is some analogue component that is electrically connected in some way to that cable? |
I use the i2s stock cable.Would upgrading this type of cable help my Dac & Transport sound better? If so, HOW? Are silver conductors better then copper for this type? |
Sean, "departure"? What did I miss? |
For sake of clarity, i am not Sean125. I didn't want anyone to confuse my departure with his arrival. Given our different points of view on power cords, it should be obvious that we aren't one and the same. Toodles... Sean
> |
Unlike other areas in cable design that are questionable to say the least there are actually scientific reasons why digital cables have to be correctly designed.So unlike power cords differences actually can exist in digital cables... |
As there are in power cords great differences are to be heard in digital cables..Much to do about time and phase coherency. Tom |
Check out bryston's website.They have a simple yet accurate response to your question... |
I was one of the original people to post on this topic, almost 3.5 years ago. My position, that any "properly constructed" digital cable should be indistinguishable from another was predictably met with scorn and ridicule. My ears and even my choice of a mate were questioned. Not that the later has much to do with the topic. Unfortunately, some people when confronted with someone elseÂ’s views that donÂ’t agree with their own canÂ’t stay objective.
While I still hold my beliefs, letÂ’s look at some of the other factors that might be in play.
Joemazzaglia makes the point that when he heard optical fiber and coax cable through the same unit, there was a big difference, in favor of the coax. I believe that he heard that difference and it is not surprising. A number of manufactures warned that maximum fidelity could not be obtained from the Toslink connections using their equipment. I believe that is because they hadnÂ’t optimized for optical digital transfer.
There is also the issue of true optical glass cable, vs. plastic. IÂ’ve purchased some true optical glass cables and their performance clearly exceeds a cheap plastic one I bought as a comparison.
I would be interested in whether Joemazzaglia would notice the same difference
if two or three coax cables, all true 75 ohm and made by reputable manufacturers were compared in a blind test? That would say more about the cables, since they are of the same type.
My own experience with a very fine DAC the Levinson 30.6, which has coax (SPDIF), balanced (AES/EBU), as well as Toslink inputs is that the type of cable shouldn't matter. I have compared AES/EBU digital cables from Cardas and Madrigal, with SPDIF cables from Kimber and Madrigal and true Corning fiber optic cables through the Levinson and have not be able to detect any noticeable differences. (The cheap plastic cable notwithstanding)
While none of these cables was a bargain basement item. They are also not cost is no object (or maybe cost is the only object) designs. They are all good well engineered and manufactured products that do exactly what they claim to which is move digital data from one component, the transport, to the next, the D to A converter. They do so properly and predictably and the DAC then can produce the analog signal it was designed to.
I think people need to remember, digital cables transmit 1Â’s and 0Â’s. They donÂ’t transmit anything that a speaker would make into sound. That all happens after the DAC converts the signal to analog.
Also, my argument wasn't that any piece of copper wire from Radio Shack will do as a digital cable. I was pointing out that once a design has "got it right", which can be achieved as fairly moderate cost, “Gilding the Lilly” by adding meaningless features (super fancy connectors, braiding around the insulation come to mind) that add cost but change nothing about performance doesn't make the sound better.
I also understand that pride of ownership may make us prefer the fancier cable and if that is so and we can afford it, go for it. But don't try to convince ourselves or others that the sound it can produce changed.
I know that will probably start another round of name calling and if so, so be it. We have a right to our opinions, but I find it discouraging that people attack when confronted with ideas that they don't want to accept. Also too much is made of too little in this hobby. I hate to see folks be convinced that they can buy something that isnÂ’t there if they just spend more money.
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Over spring break, I had a chance to listen to my Rotel/coax vs. my Panasonic/optical.
The first impression was one of much more oppeness, and a deeper sound around instruments with the optical output.
*HOWEVER*, cymbal crashes, esses, fingered strings, etc all sound FAR better via the coax!
For highly digital source material, like "The Matrix," the optic input adds a nice effect, making everything "feel" digital. It also seems to portray a tad mote bass (but it actually just muddies it up, when compared a/b).
I used my own, and a friend's copy of the Rolling Stone's "Love You Live" disc, and could simply switch between the two units on my Rotel DAC. The crowd sounds bigger, the clapping more real, and the music more convincing via the coax.
I'll admit I was hasty the first time I posted, and I'll think twice, er, listen twice before running my mouth next time! |
Here is my digital cable upgrade path over last 3yrs, getting slightly better sound each step at lower prices:
AZ Mc2 $300 --> WW Supernova III glass toslink $100 --> Sonicwave glass toslink (280 fibers) $25 --> Stereovox HDVX $100 currently used.
Still have my $25 glass toslinks around one of best examples of "bang for the buck" ever in my cable buying experience. |
I have noticed that there ARE differences, but that they do not seem to correspond to price, etc.
Fiber optic, IMO, is the best, and the cleaner the optics, the less distorted the signal , so theory would go.
But I do notice that, even my $20 Monster Toslink outperforms my $200 "digital" coaxial cable.
The cheif designer of Musical Fidelity products swears that ANY transport will do when using digital links to the DAC! I am not an expert, but I did notice that my Panasonic DVD player soundes better using the Toslink than my Rotel does via the coax digital. This is odd, since the Panasonic set me back around $70.00, and the Rotel about $450...USED!
In Rotel's defense, however, I simply LOVE their DAC (RDP 980). I have compared it to a few others consting WAAAY more, and the Rotel seems to hold its own NO PROBLEMS!
Water dripping sounds, drum beats/cymbal decays, and plucked strings are all quite remarkable via the Rotel DAC.
Mickey Hart, "Planet Drum" and Bela Fleck "Live Art" are two of my fave discs to show-off my Hi Fi system.
Bela Fleck "Perpetual Motion" is another great disc, and I can hear subtle differences between interconnects SIMPLY by listening to Fleck's fingers slide up and down the banjo strings.
Even the cheapest fiber optic cables reveal THREE times more detail than the coaxes, in my system...
your mileage may vary.
Joe M. |
This discussion died off. Why?
I'm in the "I can hear a difference and I don't know why" camp between Coax/RCA terminated cables. Some expensive digital cables sound better than the HAVE Canare, some sound worse. I dunno. |
I want a DAC with a slot for PC133 SDRAM, a 5 1/4 drive bay and an IDE controller. Those features should cost less than $60 retail (motherboards do that and a lot more for $60). I'll drop in 128 megs of RAM from crucial for $35. I'll plug in a 40x Plexwriter for $90 with a $3 IDE cable. Then the DAC can expect 100% accurate data to pour into the buffer at several times the playback speed. All it needs to do is pull out samples every 1/44100th of a second and do its thing with them.
If you think streaming bits from a buffer is somehow inferior to streaming them from an external component... well, uh... good for you, I guess. I hear that buffing bits smoothes them over and takes the harshness off the music, but to each his own.
Seriously, if $300 digital cables make a difference, we need to demand better hardware, plain a simple. |
here's what you need to do... go to your local wal-mart and purchase you a GOLD-PLATED OPTICAL cable, THAT'S WHERE IT'S AT!!!!! HA HA HA. (believe me, they make 'em) |
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Expensive Digital cables sound different because you sit lower in your chair when listening to them. This is because your wallet is empty. |
Impeadance matching is the most important factor.
This is going to get a little tekkie, so hang on.
It does not matter if the signal is analog or digital, the higher the frequency, the more important impeadance matching becomes. At RF, when a signal encounteres an impeadance mismatch, the signal is replected baclk up the line. The more the mismatch, the larger the reflection, and the higher the frequency, the greater the reflection. The specification for SP/DIF is for a 75 ohm impeadance. This means you need to use 75 ohm cable AND 75 ohm connectors. Well folks, most RCA connectors are NOT impeadance matched. The one big exception is Canare, which makes precision 75 ohm crimp on connectors for a very reasonable price.
Canare has premade 75 ohm cables available, and there are several people out there rolling good 75 ohm cables using Belden 1694A or 1695A with the Canare connectors, typically between $20 and $30.
For the best results, use BNC instead of RCA connectors. The BNC was designed to be an RF connector, and though originally designed to be a 50 ohm connector, there are 75 ohm available, such as Canare. The problem is that only the really good high end equipment usually comes with BNC's as an option, so the modestly priced CD or DVD you want to team up with a good DAC is not going to come with BNC's available.
I am not saying whether other factors, such as silver vs copper, or teflon vs. pe affect sound quality, but I am saying that if you are not impeadance matched, all the other cable snake oil will not matter. |
Simply look at how many digital cables are being sold and bought daily on Audiogon, and all you will know if digital cables makes any difference with sound reproduction. Don't tell me that the 100-200 idiots buying the cables are real idiots! There gotta be reasons. Cable design minimize signal loss and interference, this theory apply on kinds of cables! I used a 2M XLO Digital Cable instead a 1M one day, and I noticed the difference. Its the same model, same spec. same measurement, why did it sound different? You all tell me... |
Foreverhifi: I might agree with John Dunlavy regarding his comments "God himself couldn't hear the difference in two wires/cables measuring the same values!"
If the cables presented identical loads to the source component with the same level of signal transfer into the load component, they "should" sound identical. However, it would be possible to measure cables and have them come up the same yet be electrically VERY different from one another.
One could use a cable constructed like Kimber 8TC and insert inductors in each leg of the run and then take a run of 10 gauge zip cord and insert capacitors in each leg of the run. The total resistance, inductance, capacitance and impedance for both cables may measure the same on a meter but the fact that some components were lumped and others were evenly distributed would cause current to flow VERY differently along the length of the lines. As such, "tuned networks" within a small area may not be as effective as achieving similar electrical characteristics over a wider spread and vice-versa. Much of this will depend on what you are trying to achieve and the rest of the circumstances involved. Whether or not we can actually hear such things is a good question and one that i would love to be able to know ( and prove ).
As to the cables that Dunlavy was using, i would have thought that he would have been using some of his own speaker cables and interconnects. As to his choice of gain stages, he should have known better than to say that "an amp is an amp" etc... He used to design and market components back in the 1970's and they were very well thought out in terms of circuitry. I don't know how well they were constructed or if he paid attention to parts selection, but he knew enough to try and design something that was "technically correct" to say the least. He is a very smart guy but whether or not he applied everything that he knew at all times is another matter. Sean
> |
Does anyone know where i can find some THX certified
lamp cords? |
Leme@lone.com
No actually you'd be wrong!...I swear!!!
In one of John's (Dunlavy's)systems, HE WAS USING LAMP CORD FOR THE SPEAKER CABLES!!!!(all-be-it 12awg).
Now don't get me wrong, I think the gear he was using was not so good either. Infact the refernce system, which used SCIV's, had Marantz MA500 mono blocks, and a digital Marantz pre/pro up front!!...OH THE GLORY OF SOUND!!!! Patch cord IC's, Lamp cord for speaker wire, mid-fi(at best) gear...I couldn't believe what I was seeing/hearing!
He believed(JD) that a good amp was a good amp, and all wire was the same if it measure(with his test equipment) proper impedence/inductance/capacitance or whatever, it HAS TO SOUND THE SAME he stated! Infact, he went on to often say that "God himself couldn't hear the difference in two wires/cables measuring the same values!!!!! Ehem...... |
I'm jumping into this late but here goes.
First, don't assume that people who can't hear the difference in digital cables have a bad system or bad ears. Maybe they are just fortunate that their system components match well enough that the cable isn't much of an issue. Having a system that is very sensitive to cable changes may not always be such a good thing.
Second, I've tested a few ADC's (similar to DAC's) and clock jitter is extremely important in reducing harmonic distortion at audio accuracy levels. As mentioned above, errors in the time position of the sine wave samples will distort the sine wave, just as voltage errors will.
What we're talking about is two issues: data recovery from a serial bit stream and also clock recovery from the same serial bit stream.
Although data bits errors should be rare in a well-designed and well-matched system, they will cause serious problems when they occur. As noted above, there is no error detection or recovery mechanism for data errors. All serial data transmission schemes will miss data occasionally and the measure of that is the "bit-error rate".
Clock recovery circuit design is almost an art. I'm not familiar with the CD standards. I assume that the DAC's clock recovery circuit uses a PLL. Is this correct? The PLL will filter out alot of the high frequency jitter. Better PLL designs will filter out more jitter.
We have equipment to measure jitter where I work. The accuracy is in tens of picosends. The inputs are all 50 ohm. This equpment should be usable for testing the transport/cable jitter. Does anyone have ideas for some easy experiments using this equipment? It might be interesting to measure several types of transports.
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"Yes" is the short answer. I don't necessarily agree, but that seems to be the point of view of this community. |
I am not well read on auditory theory, but I seem to remember reading somewhere that auditory memory is good for a few seconds at most. Also that non-blind tests are statistically worthless since visual clues, foreknowledge and so forth demonstrably invalidate the test. Also, that expectation ("I am now changing the cable") leads to perceived differences in sound (even when the cable is not actually changed). Or do we dismiss this overwhleming scientific evidence under the illogical rubric that music is so very complex and objective undestanding so very limited that subjective experience outweighs it, and hence cables must make a difference? |
forever, are you saying the reason the systems you site as sounding lousy was because of the attitudes of the owners, or that they chose bad cables? I douby in either case they were using lamp cord
steve. |
Sean,
Can i still have your tax guys number? just kidding.
I agree...audio and high speed are two different worlds
and I do get them confused sometimes...
Forever,
I think you brought up some very good points. The more
i think about it the more im thinking perfect length
of wires might not be ideal for audio. Hmmm.. So what do
you say guys lets sell different lenght MATCHED cables
and get rich?
Best regards |
Spluta: I was primarily refering to audio and low frequency RF gear. When you start hitting high frequency RF, you can have it. Things can get WAY "squirelly" REAL fast. I can understand that you have to take what you can get in situations like that, as there just aren't a lot of alternatives out there.
Foreverhifi: Out of curiosity, what was Dunlavy running for electronics at their factory ? I would assume that he had his Z-6 speaker cables, but do you know what was used to connect all of the gear ?
This brings up another topic. If the gear / speakers being used within a manufacturer's reference system sounds like "hell" and / or is set up poorly, how is it that they can really tell if something sounds "better" or if they have actually made an improvement to a product during listening tests ??? This is not to pick on John Dunlavy in specific, as i surely respect him for his knowledge and experience. Sean
> |
I would probably have to agree with the possible theory that differences in cable geometry(or connections) will cause differences in the delicate structure of sound to different degrees.(my theory..and only a theory). I DO FIND that cables sound very very different often, and DO AFFECT THE SOUND MORE DRAMATICALLY IN BETTER MORE RESOLVED SYSTEMS for sure. I am indeed a believer, from working in 5 high end stores, and being around high end audio and custom install for more than 20 years now, that cables make a difference!
I also find that the trend amoung those advocates that cables ALL BASICALLY SOUND THE SAME(if matched from a mechanical impedance standpoint, etc), THEIR SYSTEMS TEND TO ALL SOUND POOR OVERALL!!!
I once had a long conversation with John Dunlavy(Thanks john for the contributions to audio) at his factory about cables. He is an advocate that "CABLES IS CABLES" basically. The irony there is that his systems in his factory where, HANDS DOWN, the single WORST SOUNDING set-up's I've ever heard from a "HIGHER END" speaker manufacturer!!!! Infact, other people touring the factory were asking me why his speakers sounded so bad....I litterally had to defend his speaker's "merits", and say that "I'd gotten much better sound from his speakers elsewhere!"
On another occasion, I was at CES in Vegas a year ago, where I stumbled in to a room set up with Merlin VSM's with some nice high end gear. I asked the guy running the room "what wires and cables are you using?" He actually looked at me and said "you're joking right?"..."surely you don't think cables make a differnce, do you?" I said, "no comment!" I proceeded to listen to the set-up when he started playing the music, and IMMEDIATELY BOLTED UP FROM MY CHAIR!!! Not only did I hear the most bright and analytically sound from any of the rooms I encountered during the trip, but everyone else in the room immediately took issue with the sound as well! I didn't even stop to say anything to him...I think he pondered it all on his own from that point out.
My experiences, basically, is that the "cables-is-cables" advocates, are usually "closed-in" with their thinking, and can't see past their brains!!(limited processor and hardrive I presume...) These are the one's I usually find have the poorest sounding, if even well intentioned, systems!..go figure |
Sean,
I need to talk to your tax guy :-)
I agree with you 100% its all about the benjamins$$$
I never said i like mismatches... Why do you think i
spent big bucks on my cables... But there are still
other problems on the chain and unless you youself
are designing AND fabricating the product including
the transistors you are at the mercy of ACCEPTABLE
tolerances. Welcome to the real world.. I dont like
it either BUT its reality.
Maybe you can explain to me how to get better impedance
control than what the fab houses can fabricate the product
for? In other words we design these "low tech" 48 layer
10 thousand connection PCB's running at 10ghz and 50 ohms dead nuts on... Its like having 10 thousand interconnects in a 5 sq foot area... What are we gonna do have kimber cable wire us up something?
We are forced to rely on material and fabrication tolerances. There are only two board houses IN THE WORLD that can fab that PCB for that"low tech" backbone router.
How would you do it? |
Taxes are not "mandatory", nor are "mismatches" and "reflections". The more that one becomes familiar with and "works" the system, the less of each they may have to deal with. If one is really good and / or dedicated, it is possible to reduce any of the above to a point of negligence. Just because they are there does not mean that they should be "accepted".
Such is the case for signal reflection and vswr within a system. Why settle for 90% efficiency when one can have 99 - 100% ? The only difference is how much effort and money one is willing to invest to achieve that 10% improvement? When dealing with mass manufactured products, it is probably good enough. When dealing with small quantities of "high tech" products ( such as "high end" audio ), that is too much "waste" as far as i'm concerned. "Settling for" mediocrity or what is "good enough" never made me happy. Sean
> |
Sean,
I can tell you there is an 80-90% chance that the
last reply you sent went thru a OC192 backbone router that is running at 10ghz and the 10% mismatch rule applied to that product too and it works real good. We have to allow for material caused mismatches.. PCB fabrication is a major
contributor as is cable construction.
Sorry if im overstating the odvious BUT Its a fact of life...mismatches will always be there just like taxes. |
A 10% mismatch ( VSWR of 2.0 ) at RF frequencies ( which digital data is transferred via RF ) can begin to play MAJOR havoc with some circuitry. I know that text-book theory states otherwise, but you have to realize that much of that theory was based on tube circuitry that made use of matchingn networks and tank circuits. Most SS gear does not have this type of circuitry, so loading characteristics become far more critical.
The severity of how much the operation of a device would be affected obviously depends on the stability of the circuitry and whether or not the impedance mismatch was of a reactive or resistive nature. Not only would the sending unit have to deal with the reflections playing games with the output section, signal loss is increased. The resultant decline in detail is increased due to loading via the "back-pressure" within the transmission line. This is not to mention that line loss increases as VSWR increases, compounding the factors involved drastically. As such, ANY type of vswr / signal reflections tend to start a very drastic downward spiral effect once they come into play. This is FAR more apparent with SS gear, but then again, i don't know of any transport or CD player that uses a tubed digital output section.
As such, i would think that all of my experience with RF loading characteristics and impedance mismatches would directly apply to digital data transfer. I would suspect that the output section of most transports / cd players would not be very stable due to lack of a buffer circuit or complex impedance matching network. Therefore, ANY gains in terms of increased power transfer / minimization of reflections would be of multi-fold benefit for the aforementioned reasons. I have not verified this personally, but see no reason why this would differ from any other RF based SS circuit loading up. I really do want to start measuring and experimenting in these areas, but just have not had the time to do so. Sean
> |
Gentlemen,
There are always Impedance mismatches.. the trick is
to minimize... usually we have tolerances at 10%...Even
for high speed stuff. Impedance mismatches do cause RF current problems BUT if terminated it will be minimized. |
Audioengr: Tara worked with terminating impedances years ago. Silver center conductor, Teflon insulation, Silver braided shield, 75 ohm resistor from center to shield at the load end. It is a very nice sounding cable and not bright in the slightest bit.
As far as having to use a resistor(s) to terminate the load, you would not need to do that IF the actual output impedance of the transport is 75 ohms, the cable is 75 ohms and the input of the DAC is 75 ohms. The problem is that none of these are exactly the correct impedance, so you have VSWR at the feedpoint, VSWR within the cable and VSWR at the load. If one could find out the actual output impedance of the transport and the input impedance of the DAC, it would be easy to make up a section of cable that would act as an impedance transformer. This would minimize standing waves / reflections and maximize power transfer. THAT was the point that i was trying to explain. Obviously, you could not market this cable as the specific values would change component by component.
As far as terminating the cable with a resistor at the load end, that would NOT solve an impedance mismatch at the source end. An impedance bump ANYWHERE along the signal / data path will result in reflections back to the source. Sean
> |
Forgive me for coming late into conversation...
From what ive seen there is a major flaw in the spec itself The #1 contributer of EMI is the rise time. The quicker it
is the more EMI emitted. A common misconception is the Freq
is the culprit not true. Its seems to me that the rise times
are faster than needed for application. If some one knows why they used the 3mhz with a 12-15ns rise time... Please let me know? It doesnt make sense to me. Although those numbers are prehistoric it is asking for excessive EMI with
faster rise times than needed. Typically you would see a rise time more like 30 maybe as high as 80 for that type
of frequency.
Its like saying take 15min to drive to the store (a 5 minute drive) and then RUN in side and try and make up for lost time when you get there.
As far as termination I agree with Audioeng. You have have
termination in place at reciever end. Its the reflection back thats the killer... Backwards crosstalk is evil sh*t in any system. |
Sean wrote: If one could resonate a cable that introduced a perfect non-reactive 75 ohm load at that frequency, you would end up with no standing waves. There may be no standing waves to a pure sine wave at that frequency, but you would certainly experience reflections with digital edges. There are only a couple of ways to eliminate reflections with digital signals, including: 1) series terminate (75 ohm driver) into a 75 ohm Zo cable - reflection from the end is absorbed at the driver. 2) parallel terminate a low-impedance driver into a 75 ohm Zo cable using a 75 ohm resistor at the end - no reflection. 3) Series and parallel terminate - (75 ohm driver) into a 75 ohm Zo cable with a 75 ohm resistor at the end - this cuts the signal in half, but eliminates all reflections. |
Picking up on what Redkiwi said about coax vs. digital, I and a friend were able to A/B the new Audioquest Optilink 5 with Audioquest's digital RCA, the VSD-4. The result was truly surprising. The Optilink glass fiber Toslink was significantly better in every way; soundstage, bass, "openness," highs, "air," -- you name it, the glass toslink was better, even though the data is just the same 0's and 1's. I don't have a clue why it sounded better, technically, but this was not even close. We were startled, honestly, at the difference. Moreover, the toslink was significantly louder at the same volume level, requiring us to match volume for the two. So bottom line is, for whatever reason, digital cables sound different, no question whatsoever. |
Looks like you're right audioengr. I got into an argument with Monstrous Mike one time about Manchester Biphase encoding. He was trying to say there was no digital edge, only FM modulated sine waves. From what I can find, you are right -- the signal is still sent as square waves from what I can tell (but not with sharp edges -- the standard should likely work even if the signal were closer to a sine wave, but with more jitter).
As for who I am Drubin, just an audio enthusiast, who lurks on the audoasylum cable forum under "audioNeil". |
While it is strictly a matter of semantics, the actual digital out signal from a transport is sent at 2.822 MHz. If one could resonate a cable that introduced a perfect non-reactive 75 ohm load at that frequency, you would end up with no standing waves. Bare in mind that terminating and feedpoint impedances are a LOT more complex than just selecting a 75 ohm cable ( we wish it was that simple ). This would allow the transport to load up easier due to a complete lack of reflections and a clearer signal path to transmit the data, which theoretically should drastically reduce jitter. This takes for granted that the transport and dac would both have nominal 75 ohm inputs & outputs though. This is something that i've been meaning to check into but just haven't gotten around to it. Sean
> |
Actually the SPDIF signal is digital, not dual-phase sinewaves. It appears more like a "trapezoidal" wave, which is a square-wave with controlled slew-rate rising and falling edges. This is to minimize radiated emissions for FCC regulations. The signal is around 3 MHz with 12-15 nsec rise and fall times. |
Jeez, that was terrific. Who are you? |
This is a long and old thread. Read 1439bhr and audioengr for good answers. I disagree with 1439bhr that fifo buffering is the best solution, though it is a pretty good one.
There are many references on the web about this. Understanding in the audio community has come a long way since the thread started.
It is clear that digital cables make a huge difference on most but not all DACs. Yes, it's because of jitter, not data errors. The cables carry not only the data, but also the clock information. For SPDIF the clock is recovered from dual phase sinewave encoding of the data, and I'm not sure about the Toslink mechanism (square or sine?). This is a terrible way to get the clock.
Imagine, a nice clock signal controls a transport with a FIFO. There is some jitter coming out of the FIFO, but at least the timing is controlled by the master clock right there at the transport. Then the data is encoded, sent to transmitter that changes it to dual-phase sine waves, through a lousy digital cable, into a DAC receiver, where it is put back to digital where the clock and data are recovered. We then send the data and clock straight into a DAC or upsampler.
Now, how good is that clock? An SPDIF receiver uses a phase-lock loop (PLL) to recover the data and timing. It typically has a time constant that passes all jitter below 10 kHz directly onto the DAC. Yup, your music is at the mercy of the transport clock, transport jitter, cable quality, and SPDIF trasmitter/receiver circuitry. It's a wonder we get decent sound at all. Well, in fact, in a lot of cases we don't. The SPDIF standard was made at a time when 2 ns of jitter was considered to be inaudible. This is totally not the case. I have read that "inaudible" is more like 25 ps of jitter.
The problem is that the DAC needs an accurate clock to prevent distortions in the sound. There are 3 solutions that I know about:
1) Use a huge FIFO buffer to buffer up about 1 second of data. You can then reclock with any jitter being around 1Hz or lower in frequency. This can work, but I think there can be jitter problems with FIFOs themselves.
2) Throw away SPDIF since it is junk, and go for a bidirectional communication standard like firewire. This allows the master clock of the system to be in the DAC, not in the transport. The DAC tells the transport to go faster or slower, and only a small FIFO is needed in the DAC.
3) Reclock the data using an upsampler.
Number 3) is most commonly used now, with the proliferation of the 24/192 upsampling DACs and converters. Now, you can do oversampling without reclocking (4x, 8x for instance), but since these new "upsamplers" are not a multiple of 44.1 in their timebase, they are called asynchronous upsamplers. They take samples of the data on their own clock. Yes, this reclocks the data, but unless it is done right, you just resample the jitter into the data stream now instead of the clock.
In order to for an upsampler to reclock without jitter getting into the datastream, the upsampler must "track" the incoming jitter. My Bel Canto DAC2 has an upsampler chip that acts like a digital PLL, which digitally tracks the incoming jitter and acts like a PLL with a base frequency of 3Hz. Audio-band jitter is highly reduced.
You know what? Cables don't make much of a difference on my Bel Canto DAC. But on my Denon AVR-5800 receiver, the differences are so clear. Toslink sound very different from SPDIF as well on the Denon (haven't tested the Bel Canto -- just love the SPDIF on it).
So, I hope this cable debate goes away in a few years, because frankly I think it's nuts that I spend a few hundred on a Harmonic Tech Cyber-link platinum. And I'll be darned if I'll spend thousands on a transport. The clock must be accurate right at the DAC chip, and a good cable is only part of the battle. With newer DACs I hope to be able to happily agree that "digital cables make no difference". Right now though, on most DACs, it just isn't true. Cables make a difference, as do everything else in the digital chain (yup, even power cords ... but I won't go there in this response). |
John: I don't think that you'll find anyone here that would deny that brands / prices / cosmetics may have more influence over what we hear than many of us would like to admit. I try to forget about all of this stuff and just listen. Obviousy, i try to start off with products that are at least well designed to begin with, but one sometimes does not even have that much info to work with when auditioning specific items. Sean
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Frogman's insistance on believing that Kimber's digital cables exhibit the same sort of characteristics as their analog ones may be true, or it may just be that he associates those characteristics with the Kimber brand, and his brain tells him what to expect.
We can't overlook the affect of our preconceptions on what we hear. I would bet that many of the "golden eared" of the world would be shocked to learn their conclusions in true double-blind tests. (especially if not told WHAT they're testing!)
As a reviewer in one of the high-end mags wrote (about 20 years ago) "The amplifier delivers 300 WPC into an 8 ohm load and is housed in rich persimmon wood." He then went on to describe the amplifier's sound as "warm" and added, tongue in cheek, "which is typical for amplifiers housed in rich persimmon wood..."
Or, as I responded to my friend recently who asked, "Can you really hear the difference between a stock power cable and one costing $2000?" I replied, "If you just spent $2000 on a power cable, you'll hear a difference."
No hate mail please - I have upgraded power cables, interconnects, etc. My point is simply that our brains can convince us that we hear almost anything. Heck, that's why our systems sound good at all - because our brain fills in what's missing. Therefore, be sure YOU hear it - don't take someone else's word for it. |
Ooop's, I was commenting on speaker cables and interconnects and missed the word digital. |
I would have to agree with the impedance argument, if a digital connection says 75ohm you need a 75ohm capable cable that is purpose designed to sound good doing what you want to use it for. I would suggest looking at Canare's RCAP true 75ohm RCA connectors $3-$5 each and some of the canare LV-77S cable I think it is $2 a foot or so. That combination will outperform $60-$100 digital interconnects. Canare takes cables very very seriously and they are highly regarded by the profesional audio world. |
Sean - Transmission-line effects are the main concern with digital cables. Characteristic impedance matching is a big part of this. However, dispersion of the digital signal is also caused by dielectric absorption, which can cause jitter, so just matching to 75 ohms is not sufficient to minimize jitter.
"Shouldn't a cable that has a higher velocity factor be less prone to signal deterioration / absorption due to the signal spending less time in the cable ?"
There will generally be less absorption in a high-velocity cable because in order to get high-velocity, you need a low dielectric constant. Low dielectric constant results in lower capacitance and lower dielectric absorption. The time that the signal transits the cable (propagation time) is really of little consequence itself. This will obviously change depending on the length of the cable. The rise-time of a SP/DIF signal is on the order of 20 nsec, so you would have to have 100 feet to equal the risetime. Technically, this makes impedance a non-issue for a 6-foot SP/DIF cable. However, in practice, impedance discontinuities do impact the sound, particularly the image focus and detail, by adding to the jitter.
As for dielectrics, PVC is at the bottom, getting progressively better with foamed poly, solid Teflon, foamed Teflon, expanded Teflon and finally air. I use Expanded Teflon in my Digital cable. It is hard to put a percentage on the improvement without measuring it. I have plans to purchase a Tek CSA803 communications analyzer, which will measure jitter accurately to a few picoseconds, so I will eventually be able to measure this. |
Jt25741 - You are dead-on when you say that jitter is the main issue with digital cables. However, with some cables, you get what you pay for. Every cable manufacturer has a digital cable and many of them are not even close to 75 ohms characteristic impedance. They need to be 75 ohms. As for the connector, the best you can do with an RCA is to get the impedance right up to the entry point of the jack. After that there will be a discontinuity. Jacks are never 75 ohms. Some manufacturers do this. I do. |
Zilla - it is impossible for a digital cable to act as a "tone control". What happens is that the jitter caused by the cable losses results in frequency modulation of the analog signal. This can cause lack of clarity and focus. In general, the better clarity and focus, the better the digital cable. |
