jkeny

I didn't want to be dragged back into this thread but feel I have to based on this post.<br /> <br /> Firstly should your sig not read "Auraliti, vendor of USB DAC" I've been told that is what all commercial bodies should do - like I had tagged to my sig? So now that we have established that you have a commercial interest in selling your USB DAC it will give us a reference point.<br /> <br /> Secondly can you say what your qualifications are? Chris believes that we need to evaluate your technical abilities in order that we can judge your statements or as Chris puts it "Can you share a bit of your technical background so we can put some of your comments into perspective?"<br /> <br /> <i>If the output of he HiFace is excessive it will cause problems with input receivers.</i><br /> <b>Can you specify what you consider excessive?</b><br /> <br /> <i>However different receivers will react differently. in te receiver<br /> the time to propagate from the input to the output is dependent on the magnitude of overdrive. This is not spec'd for the Crystal receivers but a scan of general purpose comparators will show the influence of overdrive. Its probably not symmetrical either so alternate edges will pass through at different times.</i><br /> <b>So your surmising here - the correct formulation of your statement should be " Its probably not symmetrical either so alternate edges <i></b>might</i><b> pass through at different times." </b><br /> <br /> <i>Whether this will have a significant influence on the output<br /> jitter or anything else is difficult to quantify. Each dac will respond differently based on a slew of factors including the input isolation (if any)the nature of the termination and even the layout of the traces from the connector to the input pins of the chip. The very high speed comparators degrade rapidly with overdrive but the more general purpose ones get better.<br /> <br /> <i>If there is a reflection from the input circuit (and there will most likely be one) the next question is will it be absorbed by the attenuator (not 100%), will it affect the receiver (comparator) and how?</i><br /> <b>The attenuator will absorb it somewhat close to twice the dB speced so if you are using a 10dB the reflections will be attenuated by 20dB (a power ratio of 100). This is significantly better than 0dB.</b><br /> <br /> <i>If the cable is long enough the reflection will not get back to the receiver at a time where it can influence the switching.</i><br /> <b>So what length do I need? Oh, yes this has to be calculated based on the various parameters. I can't do this myself so let's go to a cable vendor who knows how to do this. OK, Mr cable vendor "What price did you say again?" "Oh, OK, it's so expensive because I need 16 feet of cable, I see" "It has to be 75ohm without impedance variations, I see, so that's why it's so expensive per metre". "Jeez, it's beginning to cost more for the cable than for the devices connected to it!" "Can you tell me if I need a different length of cable for different data rates" "Why? Because I was told that if I have cables of certain lengths it will avoid the issue of reflections which I was told are on all digital lines" "Ok so I'll get a cable length of 20 feet, that way I can put my DAC where I want to" "What, I can't use 20 feet because now the reflections will hit the DAC at the next transition window & cause jitter?" "So it's only certain discrete cable lengths that work?" "My head has melted, I think I'll just buy an attenuator, I heard they work great & I'll save a bundle :)" "What's that you say, you want me to prove that they work?" "I think I'll risk it" </B> <br /> <br /> <i>A very short link with a big mismatch COULD influence the transition but may not.</i><br /> <b>"Huh, what's that you say? Hold on, a minute ago you told me I needed 16 feet of cable, so why are you trying to sell me a long cable, if the reflections are of no consequence, Mr cable vendor?" "My brain is truly melted now - I'm off before you do it any more damage"</b><br /> <br /> <i>Some systems have been designed to use the mismatch to boost the signal going into the receiver.</i><br /> <b>"Huh! what are you telling me this for? I'm already confused by the cable scenarios" <br /> <br /> <i>I looked for the "kick back" you mentioned on a receiver and could not find evidence of it. One could speculate that the hysteresis could couple through the input transistor pair and kick a minute charge back through the input but I couldn't find it. Perhaps you can provide more details of how to see it?</i><br /> <b>Don't believe me, Ask Jocko he knows all about it - he posts here as Pat from ART - look here http://www.diyhifi.org/forums/viewtopic.php?p=5879#p5879<br /> "In case you have not heard me before..........Schmitt trigger inputs are no bueno. Hysteresis couples energy back to the input. <br /> Yes, it screws up the timing by adding some anomalies that could occur at the decision point. " <br /> or here http://www.diyhifi.org/forums/viewtopic.php?p=5727#p5727<br /> "This is due to the presence of a schmitt trigger on its input, which according to Jocko Homo and others will tend to inject trash back into the transmission line."</b><br /> <br /> <i>My experience with the HiFace was very similar to Chris's. Further no Linux support made it uninteresting for my efforts. But it may provide a good experience for others.</i><br /> Great thanks for your opinion! <br /> <br />

I'm outta here - it's the only option as I won't pussy-foot around & I won't shut-up. All your advice is good & I should take some time-out. I can understand how my enthusiasm at the potential of the Hiface could be construed as self-serving but that's just something I can't avoid - maybe I'll stop putting out info & start behaving like a real manufacturer giving no information away except what is in my own interest. <br /> <br /> Maybe Juergens & barrows will report their findings in time & I'll return? Sayonara!<br /> <br />

for your support. I'm an ideas guy - I judge an idea on it's merits irrespective of the credentials of the messenger. I don't hold much store by credentials even though I have a B. SC. degree in Biochemistry & Maths & have spent my working life in computers as owner of a computer company. I'm not giving a CV here as I don't think it has anything to do with what I say. <br /> <br /> If Chris (or others here) are not able to evaluate a concept on it's merits alone then I'm sorry for their lack of objectivity & their lack of analytic capabilities - it's just more evidence of blindjim's assertion of elitism on this forum.

Let's take the overshoot out of the picture & here's a scope with no attenuator & with attenuator overlaid on it. I'm sure you will see what are reflections & how effective the attenuators are:<br /> <br /> I don't need to show my credentials to post these!<br /> <br /> <a href="http://www.4shared.com/photo/uQR5dMxB/att_noatt.html" target=_blank><img src="http://dc192.4shared.com/img/376261915/98e96d79/0.05507015523457515/att_noatt.jpg" border="0"></a><br /> <br /> And here's the technical details of the measurement set-up according to Joseph K:<br /> <i>Here it is the difference before / after the insertion of an attenuator into a deliberately "wrongly" terminated 75ohm transmission line. <br /> <br /> <i>the setup is:<br /> Upper trace <br /> generator - 75ohm line - BNC tee (real 75ohm) - 75ohm termination. <br /> The BNC tee goes into the 1Mohm input of my scope.<br /> Lower trace<br /> generator - 75ohm line - 10dB 75ohm attenuator-BNC tee - 75ohm termination<br /> The BNC tee goes into the 1Mohm input of my scope<br /> <br /> <i>As you see, the only diffeence is the attenuator. The traces are normalized, so as to see the same percentage of the reflections.<br /> <br /> ..........<br /> <i>The driver used is a 250MHz Hp pulse generator. The rise & fall times, ~700psec are limited by the scope. <br /> The Hiface is only slightly worse, 1.8nsec rise time.<br /> Last but maybe most important note: I show this setup & graphs again and again, because EXACTLY THIS IS the difference what your dac sees on it's input.</i>

Good man - I like somebody who is open to experiment & not just arguing over the technical issues - as I said it's all clap-trap - the real world has a tendency to not follow theoretical exactitude!<br /> <br /> I would suggest a 6dB one on your set-up but you may get away with a 10dB.<br /> <br /> The schematic of the device is given on the datasheet. Specs & graphs of the performance are also given on the minicircuits site. These are precision manufactured devices effective out to 2GHz & I wondered why Gordon asked about them upsetting the impedance of the digital link - it really exposed his lack of knowledge about them (even though he has been using them since before I was born )

Can you post the link & I will PM you the dB units applicable?<br /> <br /> I presume it is the Germany subsidiary of minicircuits you are looking at? You want the fixed attenuators 75ohm precision at the bottom of this page http://www.minicircuits.com/products/attenuators_coax_fixed.html<br /> <br /> 6dB, 10dB & 15dB would be the best selection to experiment with.

You are at it again - deflecting from the point - the point about the shots is to show the attenuators effectiveness not the SPDIF waveform. These shots are, in some instances, deliberately mis-terminated to over-emphasise the reflections & hence to show the effectiveness of the attenuators.<br /> <br /> You haven't answered my challenge to you to try the attenuators?

Yes, there is an overshoot in the signal which is coming from the Hiface but the intention of the scope shots was to show the effective reduction in the reflections which are all the other squiggles (to use a technical term ). <br /> <br /> So let's not get distracted - the overshoot is something that can be handled separately. <br /> <br /> The attenuators can be seen to effectively reduce the reflections - can this be seen by everybody?<br /> <br /> Juergen, minicircuits have a UK branch which you will find on their distributors page!

<i><b>I see nothing wrong asking John for his technical background. This is much more than a $12 piece of equipment. He mods this very product.</i></b><br /> <br /> I think you are again mixing up my modified Hiface (which I haven't referred to here, BTW) & the attenuators. I don't do any modifications to attenuators - I simply suggest their use. So asking for my credentials in order to evaluate my suggested use is a bit rich, don't you think?<br /> <br /> This thread is not about my modifications to the Hiface is it?<br /> <br /> I did ask about advertising rates, etc & this is the first time you have responded to this request

While your aspirations are laudable & probably no different from the majority of inmates here, I would respectfully suggest that you are living on "blind faith" & manufacturer's claims about your system. <br /> <br /> In the real world even the very best 75ohm cables have impedance variations that cause reflections. You might be surprised how many devices do not have properly terminated SPDIF interfaces - ask Pat, Jock, one of Gordon's buddies to confirm this! Have you got any scope shots for your equipment similar to what I posted for the Hiface or for any of the equipment you are aspiring to? <br /> <br /> Your point about cable length obviating the need for concern about reflections is a whole other debate & is back in the land of buying expensive cables (with a nice & high price per metre) & usually of a long length. Why bother when you can buy a $12 attenuator which bypasses this expense?<br /> <br /> I didn't treat Gordon in any manner that I was not treated by him - he basically accused me of not knowing what I was talking about but did nothing to show that - he asked if I even knew what a schmitt trigger was thereby impugning my credibility & technical knowledge. Any confrontation was a direct result of this. I still ask him to answer, who invited him to clear up things & exactly what did he clear up? It very much seemed like a hit & run tactic to me. <br /> <br /> Barrows, I challenge you to try a 6dB attenuator on your CD system & report back here. As I said I will refund you if you are not keeping it. How about it?<br /> <br /> To find out more about the conditions of these scope shots here's a link: http://www.diyaudio.com/forums/digital-source/168901-rf-attenuators-jitter-reducers-5.html#post2222973

Chris, just some egos, maybe & we all know egos are more important than babies or puppies .

<br /> <br /> Here's some more of Joseph_K's scope shots & here's what he syays about them: <i><b>By the way, this what it looks like a Hiface driver connected directly (with a BNC "I" )<br /> to the scope input. A USB extension cable is used for the laptop.<br /> <br /> But be careful! The first shot is what it looks like when this "no SPDIF cable" is not terminated well..</i></b><br /> <br /> Hiface with no attenuators directly connected with no SPDIF cable & a bad termination: <a href="http://www.4shared.com/photo/f_GCpCq-/Hiface_direct_connection_bad_t.html" target=_blank><img src="http://dc224.4shared.com/img/376116926/83aef907/0.7806769698556696/Hiface_direct_connection_bad_t.jpg" border="0"></a><br /> <br /> <br /> Hiface with attenuators directly connected (no SPDIF cable):<br /> <a href="http://www.4shared.com/photo/hWxKOzIu/Hiface_direct_connection_with_.html" target=_blank><img src="http://dc274.4shared.com/img/376122081/20365226/0.8957177454210792/Hiface_direct_connection_with_.jpg" border="0"></a>

I wondered about that - whether the pics would show up or not BUT I'm sure those looking for some technical proof will be members over on DIYA. Otherwise, is there no way to post an img - I'll try hosting it on a open site now.<br /> <br /> Maybe, plain talking is what is required here rather than the beating around the bush, deflections, mis-directions & insidious comments that I've witnessed here? Blindjim refreshingly stated it & I'm just pointing to examples of this elitism in action!<br /> <br /> Does this work?<br /> Hiface with No attenuator <img>http://www.4shared.com/photo/kWM6gbim/JosephK_Hiface_noatt.html</img><br /> <br /> Hiface with attenuator <img>http://www.4shared.com/photo/Y8yhMnlu/Joseph_K_Hiface_with_att.html</img><br /> <br /> Nope nothing seems to work to get an image on this post

Again, you fail to comprehend & this is becoming frustrating & exhausting to explain - this isn't a band-aid only used to fix a badly designed digital link (you see how you use every opportunity to criticise the Hiface?) - I'm suggesting that most/all digital links will benefit from it, including yours. I challenge you to try it & if it doesn't improve the sonics of your system I'll pay for it! <br /> <br /> This is why I'm somewhat defensive, I posted a simple/cheap tweak that everyone can try & all that has happened is that I have been defending myself from allegations that I don't know what I'm talking about, what are my credentials, it's only for bad interfaces, etc.<br /> <br /> Nobody has bothered to look at the links I have given to check the veracity of what I say. Indeed nobody has decided to try it & report back. It seems to me that what's being engaged in is less about whether the tweak works & more about denigrating the Hiface.<br /> <br /> I gave two links to scope shots of Hiface without/with attenuator - as I was asked to provide them - anybody wish to comment? Anybody wish to even acknowledge the post or say thank you (these are the usual polite responses that one normally encounters on all other forums)<br /> <br /> BTW, barrows what system do you have that has a perfect reflection free digital interface between transmitter & receiver. Have you measured this or are you going on belief? Are your 75 ohm connectors soldered or crimp connected? Have you measured your DAC receiver's characteristic impedance? Your statements are beliefs about what you endeavour to aspire to rather than real-world measurements of what you have, no? BTW, I hope some of my comments have helped you to understand the technical side of the digital link?<br /> <br /> Why I mentioned the Wavelink thread is that you are almost fielding questions over there & directing people to Gordon's site for prices - a job that some people get paid for - I think it's called PR <br /> <br /> My question still stands to Gordon - who asked you to come over here to clarify things & exactly what things did you clarify?<br />

Now to answer your other question: <i><b>Question is: are the reflections lower after attenuation than they would be if the signal was on 'spec' to start with? That is what we need answered- nothing else matters.</i></b><br /> <br /> Firstly, the reflections have nothing to do with the signal being on spec or not on spec - the reflections are purely a result of impedance anomalies that the signal encounters on it's trip from transmitter to receiver. <br /> <br /> These can arise at the output stage of the transmitter, along the cable or at the input stage of the DAC receiver. None of this has anything to do with the Hiface high SPDIF level. In fact the 73ohm characteristic impedance of the Hiface BNC output reported by Chris in his review is the relevant figure & this is close enough to 75ohm to be a minor issue (I would contend that there are a helluva a lot worse than this).<br /> <br /> So the reflections are really down to whether this 75 ohm is maintained all along the cable & into the DAC. This is a big ask & again I would contend not realised in most people's implementations. <br /> <br /> Therefore reflections are a part of real world systems & anything that will cut them down in level is a good thing, especially something that costs $12?<br /> <br /> BTW, these attenuators will work on other SPDIF outputs too - I already reported about the squeezebox being improved by them - thare are others that I could mention.