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How the HDMI cable scam works

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Old 01-08-2008, 02:57 PM   #31  
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Quote:
Originally Posted by electrictroy View Post
Perhaps the original author ought to stop opining on things he clearly does not understand.
How true, how true . . . both for the original autor and the respondent.

It is clear that neither fully understands nor has the necessary competence to post anything better than the nonesense that has been posted.
Shame!
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Old 01-09-2008, 12:49 PM   #32  
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What do you think a digital signal is??? (Hint: Oscillating analog wave.) As any digital engineer can tell you, the actual waveform looks more like a "dirty" sine wave than a square wave.

And the quality of the cable affects how clean the sine wave. A poor-quality cable will result in distorted signals that CAN lead to loss of digital information (because the TV doesn't know if the 1.0 volt signal it's seeing is supposed to be a 0 or a 1 - it's become too corrupted).
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Old 01-10-2008, 08:30 AM   #33  
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OK, I'll bite . . . .

Quote:
Originally Posted by electrictroy View Post
What do you think a digital signal is??? (Hint: Oscillating analog wave.) As any digital engineer can tell you, the actual waveform looks more like a "dirty" sine wave than a square wave.
A digital signal bears no resemblance whatsoever to an "oscillating analog wave" by any stretch of the imagination.
An "oscillating analog wave" is a smooth waveform which swings symmetrically from positive to negative in a repeating pattern.
A digital signal (in the context of this thread) is a square wave with no negative component and a random pattern.
No matter how "distorted" the digital signal may become, it will NEVER look more "like a "dirty" sine wave" than a square wave . . . . indeed, it may not look "square", but it will always retain the obvious characteristics of a digital signal and always be recognizable as such.

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And the quality of the cable affects how clean the sine wave. A poor-quality cable will result in distorted signals that CAN lead to loss of digital information (because the TV doesn't know if the 1.0 volt signal it's seeing is supposed to be a 0 or a 1 - it's become too corrupted).
While the above statement is painfully lacking in technical precision, it is "basically" correct as a layman's explaination. For those with enquiring minds the HDMI signal levels and eye diagrams provide a definitive specification of how this works.
http://www.hdmi.org/download/HDMISpecification13a.pdf - pp 40-50 et al.
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Old 01-10-2008, 08:44 AM   #34  
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How true, how true . . . both for the original autor and the respondent. It is clear that neither fully understands nor has the necessary competence to post anything better than the nonesense that has been posted.
Wrong. I *do* know what I'm talking about. The quality of the cable affects how clean that wave is when it comes out the opposite end. A poor-quality cable will result in distorted signals that CAN lead to loss of digital information (because the TV doesn't know if the 1.0 volt signal it's seeing is supposed to be a 0 or a 1 - it's become too corrupted).

This doesn't mean you need to buy a $100 cable,
but neither should you buy a cable some guy
made in his garage. It will work as poorly as it looks.

Last edited by electrictroy; 01-10-2008 at 08:53 AM..
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Old 01-10-2008, 09:03 AM   #35  
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An "oscillating analog wave" is a smooth waveform which swings symmetrically from positive to negative in a repeating pattern.
(1) I said "resembles". That's not the same as saying identical. I said a digital signal *resembles* a dirty sine wave.

(2) There are many analog waves that don't go anywhere near negative territory. For example, the television wave which is always positive. Simiarly, a 3.3 volt peak-to-peak oscillating sine wave with a +5 volt DC offset also does not venture into negative.

Quote:
A digital signal (in the context of this thread) is a square wave with no negative component and a random pattern.
Bullshit. Digital signals have "overshoot" and "ringing" and they will shoot well past 0 volts & into the negative side (usually no lower than -1 volts, but still negative).

On the contrary the stuff I deal with looks nothing like a square wave, because it's pulsing up-and-down so rapidly. LOOK BELOW (not the best example, but it gets the point across). If you have a very poor quality cable, you're going to get garbage coming out of the end of it, and lots of bit errors.




Last edited by electrictroy; 01-10-2008 at 09:16 AM..
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Old 01-10-2008, 09:55 AM   #36  
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I hope I don't shock you too much, Troy, but I'm on your side.

I've run plenty of digital signals over cables in my life. We even had to design and build our own ultrafast buffer boards for cables on high-end In-Circuit-Emulators because they wouldn't run reliably at speed due to the cables. I am so glad they developed and implemented the JTAG specification so we no longer have to spend a fortune on ICE's that don't work.

Overshoot
Undershoot
Crosstalk
Ringing
Capacitance
Inductance
Resistance
EMI

They all corrupt digital signals over a cable. Yes, a digital signal is more robust than an analog signal--it takes a lot more to corrupt it and at low frequencies is nearly incorruptible.

But we are not talking about low frequencies--the last time I checked the maximum frequency for HDMI is equivalent to 1.65 GHz (and around 750 MHz for typical HDTV). At these frequencies you are very much talking about an analog signal (albeit conveying digital information).

HDMI has been designed to operate properly under typical conditions--The connectors and cables are specially designed to minimize deleterious effects, signals are transmitted in such a way that any transmission problems are cancelled as much as possible and the data itself has redundancy and error correction.

That doesn't mean a poorly designed or constructed cable that doesn't follow all the rules layed out by the HDMI organization will work properly.
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Old 01-10-2008, 11:31 AM   #37  
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OK, so what are you saying? What is a "poor quality cable"? Are you saying I shouldn't buy the $25 6 foot HDMI cable at Walmart?

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Old 01-10-2008, 12:46 PM   #38  
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New here, but didn't fall for the $50+ scam either. Glad I read this thread first. Got a 6 footer for $3.95 at BuyExtras, works perfect between my Olevia247 Xmas present and Dish 211(New Years present).

Just a note, the Dish installer hooked up the system with an s-vid cable and left, I rehooked it up, how many out there are really not watching hi-def???
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Old 01-10-2008, 02:37 PM   #39  
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OK, so what are you saying? What is a "poor quality cable"?
Very good question, and although there are those who do like to throw the term around as though it has some inherent meaning, you are clever to remind all that the discussion is somewhat silly without a clear definition of what a poor quality cable is and what to look for in order to avoid buying one.
It "could" include manufacturing inconsistencies in the wire, the twisted pairs, the insulation and the shielding; it "could" include inferior connectors or connector attachment methods; it "could" include lesser quality insulation materials; and it "could" include some other unknown factors. Needless to say, there is no way for the consumer to evaluate ANY of these features/characteristics - about the only thing we have to go by is whether the cable has been certified by HDMI Ltd; which they pretty much all are.
My guess is that it would be nigh unto impossible to find a "poor quality" HDMI cable in the current broad marketplace.

Quote:
Are you saying I shouldn't buy the $25 6 foot HDMI cable at Walmart?
Actually, this poster IS indeed saying that you should't buy the $25 6 food HDMI cable at Walmart . . . for exactly the same reason that you shouldn't buy a $50 or $100 6 foot cable from Monster . . .
. . . why should you when you can buy a perfectly good 6 foot HDMI cable at monoprice and similar suppliers for $5.
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Old 01-10-2008, 03:27 PM   #40  
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Originally Posted by electrictroy View Post
(1) I said "resembles". . . . I said a digital signal *resembles* a dirty sine wave.
mmmm - search as I may, I just can't find the word "resembles" in any prior posts. I do find the phrase "looks more like" of course . . . and I suppose that one could rightfully argue that "resembles" does very much resemble "looks more like" . . . or conversely that "looks more like" does look a lot like "resemble", but that would just lead to further semantic one-upmanship . . . oopps . . . I guess that's where you were headed, right?

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That's not the same as saying identical.
So who used the word "identical"??? Not me!

Hell, all I said was "A digital signal bears no resemblance whatsoever to an "oscillating analog wave" by any stretch of the imagination." and I stand by that statement, and I support the original author's use of the phrase in the preparation of an article written for the lay audience; it was sufficiently accurate and not in any manner misleading to the lay audience within the context of the article.
Was it 100% precise - no - was it sufficiently precise for the purpose of the article - yes; and there was certainly no valid reason to attack the author for his use of the phrase or the distinction that he made; nor to suggest that the author was "opining on things he clearly does not understand." (L2W's post #30)


Quote:
Bullshit. Digital signals have "overshoot" and "ringing" and they will shoot well past 0 volts & into the negative side (usually no lower than -1 volts, but still negative).
I never said they didn't . . . but, I did go on to say that "it may not look "square", but it will always retain the obvious characteristics of a digital signal and always be recognizable as such".
Again, you are so often wont to take another's comments totally out of context for the sake of silly attacks, I won't bother to defend what is not necessary to defend.

As for the posted scope photo. Sure looks like the yellow and red traces are sine waves . . . and looks like the pink and green traces are digital eye diagrams (certainly NOT "dirty sine waves").
Seems awfully difficult to confuse the two.
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Old 01-10-2008, 08:52 PM   #41  
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Quote:
Originally Posted by Ruark View Post
OK, so what are you saying? What is a "poor quality cable"? Are you saying I shouldn't buy the $25 6 foot HDMI cable at Walmart?

Ruark
Often you can tell by looking at them (especially where the connectors meet the cable) and flexing them--if they look or feel junky (stiff, brittle, if the jacket wrinkles when you bend it, if the jacket gaps at the connectors when flexed) they probably are junky.

Having worked with Chinese vendors before, I wouldn't be surprised if there are HDMI cables out there marked with the HDMI approval that aren't really HDMi approved (the HDMI organization even warns about this). Also, even if an HDMI cable was tested and approved, that doesn't speak at all to it's long-term reliability or build quality.

The best thing is to do just what you did--go on a forum and ask what other people's experience is and if they have brands of cables they are happy with.

Likely most HDMI cables out there will work fine, but, like I said, if I was running it through walls and ceiings and/or over long distances, I wouldn't choose the cheapest cable I could find.
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Old 01-11-2008, 09:11 AM   #42  
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Hell, all I said was "A digital signal bears no resemblance whatsoever to an "oscillating analog wave" by any stretch of the imagination." and I stand by that statement
Well then... you're wrong. See BobY's message below, because he explains extremely well WHY your statement is wrong.
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I hope I don't shock you too much, Troy, but I'm on your side.
(faint)
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Overshoot, Undershoot, Crosstalk, Ringing, Capacitance, Inductance, Resistance, EMI

They all corrupt digital signals over a cable. Yes, a digital signal is more robust than an analog signal--it takes a lot more to corrupt it and at low frequencies is nearly incorruptible.

But we are not talking about low frequencies--the last time I checked the maximum frequency for HDMI is equivalent to 1.65 GHz (and around 750 MHz for typical HDTV). At these frequencies you are very much talking about an analog signal (albeit conveying digital information).


I'm quoting you because you explained very, very well what I was trying to say. And yes at 1.6 gigahertz, if you look at the signal on an oscilloscope, you're not going to see a nice clean square pulses. You're going to see waves.

Last edited by electrictroy; 01-11-2008 at 09:21 AM..
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Old 01-11-2008, 11:52 AM   #43  
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Again, you are so often wont to take another's comments totally out of context for the sake of silly attacks
Look at the top of the page. YOU were the one who declared I was incompetant, and YOU are guilty of making "silly attacks" in this thread. All I've been doing since then is defending myself from the accusation.

YOU made the "silly attack".

Not me; I'm just trying to shield myself from your barbs.

Last edited by electrictroy; 01-11-2008 at 12:00 PM..
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Old 01-11-2008, 01:28 PM   #44  
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Look at the top of the page. YOU were the one who declared I was incompetant, and YOU are guilty of making "silly attacks" in this thread. All I've been doing since then is defending myself from the accusation.

YOU made the "silly attack".

Not me; I'm just trying to shield myself from your barbs.
Oh, how silly and what a gross and untrue denial: As usual, more misquoting and out of context comments.

You said on 1-08-2008, 8:55 AM in your post #30 (which by your definition would be a "silly attack": "Perhaps the original author ought to stop opining on things he clearly does not understand."
mmmm, seems you chose to judge him rather harshly, right?

I commented on 1-08-2008, 3:57 PM in my post #31: "How true, how true . . . both for the original autor and the respondent.
It is clear that neither fully understands nor has the necessary competence to post anything better than the nonesense that has been posted."


Now, I clearly owe an apology to the original author as I did indeed wrongly slight him with my post. As I said in my later post # 40; "I support the original author's use of the phrase in the preparation of an article written for the lay audience; it was sufficiently accurate and not in any manner misleading to the lay audience within the context of the article.
Was it 100% precise - no - was it sufficiently precise for the purpose of the article - yes; and there was certainly no valid reason to attack the author for his use of the phrase or the distinction that he made; nor to suggest that the author was "opining on things he clearly does not understand." (L2W's post #30)"

It might be noted that the "original author" in this case seems to have been a Marshall Brain writing in a web magazine called thesouthern.com. It was posted and duly cited by eHDMI in his post #1 of this thread. Further, there is considerable discussion following the article which treat the author with due respect in spite of some mild critical comments. No one in the orginal post or follow-up posts found it necessary to suggest that the author was "opining on things he clearly does not understand."

The author may or may not be a EE, but what he wrote was sufficiently clear and accurate for the lay audience and there was certainly no cause to attack him by suggesting that he did not understand his material. If you're going to dish it out, perhaps you should be prepared to take your own dose as well . . .
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Old 01-14-2008, 07:50 AM   #45  
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You said on 1-08-2008, 8:55 AM in your post #30 (which by your definition would be a "silly attack": "Perhaps the original author ought to stop opining on things he clearly does not understand."
mmmm, seems you chose to judge him rather harshly, right?
Attacking a person is wrong.

But there's nothing wrong with challenging the IDEAS a person puts forward, especially if that person represents a nationwide periodical that millions will read. He should be held to a higher standard, and he should have done more research, because his ideas (digital signals don't need a good cable) are clearly false.
Quote:
Was it 100% precise - no - was it sufficiently precise for the purpose of the article - yes;
I disagree. It is not precise to say a consumer can go buy any junk cable and expect no loss of signal. And I think I have a right to point-out the fallacy, rather than let it mislead people into buying POS cables.

If the cable is poorly-made, signal loss will occur.
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