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Ideas for making a USB voltage swing eavesdropper? Please! - Page 2

post #16 of 22

I really hate to shoot you down like this, but I think you are wasting your time and money. There is so much more to USB signal integrity than the voltage at which the high and low states are at the end of your cable. You need to look into signal quality measurements to really get any idea of what you are seeing and if it might affect the signal integrity. For one, I suspect you won't even be seeing anything relevant at the end of your cable before your computer and USB-device have connected. If you do, you won't have terminated D+/D- lines which will make the signals irrelevant to look at anyway.

 

If you truly want to look at only the signal levels. You should break open your headsets USB-dongle and measure the voltages at the USB jack of the device. That will at least show you what the signals look like with correct termination and load. If your scope can capture and eye diagram, that's when you start getting somewhere! :)

 

 

Best of luck!

//swek

post #17 of 22
Thread Starter 
Quote:
Originally Posted by swek View Post
 

I really hate to shoot you down like this, but I think you are wasting your time and money. There is so much more to USB signal integrity than the voltage at which the high and low states are at the end of your cable. You need to look into signal quality measurements to really get any idea of what you are seeing and if it might affect the signal integrity. For one, I suspect you won't even be seeing anything relevant at the end of your cable before your computer and USB-device have connected. If you do, you won't have terminated D+/D- lines which will make the signals irrelevant to look at anyway.

 

If you truly want to look at only the signal levels. You should break open your headsets USB-dongle and measure the voltages at the USB jack of the device. That will at least show you what the signals look like with correct termination and load. If your scope can capture and eye diagram, that's when you start getting somewhere! :)

 

 

Best of luck!

//swek

Just to recap I plan on sometimes using my new audiophile headphone equipment next to my computer which will be the source of the audio and sometimes using it near my big-screen television where the audio will be transferred over and 82 foot repeater USB cable from our reliable company called Monoprice whose cable can be seen at this link. 

 

 

I am figuring since I am using such are reliable cable vendor the integrity of the data at the end of this cable will be good should the DAC (Little Dot DAC_I Digital to Analog Converter) be of sufficiently high impedance not to draw down the voltage swing at the end of the cable too much. I would be very interested and appreciate of if those of you who build and designed DAC's can give me some perspective as to whether or not my thinking is correct. 

 

 

Have tried to make an illustration of what I will try to do once I receive my set up during the second week of March.

Quote:

Originally Posted by swek View Post

 

 If you do, you won't have terminated D+/D- lines which will make the signals irrelevant to look at anyway.

 

//swek

 

The criticism above was not appropriate I am aware of terminating in proper impedance and intend to use the actual DAC load as illustrated above.


Edited by bbmiller - 2/26/14 at 11:45am
post #18 of 22
I stand corrected. I misread your post and I apologize.

I still stand by my opinion that unless you want to get into eye diagrams and dropped data you are wasting your time. If the voltage is not high enough to drive your DAC, you will know, because it will drop the connection to your computer. Audio quality will not deteriorate.
post #19 of 22
Thread Starter 
Quote:
Originally Posted by swek View Post

 eye diagrams

 eye diagrams? what? Please define?

post #20 of 22
post #21 of 22
Thread Starter 

swek - thanks for that link on Eye Diagram Basics. The links on that page to find out even more than basic information on these eye diagrams prove bad. I could flounder finding more information by myself but I am wondering if you could be so good as to help me find this information in a very targeted way?

 

 

 On that information link it showed pictures such this, with its above-captioned, (both of which it would be good to learn how to reproduce during my little DAC testing exercise) so could you help me find out where to go to learn to reproduce the below?

 

 

As can be seen in Figure 4, an eye diagram can reveal important information. It can indicate the best point for sampling, divulge the SNR (signal-to-noise ratio) at the sampling point, and indicate the amount of jitter and distortion. Additionally, it can show the time variation at zero crossing, which is a measure of jitter.

 

I do not know if you have a guess as to whether or not my present oscilloscope and freeware audio signal generator is sophisticated enough reproduce the above, but here are some pictures and links, if you could comment back on you if you as to whether they are sophisticated enough that would be appreciated too.

 

My oscilloscope is BK Precision 1570a 70 MHz quad track oscilloscope  is very similar to this two track model.

Which can be bought on eBay at this link where they have a little more, but not much information.

 

My freeware sweep generator is "audio sweep generator version 3.6.32" and is very similar to this slightly different model I am able to get information about.

 

swek - Your help has been greatly appreciated and any further help you could find a time to give me would also be greatly appreciated.

post #22 of 22

Here's a link to setting up an eye-pattern type display on a digital Tektronix 'scope.

http://www.tek.com/support/faqs/it-possible-display-eye-pattern-my-oscilloscope

 

You have an analog scope, and I am at a loss as to whether a similar result can be produced with one of those.

 

Edit: Now that I think about it. Since an analog oscilloscope does not sample, but only displays real time measurements, I cannot see how you're going to be able to reproduce an eye-pattern with it.


Edited by swek - 2/27/14 at 2:24am
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