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In Reply to: RE: usb to spdif with BNC-Go posted by Thorsten on February 14, 2014 at 12:42:58
which is the important thing and why some want correct BNCs.Quote ''You may talk the talk, why don't demonstrate that you also walk the walk?'' Unquote.
Where have you walked the walked other than asserting that the connector equation is not important?
Contrast this to iFi stressing the 90R terminations for your design of their USB devices as being 'important' (which I think it may well be). Are you now denying this?
Edits: 02/14/14Follow Ups:
Fred,
Let us be clear, for both SPDIF and USB Cable impedance and termination (in the technical sense of transmission line termination http://en.wikipedia.org/wiki/Electrical_termination ) matter greatly.
You can read more why here:
http://en.wikipedia.org/wiki/Reflections_of_signals_on_conducting_lines
Incorrect cable impedance or termination resistance will cause reflections that will corrupt the signal. A short or open circuit will reflect 100% of the signal. Less extreme mismatches reflect less signal.
Discontinuities in impedance at the connection points also can matter IF (and that is a big if) they are significant in length compared to the wavelength of the signal or to the equivalent in distance of the rise time of the signal.
So we need to look at the signals.
SPDIF - uses manchester code so appx. 3MHz (44.1K samplerate) to 12MHz (192KHz)
USB2.0 at high speed - uses a complex bidirectional coding scheme, 480MHz maximum frequency
So USB at high speed contains 40 Times the frequency of SPDIF at 192KHz and thus can tolerate different impedances along the line of 40 timesless than SPDIF.
For reference, at 12MHz we are dealing with a wavelength of 17.5 meters! The impedance discontinuity of an RCA plug is pretty much irrelevant at this frequency.
Measured on a TDR a 75R BNC connector can show that it is actually 66 Ohm and causes a glitch of appx 4.2mm total length.
RCA Connectors (plug & socket) show an impedance of 52Ohm (this is rather more variable) and causes a glitch of around 12mm total length.
This translates to a lot less than a 1nS glitch in either case with a 12% (BNC) and 30% (RCA) of the signal reflected.
Now the transition time for the LMV7219 Comparator (e.g. the SPDIF input often used with the ES9018) is on average around 10nS. This is much better than common SPDIF receivers, which are by far slower.
If such a glitch as described arrives close with the signal causing a transition on the receiving end it will simply be swamped by the comparators (or SPDIF receivers) slow transition. If the reflections are significantly delayed they simply do not matter.
So, bottom line, both BNC and RCA cause some reflections, both reflections are short enough in the time domain to not matter at SPDIF Data rates, objectively speaking.
If you have objective data that proves different, make it public.
Ciao T
Sometimes I'd like to be the water
sometimes shallow, sometimes wild.
Born high in the mountains,
even the seas would be mine.
(Translated from the song "Aus der ferne" by City)
with what is heard and seen. Who is now talking the talk?
We can't "hear" what you "hear". But we can "see" what you can see.
Is there something that you can show us?
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Unfortunately my Tetronix 400MHz has just blown the power supply and so I cannot show right now. My historical paper records are in a different country as well.
But any serious and experienced audiophile with technical skills will know that correct terminations and cabling are important in getting the best sound out of digital replay, especially for spdif connections.
There is no point in switching arguments between connectors and terminations as is the case in my exchanges with Thorsten.
Fred,
> There is no point in switching arguments between connectors and
> terminations as is the case in my exchanges with Thorsten.
Termination is a clearly defined technical in the context of high frequency cabling, as is the use of the term. If you want to change that definition and use when it suits you, then you are switching arguments or are deliberately obfuscating things.
400MHz gives you around 2.5 nS time domain resolution.
The glitches caused by RCA Plug/Sockets together are one fifth of that. There is no way you can actually see these glitches unless you get much better gear, neither my standard set of 'scopes nor your "the dog ate my homework" Tek with blown powersupply can show the differences between RCA & BNC connectors.
If you actually see any differences using this gear you either changed more than just connectors or the method has a fundamental flaw. You should really check your work.
You can ask Steve to help. He has the instrumentation needed.
Ciao T
Sometimes I'd like to be the water
sometimes shallow, sometimes wild.
Born high in the mountains,
even the seas would be mine.
(Translated from the song "Aus der ferne" by City)
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
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