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In Reply to: RE: Mythbusting posted by Ole Lund Christensen on August 20, 2011 at 11:01:28
Let us assume that your bank account on Wednesday has $65,535 .That is written somewhere in the bank computer as $1111111111111111, the largest possible 16 bit number.
Thursday you buy the big Hi-Fi system of your dreams, and you have $0000000000000001 left.
So in that weekend you buy a $1 dinner :-)
But Monday the yearly bonus arrives to your bank account and you have $1111111111111111 again.
Replace the $ with sound levels, and you have some big transients, from maximum to almost silent and back to maximum.
Now if the bonus came on Friday, you could invite all your friends to a weekend dinner.
Clearly the arrival date of the bonus is important for your weekend dinner.
Replace again $ with sound levels, and you see that the arrival time of the big sound level of 1111111111111111 is important.
Where is the arrival time written? On your bank account it is written beside the $ amount, like this 2/28/2011 $65,535
In the S/PDIF digital audio signal, we only have one line, so all data have to be sent serial, not parralel. Plus we have a lot of extra bits giving space for stereo 24 bit audio as well as instructions and clock.
That means the signal is not anymore 44,1 kHz but several Mega Hertz. At such a frequency cables behave like transmssionlines, and impedances of cable and connectors have to match to avoid reflections (signals going in the wrong direction) Depending on the high frequency performance of the cable, it becomes difficult for the receiver IC to know, when the voltage is 0 and when it changed to 1, because it now change at a slow slope, and no longer makes a sharp change. The arrival time code is mixed in as a part of the signal and it becomes uncertain too. A lot of work have been done on reducing this problem in receiver ICs, but it just reduced, not solved.
So S/PDIF cables have to have the correct impedance, not just low capacitance and low inductance. To make life easier for the receiver IC. You can read more about S/PDIF in Wikipedia.
All this is well known in analog TV, because here the reflections are seen as ghost lines in the analog picture.
There is a much better way of sending digital audio, and it is used inside most CD players. It is called I2S and it uses 3 lines instead of just 1.
see link
Follow Ups:
Hey Ole,
I thought i2s was one of the advantages pci cards.
No one here remembers the bending of our minds
Did you think of I2C? This is a 2 wire computer standard, I2S is a 3 wire digital audio standard. They are based on the same ideas.
see link
Hi Ole,
Forgive me for being confusing. Let me explain.
I was thinking i2s. I probably have this wrong but I remember charles Hansen talking about the phillips sony guys needing to do some measurments or such and the i2s protocol was born...it was not envisioned as a connection between boxes IIRC.
I thought that all cdps had this connection internally to the dac chip. I took your post to mean that this was better than spdif and that to me at least made sense that a short connection to the dac chip would be better than a long cable connection. I had thought that by default a soundcard using its own dacs would have an i2s connection of sorts. Is that not right?
Though I think you took my post to mean external connection from a soundcard to an external dac. There was once a card that did that but the links are dead and this is the best I could find:
No one here remembers the bending of our minds
Some of use use I2S externally too, with good results. Like S/PDIF though, the devil is in the detials. The Transmission-lines must be terminated and the edge-rates must be fast in order to avoid adding significant jitter. Really low-loss high-bandwidth cables are required.
Do you know of any systems that run I2S where the clock goes in the reverse direction from the data?
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
The I 2 S-e interface is supposed to allow the DAC clock to control the data flow. Other than the Sonic Frontiers/Assemblage line and the Muse products that use it (the two systems are not compatible though), I don't think there are others. You can ask Charles Hansen -- though he hates any external implementation of I 2 S -- for a copy/link to the white papers, Chris Johnson at partsconnexion.com (he was the head of Sonic Frontiers/Assemblage) or Kevin Halverson at hrtechnologies.com (founder and chief engineer at Muse Electronics) for more info.
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