Help understanding a digital signal.

[robbbby]

I have esi juli@, i've read a bit about people modifying the digital board, I figured for a few bucks in quality caps I would give it a shot. I asked a friend of mine who is an electronics guy help with doing the board mods. He told me since it is just digital out that I am wasting my time and that the caps do nothing for a digital signal.

 

I got into a bit of a dsicussion within about digital output and he said that the digital output signal is the exact same from a $50 motherboard with onboard sound, to the digital out of my julia and comapred to anything else like a lynx aes16, he said it's bit perfect since it is a digital out.

 

I unfortunately don't know much about the subject so I could not argue. I was just hoping somebody here could exaplain to me how the digital out differs from the above mentioned various sources. Are they all the exact same up until the reach your dac?

 

[Mr.C]

No, they are not. I believe Steve N or Gordon Rankin explained that while the signal is digital, the transmission medium is an electrical impulse ie. in the analog domain. Therefore, the quality of the signal can be influenced by the quality of the electronics sending the signal. Moreover, the quality of the clock affects the jitter in the s/pdif signal which will carry into the DAC.

 

[Nicholas.Bedworth]

The issue here is that the notion of a binary digit, or bit, is an abstract mathematical concept.

 

Digital signals are electrical waveforms that just look funnily. They have extremely sharp corners, which are formed by the high frequency components in the waveform spectrum. And you better believe that capacitance alters the shape of the waveforms such that downstream equipment might not interpret them correctly information-wise, or at the right time (the jitter phenomenon mentioned by the OP).

 

In addition, if there's digital media player software involved, the device may not deliver output data which is a bit-perfect copy of the original. Typically media players do volume control dithering, resampling, and otherwise alter the bitstream. You can defeat these in some cases, but it's always something to watch out for.

 

[robbbby]

Thanks guys, keep the info coming. Even some links to some good reading would be appreciated. Like I mentioned i'm not well versed in all the science behind it but I assumed it was much more than the "it's the same, it's all 1's and 0's", that you hear people often say.

 

[Nicholas.Bedworth]

Look for digital audio entries... quantization, A/D and D/A conversion, compression, etc.

 

There's a lot of good material there. You've definitely understood the main point, the ones and zeros are conceptual, the rest is electrical. Fourier analysis shows how all the sine and cosine waves add up to form, in this case, a square wave. And these sinuosoidally-shaped, continuous waveforms are adversely affected by trace width, impedance, resistance, sun spots and on and on.

 

So there's nothing magical about digital signalling, and as you get deeper into the topic, this technique introdces all kinds of problems of its own.

 

[audioengr]

Digital signals for audio streaming are a special case. Jitter is a concern. You can read about jitter here:

 

http://www.positive-feedback.com/Issue43/jitter.htm

 

http://www.positive-feedback.com/Issue22/nugent.htm

 

http://www.positive-feedback.com/Issue14/spdif.htm

 

For a typical mass-marketed interface, the mods to improve jitter include:

 

1) replace the master oscillator (clock) with something with lower jitter

 

2) add high-performance power decoupling caps on all of the involved devices - improve bulk decoupling also

 

3) improve the S/PDIF output circuit by: upgrading signal coupling caps, improving power delivery/power decoupling, eliminating long impedance mismatched traces, wire with impedance controlled coax cable, etc...

 

 

Steve N.

Empirical Audio