[eric343]

Is this single-mode or multi-mode fiber?

(the difference is that in single-mode, the light travels in a straight-down-the-center path. With multi-mode, it bounces around the inside of the fiber, and a pulse sent at a given time will fast spread out in the time domain.)

 

[dd3mon]

Quote:

Originally Posted by HMan I am sorry if I'm coming across too strong. Trust me, I'll be the first to fall on my sword if anybody can provide a reasonable explanation. I just don't see how glass versus plastic could have tonal or sibilance changes.

I'm a recent computer science graduate and the "difference" between one digital source (using a digital output) or better yet the difference between digital cables (that actually got the data through) seemed ridiculous to me.

However, there is a key factor about digital audio that is irrelevant to computer data, the clock. The clock inside your computer (let's say on a network card that determines when exactly to send a packet of data over a network) is ridiculously inaccurate on high-end audio standards. Since audio digital to analog conversion is done on the fly, the bits arriving exactly on time is crucial to getting the conversion done well. Unfortunately that's where budget sources (computer, cheap dvd and cd players, etc) fail miserably. Their clocks are very inaccurate and therefore provide a great deal of jitter to screw up the signal. Cables can have the same effect if they don't transmit the data exactly as it's given to them (we're talking pico-seconds of error here that can make an audible difference!).

A website devoted to jitter (and more devoted to selling their anti-jitter product) has some great reading on the subject. Here's the link:

http://www.jitter.de/english/engc_navfr.html

Despite the obvious bias, they do explain the basics well.

-dd3mon

 

[HMan]

Quote:

Originally Posted by gaboo I think the sound difference is due to the different refraction coefficient between glass and plastic. This results in different jitter, despite the almost nil bit error rate. Remeber: adio (from the DAC) = bits + clock. Plenty of info here: http://www6.head-fi.org/forums/showthread.php?t=90069

The index of refraction for glass is 1.50-1.62. The index of refraction for plastic is from 1.45 - 1.60. In any case we are talking about a timing difference in sub picosecond range. Given that talking about 163 nanosecond clock rate with an allowable +-20 nanoseconds of jitter in the signal, I don't see that the refraction can make any difference.

 

[gaboo]

If someone can find the specs for TOSLINK (or ADAT optical for that matter) I'll add them to faq-in-progress page...

 

[HMan]

Quote:

Originally Posted by dd3mon Cables can have the same effect if they don't transmit the data exactly as it's given to them (we're talking pico-seconds of error here that can make an audible difference!).

My copy of the IEC958 specification states that it has a +/- 20 nanosecond margin of clock jitter. If that is not the case, then it explains quite a bit.

 

[gaboo]

Quote:

Originally Posted by HMan The index of refraction for glass is 1.50-1.62. The index of refraction for plastic is from 1.45 - 1.60. In any case we are talking about a timing difference in sub picosecond range. Given that talking about 163 nanosecond clock rate with an allowable +-20 nanoseconds of jitter in the signal, I don't see that the refraction can make any difference.

I'm no expert on optics, I was just tossing an idea around. Most of the studies on jitter have been done on electrical. It seems there are few AEs that know optics...

Anyway, I did find this paper, which seems to support your statement that jitter due to fiber is within ps.

But, toshiba has this product page, which shows much higher values. It looks like most of the jitter happens at the transmitter/receiver.

 

[HMan]

Quote:

Originally Posted by gaboo If someone can find the specs for TOSLINK (or ADAT optical for that matter) I'll add them to faq-in-progress page...

The S/PIF IEC958 specification is copyrighted and used to cost quite a bit of money. Thank you Sony and Philips. I don't know if this true any more.

A good link to the subject can be found here.

 

[gaboo]

Quote:

Originally Posted by HMan The S/PIF IEC958 specification is copyrighted and used to cost quite a bit of money. Thank you Sony and Philips. I don't know if this true any more. A good link to the subject can be found here.

I've linked http://www.audioprecision.com/bin/Tn-26.pdf on the jitter thread. It covers the updated IEC 60958, and (blush) it covers toslink...

 

[gaboo]

Toslink uses plastic multi-mode optical fiber with a
red light-emitting diode (LED) transmitter and a photo
diode receiver. The transmission distance is limited to less
than a few yards (or meters). IEC60958-3 has a section for
defining this format but it is still “under consideration.” As a
result, methods of defining receiver and transmitter
performance do not have a benchmark to evaluate against.

 

[gaboo]

So it looks like the light disperses through the fiber (multi mode) => jitter plenty.

Can anyone tell what kind of fiber was tested in the umd paper?

 

[dd3mon]

I just compared my glass to my plastic again. The glass seems more extended, clearer and more detailed than the plastic. The plastic sounds a tiny bit thin (due to less bass extension) and seems to have some "haze" to the sound.

My plastic optical is super-duper cheap however, it's a <$10 GE model. My glass is the glass optical from eBay (reasonably priced, great looks and build quality).

-dd3mon

 

[eric343]

Quote:

Originally Posted by gaboo Toslink uses plastic multi-mode optical fiber with a red light-emitting diode (LED) transmitter and a photo diode receiver. The transmission distance is limited to less than a few yards (or meters). IEC60958-3 has a section for defining this format but it is still “under consideration.” As a result, methods of defining receiver and transmitter performance do not have a benchmark to evaluate against.

Well, guess it's time to try using a single-mode fiber...

 

[gaboo]

Quote:

Originally Posted by eric343 Well, guess it's time to try using a single-mode fiber...

Is there plastic single-mode? Is it cheap? Will it work despite the spec?

 

[tomek]

Quote:

Originally Posted by dd3mon I just compared my glass to my plastic again. The glass seems more extended, clearer and more detailed than the plastic. The plastic sounds a tiny bit thin (due to less bass extension) and seems to have some "haze" to the sound. My plastic optical is super-duper cheap however, it's a <$10 GE model. My glass is the glass optical from eBay (reasonably priced, great looks and build quality). -dd3mon

I guess we have different preferences. I find the glass to be overly analytical. It makes a digital cable feel 'digital', and not in a good way. Like the equivalent of a silver cable.

My plastic is more musical, and I find myself listening to my music not analzying my gear more when I use it, and that's what we want as the bottom line.

BTW, mine isn't as cheap, so that might be the issue.

 

[gaboo]

Quote:

Originally Posted by tomek My plastic is more musical, and I find myself listening to my music not analzying my gear more when I use it, and that's what we want as the bottom line. BTW, mine isn't as cheap, so that might be the issue.

What is your DAC?