Newsgroups: rec.audio.tech
Subject: Re: Digital audio connections help?
References: <[email protected]> <[email protected]>
[email protected] (Dave Platt) writes:

> In article <[email protected]>,
> Sseadoubleyou  wrote:
> 
> >Was wondering if anyone knew anything here about digital audio
> >connections on audio systems?  My system has both optical (fiber optic)
> >as well as coaxil.  I want to connect my DVD player up digitally to the
> >system.  Which digital connection is better, optical or coaxil?  I
> >thought I heard somewhere that coaxil is a 'slower' connection and
> >optical gets the signal thru faster?
> 
> >Can anyone confirm this?
> 
> In one sense, coaxial may be "slower".  The electrical signal
> propagating through a typical coax will be travelling at somewhere
> around 66%-80% of the speed of light in a vacuum, while the signal on
> an optical cable may be travelling faster than that (it depends on the
> index of refraction of the plastic). 

Basic Principles of Fiber Optics
http://www.corningcablesystems.com/web/college/fibertutorial.nsf/appprin?OpenForm

The index of refraction (IOR) is a way of measuring the speed of light
in a material. Light travels fastest in a vacuum, such as outer
space. The actual speed of light in a vacuum is 300 000 kilometers per
second, or 186 000 miles per second. 

Notice that the typical value for the cladding of an optical fiber is
1.46. The core value is 1.48. The larger the index of refraction, the
more slowly light travels in that medium. 

I think those numbers are for fiber optics made of glass 
(the fiber optic types used in telecommunications). 
The speed of the signal in optical fiber would be then around
200 000 kilometers per second. 

Plastic Optical Fiber, abbreviated POF, typically uses PMMA (acrylic),
a general-purpose resin as the core material, and fluorinated polymers 
for the clad material. This should apply to fiber optic cables 
used for digital audio.
The simplex cable with PMMA fiber finds application for short distance
analog and digital signal transmission. The step index fiber has a
large area of cross-section and a high numerical aperture,
facilitating easy coupling with transmitting and receiving devices. 
Core Refractive Index for such cable would be around 1.492 and 
Cladding Refractive Index 1.405 to 1.417 according to 
http://www.telenet-systems.com/cableSpecs.php 
There could be pastic fiber optic cable with sligtly different 
Refractive Index and so slightly different speed. I don't expect 
large variations on this. 

So for the plasic fiber optic cable the speed would be also in 
that around 200 000 kilometers per second. Around 66% of the speed
of light in a vacuum. So certain coaxial cables could carry 
the signal at same speed or even faster thn typical fiber optic cable
types. So the fiber optics is not faster than coax. 

>  Hence, a coax cable may take a
> few nanoseconds longer to get the signal to your A/V receiver/preamp
> than an optical cable would.

So the signal might be slightly faster in one of them. 
Generally in commonly used cable type there should not be 
any considerable speed difference between coax and fiber!
 
> You are *not* going to hear a difference from this effect, even if it
> occurs.  Period.

You are absolutely right on this. 

> There are those who feel that consumer-electronics-type optical links,
> using the inexpensive plastic TOSLINK cables (or even the expensive,
> boutique TOSLINK cables), may sound somewhat inferior, because the
> inexpensive LEDs used to drive them can have a slow rise time, and
> that this can lead to timing "jitter" in the receiver, if the optical
> receiver circuitry and DACs are poorly designed or implemented.  A
> good coax-cable driver can have a very fast rise time and would not
> suffer from this problem.
> 
> I don't know of any actual, reliable, repeatable tests which show that
> this is actually an issue.

I can agree on this.

> In some situations, with some equipment, using an optical connection
> might produce better sound, because it allows the DVD player and the
> A/V preamp to be electrically isolated from one another.  A coax cable
> connects the two components via its shield, and if they're plugged
> into outlets on different electrical circuits this could create a
> ground loop and introduce some amount of hum into the sound (and not
> just the sound on the digital input, either).

I can agree on this also. 
 
> In most situations, however, I believe that the two connections would
> result in sound which is indistinguishable - it doesn't matter which
> one you use.  Use whichever is most convenient, and don't let yourself
> be seduced into spending large amounts of money on an expensive
> "pro-grade" coax or optical cable with which to make the connection.

Absolutely right. 

> If you only need a short cable, I see that Radio Shack currently has a
> 3-foot "composite video cable, and TOSLINK optical cable" combination
> on sale for $2.48 (!!).  Catalog nubmer 15-1591.  You could buy this,
> hook up *both* cables (a composite-video cable is 75 ohms, and is
> close to ideal for a digital-audio coax link), and switch your A/V
> receiver back and forth to see whether you can hear a difference.

75 ohm ciaxial cable designed to carry video signals is 
the cable type the digital-audio coax link has been designed 
to wotk with originally in the standard. 

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/