Optical versus Coax

We had the same discussion on the TIPS list and this was the result plus some additions. Toslink is an inferior connection to coax. There is another element called jitter. In audiophile circles coax is considered the more precise connection provided you use the correct wire and connectors. 75 ohm DVD video cable works great.

digital is digital only in the digital domain. The products of jitter do not seem to appear until you actually decode the signal into analog. It changes the sonic signature of the D/A convertor. This is why you can digitally record something and have a better sound quality due to the player having less system jitter during playback. Jitter errors can also be introduced by the CD itself.

In the end don't sweat it because few even setup the speakers and viewing position correctly anyway which means so much more than this issue. I use toslink also for my HDTV receiver. Home Theater is not what drives me as much as two channel audio. In this case I would use coax and if possible use "AT&T glass" optical connectors (very unpopular so coax is king). This is also why I wanted a 75 ohm digital coax out on the receiver - just in case.

If you can hear the difference between Dolby Digital and DTS then you might be able to hear the difference between toslink and coax. (The point here is the ability to hear a difference and not to make some sort of comparison between connections and encoding processes)

Richard F. Fisher

Richard,

Have you done an A-B and can you hear the difference?

ToddK

Yes

Richard

Richard,

In summary:
- Toslink is inferior.
- You can hear the difference.
- Don't sweat the differences.
- You use Toslink for HDTV.

I think understand now. Thanks for the insite.

ToddK

Coax probably provides a more direct path, eliminating the fiber-optic conversion on each end. Not a biggie - the datastream is recovered with no errors with a good interconnect. Optical fiber has no EMI or noise coupling concerns and a smaller, light-weight cable. Less chance of signal distortion over longer cable lengths with optical. But if equipment only a few feet apart either one is fine. I use optical, but have had problems with the connectors being easier to break than with coax.

EMC Guy

In terms of what you hear, coaxial audio and optical audio are perfectly identical.

Ken - KQ6QV

Ken,

In terms of what you hear, coaxial audio and optical audio are perfectly identical.

thought so too, and even did an A/B. I've got both outputs on my DVD player and I configured two inputs on my receiver so that I was able to switch back and forth quickly.

Other posters have disagreed. Back in September, one said:
"Toslink is an inferior connection to coax. There is another element called jitter. In audiophile circles..."

Maybe the DVD was not an adequate sound source, but it's probably my equipment or my ears.

a

That was me.

I can't find the thread. The quick version is most any connection can be used to transfer bits provided you are recording those bits to another medium. If you are listening to that stream then the jitter products of your connection could affect the final D/A conversion process. Audiophiles have prefered coax, 75 ohm cable and connectors just like video, always over toslink due to potential jitter issues with AT&T Glass being the best but seen only on some hi-end products.

Bottom line is don't make a big deal out of it. Try both if you want to play. If you hear a difference then go with the coax. If you don't then move on.

Richard F. Fisher

Dan, I agree with you regarding the following:

The interference caused by a digital coax cable is not distinguishable by the average human ear.

The digital audio itself won't be affected by interference. What was going through my mind was that digital data is inherently noisier than analog and the noise is spread over wider frequencies (digital will always appear as noise to another unrelated system). If it leaks off a poorly constructed shielded cable it could get into other analog video/audio cables and degrade the SNR. You might hear some hiss in the sound or interference in the picture. I doubt if it's a serious concern, but when people say that noise on power cords affects things, this digital coax is even a worse source of noise in a system. Just make sure a good cable is used.

emc guy

I'm not an expert on toslink, but usually jitter is anticipated for in a comm channel and the data is resampled with a clock on the receiver end. Therefore several nanoseconds of jitter are not seen because the data is resampled when the data is known valid. Here's something I found at http://www.epanorama.net/documents/audio/spdif.html :

The AES/EBU standard for serial digital audio uses typically 163 ns clock rate and allows up to +-20 ns of jitter in the signal. This peaks to peak value of 40 ns is aroun 1/4 of the unit interval. D/A conversion clock jitter requirements are considrably tighter. A draft AES/EBU standard specifies the D/A converter clock at 1 ns jitter; however, a theoretical value for 16-bit audio could be as small as 0.1 nsec. Small jitter D/A conversion is implemented by using separate PLL clocks for data recover and DAC and by using a buffering between data recovery and DAC.

In my opinion I seriously doubt you could hear the difference between optical and coax, but I guess some people could if they can also hear differences with different power cords.

Another interesting item related to my earlier post - the bandwidth of this digital channel can go up to 6 MHz. That easily covers baseband video in all flavors so keep those cables quiet.

emc guy

I dug this up. the effects of Jitter are very well documented and the credit for the initial findings of this phenomena for audio go to Stereophile and Ed Mietner in the early 90's.

http://www.digido.com/jitteressay.html

You will get a webpage. On the left box click on "articles" then "jitter".

Richard F. Fisher

Richard, hopefully this has improved since 1996 or so when your reference was written.

Here's a quote from another article that indicates improvements in the D/A converters used today:

"Toslink's optical conversion eliminates this interference completely. Talking also with John Stronczer of Bel Canto, at least two respected digital engineers acknowledged that today's D/A chips, sample rate converters and phase-lock loops exhibit inherent jitter rejection far superior to what was possible just a few years ago. Hence RCA's touted jitter advantage should play out as less of a real boon. If a plastic or glass fiber optical link were properly engineered, one should indeed expect smaller rather than greater differences."

This is from http://www.6moons.com/audioreviews/toslink/toslink.html for those wanting to read the complete article.

Who knows - it may be the interface type is unimportant as long as good cables are used.

emc guy

Check out Stereophile for the last 1-2 years and you will see that while it has improved it is not a "for sure" regardless of how much you spend. I do not know of a comparable way to measure this in a surround decoder and they have not done this as of yet. There was an audiophile company a while back that made a jitter reduction interface between the CD player and the D/A convertor and many other products but they sadly went under and possibly unnecessary now as you point out. As always in audio it all depends and nothing can be just taken for granted - synergy due to lack of standards. Correct and consistent analog audio reproduction beyond headphones is yet to be defeated and common. It doesn't have the simple scientific discipline of digital where bits are bits. The good news for all consumers is it all works and you will have fun regardless but if you want to push the envelope there are many, many things to take into consideration and this is just one of them. I guess that is why when you go beyond fun it really is a hobby since there is so much to learn about and plenty of snake oil on the way. Fun is obviously much easier so use that optical - I sure do! I only sweat about my 2 channel rig.

Richard F. Fisher