A basic question about optical digital output - AVS Forum
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post #1 of 5 Old 07-29-2012, 03:32 PM - Thread Starter
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Disclaimer! Forgive any egregious newbie oversights/misunderstandings - My technical audio knowledge is only casual!

I use a Harman Kardon HK3490 and a couple Infinity Primus 162 speakers for my personal audio setup. In the past I have, for the most part, listened to vinyl and CDs through this setup, and would occasionally plug in my laptop or ipod with a 1/8" to RCA cable.

After purchasing a NAS device and consolidating and organizing a large digital library of lossless audio, I have now shifted to almost exclusively using my stereo setup to play music through a laptop. Having just purchased a new computer, I have allocated my old laptop (late 2007 macbook) as a permanent part of my stereo cabinet.

Now, I have just continued on using the 1/8" to RCA connection as I always have, but I just recently read about optimizing sound quality by connecting the laptop via an optical digital toslink cable (macbook's 1/8" outputs double as digital audio out via a mini adapter).

As for my questions, let's start slow:

1. First, am I correct in my understanding as follows?: When I have a laptop or mp3 player connected to a stereo receiver via a 1/8" to RCA cable, the laptop or mp3 player has a built-in Digital-to-Analog converter that takes the digital sound information and converts it into an analog signal, which is then passed along to the receiver where it is amplified and sent to the speakers.

And with a optical digital cable running from my laptop, the digital file information would be streamed over to the receiver, and then a Digital-to-Analog converter in the receiver itself would convert the digital info to an analog signal.


I just want to make sure I have that correct - let me know if I'm off at all.


2. Second, after searching around a bit, I seem to be finding contradictory advice. My first instinct when I learned my laptop had optical digital output was to forego my current connection and get a toslink cable, but maybe that's not the case. What are the differences in my current setup vs. a optical digital setup? Is there any advantage in converting the signal to analog in the receiver itself, and why? Which will result in better sound quality?

Finally, what exactly is "jitter" ? - this seems to come up quite a bit

Hope that makes sense. If this helps/is relevant, the specs on the macbook's digital out are as follows:

"The headphone / line output jack accommodates optical digital audio output, analog audio output with a 24-bit, 44.1-192 kHz D/A converter, digital audio output up to 24-bit stereo and 44.1-192 kHz sampling rate and supporting encoded digital audio output (AC3 and DTS)"

Also, in the event that I do go for the optical digital connection, I was going to go with this:

http://www.amazon.com/AmazonBasics-Digital-Optical-Audio-Toslink/dp/B001TH7GSW/ref=sr_1_1?s=electronics&ie=UTF8&qid=1343594369&sr=1-1&keywords=optical+cable+digital+audio

and this:

http://www.amazon.com/gp/product/B000FMM8TO/ref=pd_lpo_k2_dp_sr_1?pf_rd_p=486539851&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=B0002J24OO&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0SYWN2C850KHXNC1B6X0

Are these cables good?

Thanks!
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post #2 of 5 Old 07-29-2012, 05:02 PM
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Quote:
Originally Posted by machajew View Post

1. First, am I correct in my understanding as follows?: When I have a laptop or mp3 player connected to a stereo receiver via a 1/8" to RCA cable, the laptop or mp3 player has a built-in Digital-to-Analog converter that takes the digital sound information and converts it into an analog signal, which is then passed along to the receiver where it is amplified and sent to the speakers. And with a optical digital cable running from my laptop, the digital file information would be streamed over to the receiver, and then a Digital-to-Analog converter in the receiver itself would convert the digital info to an analog signal. I just want to make sure I have that correct - let me know if I'm off at all.

Yes you have got the basics correct. However with your receiver which I am not familiar with, it may convert analog to digital for its processing of sub management or EQ or other such things. It depends if your receiver is just like an AVR but only with two channels... or it may just keep everything as analog. If it does do things to the audio signal in the digital domain, then you would be better off feeding it a digital signal to begin with to save unnecessary conversion steps. Ideally you want to keep things digital for as long as possible and only convert to analog at the last stage in the chain.



Quote:
2. Second, after searching around a bit, I seem to be finding contradictory advice. My first instinct when I learned my laptop had optical digital output was to forego my current connection and get a toslink cable, but maybe that's not the case. What are the differences in my current setup vs. a optical digital setup? Is there any advantage in converting the signal to analog in the receiver itself, and why? Which will result in better sound quality?

Optical will certainly be worth trying as it won't cost much for a cable so you can hear for yourself. But as you are using a laptop and can't add a good soundcard that may perform better than the on-board audio... your next option would be to try a USB to SPDIF converter.

Quote:
Finally, what exactly is "jitter" ? - this seems to come up quite a bit

It is the search for lower jitter rates that has everybody trying different connections from their computers to DACs.
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post #3 of 5 Old 07-29-2012, 05:07 PM
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I would definitely think sending the audio digitally to the receiver would be better, no chance of interference
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post #4 of 5 Old 07-30-2012, 01:51 AM
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The mini Toslink to Toslink cable is sweet with a mac.

http://www.amazon.com/Optical-Toslink-Fiber-Plated-Digital/dp/B001VJ680U/ref=pd_sim_e_8

With an older mac, you will be limited on the USB capability. Adding a halfway decent DAC to your rig would be a nice thing to do. I have used the Grant Fidelity TubeDAC 0.9 and 1.1 as well as the Maverick Audio TubeMagic D1 with great results. Add in a copy of Pure Music to your mac as the front end for iTunes and you will have a very nice sounding rig as well as the ability to play high res FLAC files (mac no like)!

Surfing the Bleeding Edge!
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post #5 of 5 Old 07-30-2012, 04:26 AM
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Quote:
Originally Posted by machajew View Post

Finally, what exactly is "jitter" ? - this seems to come up quite a bit

Looks like your other questions were answered well, so I won't belabor them.

This one, not so much. ;-)

What is jitter?

Jitter is to me better described as FM distortion. It is a distortion that artificially changes the frequencies contained in an audio signal, as opposed to AM distortion AKA Harmonic and IM distortion, which artificially change the amplitude of an audio signal. In both cases the changes are usually short term although they don't have to be.

One example of FM distortion is Doppler distortion that causes a change in the pitch of a train whistle as it goes by. Another example of FM distortion is the vibrato that musicians apply to their voices as they sing or the instruments that they play.

If you want to hear audible jitter, listen to a LP. It is nearly impossible to play back a LP record without adding audible amounts of flutter and wow which are kinds of jitter. Flutter is a rapid pulsating change in the pitch. Wow is a slower change that is more like what happens to the train whistle I just mentioned except it always comes back to the same frequency.

There are two frequencies that are associated with jitter. One is the frequency of the signal that the jitter is applied to, and the other is the frequency of the artificial change in frequency. For example a violinist applies vibrato to his playing by rapidly moving his hand in such a way as to make quick changes to the tension of the string. The frequency of the signal is the note that he is playing, and the frequency of the jitter is the rate at which he moves his hand this way.

At low and medium frequencies, jitter makes music sound watery. At high frequencies jitter actually adds additional tones that make the music sound more complex or full, but do so in ways that are not harmonically related to the music so it often ends up making the music sound sour. Another term that is used to describe the audible effects of mid and high frequency jitter is roughness which I think is related to the sourness that I previously mentioned.

In digital audio, jitter is due to artificial alterations of the time base of the samples that make up the music. This time base is usually called the clock. One interesting aspect of digital audio is that jitter can always be reduced to vanishing amounts by regenerating a new clock that is a smoothed out version of the existing clock signal which has some how become unsteady. Of course if jitter is truly massive, the whole signal may be lost.

The current concern over jitter is probably due to the errors in judgement that come from doing sighted evaluations of audio gear. Sighted evaluations, that is comparisons where the listener knows what he is listening to at any moment are well-known to be strongly affected by the state of mind of the listener. If you are feeling bad it is less likely that things are going to sound right. If you are told that something is going to sound a certain way you may believe that it does, even when it actually sounds differently.

Back when digital audio was first being introduced, the first thing that people noticed is that digital recordings generally sounded significantly different from the analog recordings of the day. Compare and contrast how quickly and profoundly CD and DVD took over the marketplace for recordings of music from analog LPs and tapes and video tapes as compared to what is happening with SACD and Blu Ray. SACD completely failed to gain traction in the mainstream, while Blu Ray is only relatively slowly taking over the marketplace for video. In fact physical media itself is in question. This is signficant because transmission of audio and video over networks inherently causes massive amounts of jitter. However this jitter is usually dealt with quite efficiently.

However, people are creatures of risk aversion and resistance to change. A tiny vocal minority starting perceiving bad sound from digtial recordings. They started looking for reasons why digtial which measured to orders of magnitude better performance than analog sounded bad to them. It was easy to show with blind tests at the time that most digital equipment of the day reproduced music in ways that were either far more perfect than analog, or just plain indistinguishable from the source.

However, the doubters continued to complain and it was thought that some previously unmeasured form of distortion was the problem. Fact is that measuring jitter had been done for decades up until this time, particularly as flutter and wow and it was also known that even the best analog equipment either have audible jitter or are close enough to have audible jitter that a little slipup in the ongoing need of analog equipment to be maintained would lead to audible jitter. If anything, the fact that even the digital equipment of the day had very low jitter made it sound different than the cherished and trusted analog gear that people had been using for years.

High end audio had a big problem with digital equipment because it worked too well and digital equipment was quickly becoming too inexpensive to profit from. This cut their flow of cash. Their response was to create a myth about the difficulty of making audio work well in digital environments which they purported to cure by separating the DACs from the transports in high end music players. They further made this myth apparently truthful by often doing this separation of function very badly, technically speaking. A self-fulfilling prophecy was created.

Today, many people who want to sell overpriced new or retro technology use jitter as a justification for more complex equipment and higher prices. If fact the original Sony CDP 101 from 1983, one of the first two digital players marketed to the mainstream in the world has very low jitter. It does have some very minor possibly audible problems, but jitter is not usually one of them.

One current Audiophile Myth relates to the use of optical connections as compared to using coax. Hands are waved and words about reflections in the fiber are thrown about. We hear the J-word (jitter). In fact, all things considered, optical connections are more likely to be trouble free because they are both immune to being sensitive to or they themselves creating electrical interference. Coax connections can do both and in clearly audible ways.
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