Rega DAC filters and coax vs optical

[jim1961]

I recently got a Rega DAC and is feed by a Rega Apollo. I was curious if anyone had played with the filters and reached any conclusions.

In addition, I was curious whether people here preferred coax or optical.

[PewterTA]

I don't have the Rega DAC so I can't state on the filter selection. From talking with Brian Zolner of Bricasti Design (M1 Dac), he was talking that basically the best filter is basically no filter....however you must have a filter, it more depends on the quality of the filtration done and there should only be a need for a single one. However, with peoples equipment being all different most people want some options to get what they perceive as the best.

So I'd thinking it'd be some demoing to find what you think sounds the best.

For Optical or Coax, it really depends upon the DAC and what's was given the "priority" of construction and design. Some manufacturers really gear towards one or the other and the "lesser thought of" one tends to be a more generic way to route. Now of course many make both as close together as possible so it doesn't really matter.... Only thing you can do again, is unfortunately try both ways and see what sounds best.

There's no "right answer" to either question really.

[Chris D]

Coax.

For filters, my answers is as "transparent" as possible.

[JonMarsh]

Between coax and optical, coax will usually be much preferred, as the inherent jitter of the interface with a reasonably decent cable is much better- this is a function of the transmitter/receiver units used and the intrinsic bandwidth of the cable medium, when properly implemented. Now, there are situations where the isolation quality of the S/PDIF input or output may be poor, or their may be common mode noise issues (think PC power supply with coax output output from an audio card or motherboard) where the electrical isolation advantages of optical will come to the fore, especially if the overall DAC or music transport function is, shall we say, midrange at best. Then, optical may be your better solution- only way for the average person to tell (without some pricey test gear) is just listen...

My personal experience with a variety of DACs in the last 7-8 years doesn't necessarily follow common wisdom, as the that would state that AES/EBU will not be as perfectly cable terminated as a well designed S/PDIF cable and connector. However, on really high end DAC's and systems, the AES/EBU as always been best, S/PDIF next, and optical generally trailing. AES/EBU has about 10 times higher voltage levels, as well as being balanced differential, and these factors seem to work in it's favor. If you do use optical and need a longer run, try to find an optical cable with glass fiber; they have lower loss and generally optimize the performance of the interface, including jitter.

Just my 0.02.

I think it's about time I should start I thread myself about some of my "Adventures in Digital", though they're by no means concluded! :W

[jim1961]

These 5 digital filters are built into the Wolfson WM8742 D/A chip used in the Rega DAC

These are the filter choices:

1 Linear phase half-band filter
2 Minimum phase soft-knee filter
3 Minimum phase half-band filter
4 Linear phase apodising filter
5 Minimum phase apodising filter

I thought maybe those not familiar with the Rega DAC specifically might be familiar with how these different filters translate.

--- --- ---

Can any quality RCA patch cord work as a coax digital cable?

[Chris D]

Wow, Jim, me not being a DAC expert, I don't have a clue about those filter phrases. To me, 1-3 sound like sports medicine stuff, and "apodising" to me sounds too close to the fat word "adipose". I'll let someone else talk about those. As far as the cable, though, we're talking a different impedence, so no, you'll wan to use a dedicated coax cable. (All mine are custom from Cat Cables, one of the forum sponsors here)

Jon, I am definitely not alone in saying that if you post your digital experiences here, the rest of us schmoes can definitely learn from it!

[jim1961]

Did some googling on the matter (RCA audio cables used for a digital coax connection) and it seems the consensus is that for short runs (< 1.5M), RCA audio cables usually work fine. Most did say also that 75 ohm digital coax should be used though. I am using a 0.5M RCA audio cable and it sounds fine to me.

Anyone agree, disagree?

[JeremyG]

Chris, agreed!!! I would love to see Jon's adventures in on spot, as opposed to searching through half the forum when I have questions!!!
----------------
Jim, nice to see another Texas member!!!

As long as your cable is 75 ohms no sweat. The 50 ohm stuff not only is a different impedance, but also has a different capacitance as well. Now will it matter in a six foot run? In the frequency and bandwidths we use in home audio, probably not. I would worry more about the terminations on the ends at that short of a length if it were my cables. Now 100' of low loss cable passing 20GHz? We would need to talk...

[jim1961]

Eventually, I will probably just go get the recommended cable. This logically leads to whom to trust to buy the cable from. Im not sure Walmart or Radio Shack strike confidence in me.

BTW, The Rega DAC is an improvement over the Apollo. I am still sorting out the differences, but I would generally say the new DAC sound less digital. More natural and analog sounding to my ears. Makes me wonder what the cutting edge $5K and $10K DAC's sound like. One can always dream.

[Chris D]

Jim, sure, you can jury-rig a system with cables designed for other purposes, and it'll work. I think big picture, though, if you're at the level where you're buying a stand-alone DAC (which 99% of consumers don't even have a clue what one is), you should be using the correct cables to feed it. Why buy a higher-end purposed component, focused on intricate nuance, and then feed it compromised stuff? Kind of like using a $2 oil filter from the dollar store in a car. If you happily drive a 1980 Toyota Corolla, you might not care. But if you drive a brand new BMW 5-series, sure, you could use that filter in your car, and it'll function as an oil filter. But do you really want to do that?

[jim1961]

Jim, sure, you can jury-rig a system with cables designed for other purposes, and it'll work. I think big picture, though, if you're at the level where you're buying a stand-alone DAC (which 99% of consumers don't even have a clue what one is), you should be using the correct cables to feed it. Why buy a higher-end purposed component, focused on intricate nuance, and then feed it compromised stuff? Kind of like using a $2 oil filter from the dollar store in a car. If you happily drive a 1980 Toyota Corolla, you might not care. But if you drive a brand new BMW 5-series, sure, you could use that filter in your car, and it'll function as an oil filter. But do you really want to do that?

Kinda funny that just the other day, a friend of mines wife asked him "You keep saying DAC, what does that mean exactly?"

In the audiophile world, seemingly at every turn, there are claims to what is necessary or what makes a system better that either have no audible difference or worse still are snake oil. If a $2 oil filter cleans the oil system just as well as a $12 one, and lasts as long, then your just simply wasting money in my way of looking at things in using the $12 oil filter.

Likewise, if a 75 ohm digital cable only makes a difference at runs longer than 10', then how exactly is another cable degrading anything at 2'?

Now, it may turn out to be the case that the recommended cable does have an impact at short runs. If so, I will happily embrace getting one. What interests me are the facts, not unproven claims.

[jim1961]

I called a Rega dealer in Dallas just to see what they would say. Of course I got a salesmen who tried to sell me on a $175 cable (a VERY expensive oil filter). I went to Blue Jeans cables and theirs is $16. But an interesting new wrinkle is that they said the cable must be at least 1M in length. He said a shorter one could cause errors or problems. First ive heard too short a cable can be a problem.

[jim1961]

http://www.head-fi.org/t/581931/audio-gd-digital-filter-flavor-switch-for-nfb5-nfb9-nfb10se-nfb3-1

Here is an informative link to the filter types. The Minimum phase apodising filter looks like the winner looking at those graphs.

http://www.audiostream.com/content/qa-charles-hansen-ayre-acoustics-page-2

"When a linear-phase filter is used, there is no phase shift at all, but then half of the ringing occurs before the impulse, and the other half occurs after the impulse. This never happens in the real world. In the real world there are always echoes (sound reflections from nearby objects), but they always occur after the impulse. It is impossible for echoes to occur before the event. This is one reason that standard digital technology tends to sound un-natural."

"...a minimum-phase filter has all of its ringing after the impulse. This is the only way that echoes occur in nature, and therefore a minimum phase filter generally sounds more natural than a linear phase filter."

Interesting.....

http://resonessencelabs.com/digital-filters/

"All the various trade-offs discussed here, but particularly Apodizing filters, show us a fundamental relationship: as a filter is designed to be optimum in the time domain, it cannot be optimum in the frequency domain. The two are related in a very fundamental way, it is the same relationship that governs the whole world of physics: Heisenberg’s Uncertainty principle that says if you know the particles position exactly, you cannot know its momentum, or vice-versa, is because mathematically one (the position for example) is the Fourier transform of the other (momentum for example) and as the “width” in position becomes better defined, the “width” in the momentum domain, being the Fourier transform, becomes less well defined.

In our perhaps less profound world of audio, the behavior in frequency is the Fourier transform of the behavior in time, and because of this, one gets worse as one gets better."

[wkhanna]

During our dinner conversation with Brian Zolner at the Capital Audiofest last week I asked him his thought on very same subject.
Any filtering that contains pre-ringing is bad. Pre-ringing alters the perceived audio performance (to his ear) more than just about any other thing in the DtoA conversion process.

[jim1961]

During our dinner conversation with Brian Zolner at the Capital Audiofest last week I asked him his thought on very same subject.
Any filtering that contains pre-ringing is bad. Pre-ringing alters the perceived audio performance (to his ear) more than just about any other thing in the DtoA conversion process.

So are we talking about Linear Phase Filters in regards to pre-ringing? As opposed to Minimum Phase Filters which are described as having all their ringing post?

[wkhanna]

Again, according to Brian, any filtering will change the original data.
Thus far, any D to A process will cause aliasing in the upper audio range (around anything over 10kHz iirc).
Check out the photos at the right in the link for a visual example of what aliasing is.
This 'distortion' must be dealt with in any DAC by the use of filters.
Pre & post ringing filters are optional.

[JeremyG]

I called a Rega dealer in Dallas just to see what they would say. Of course I got a salesmen who tried to sell me on a $175 cable (a VERY expensive oil filter). I went to Blue Jeans cables and theirs is $16. But an interesting new wrinkle is that they said the cable must be at least 1M in length. He said a shorter one could cause errors or problems. First ive heard too short a cable can be a problem.

Hmmm... they may be the same guys I bought my Maggies from. Did a demo using one of their Riga CD players. They were actually pretty down to earth, all things considered. Their location kinda lends them to having big spenders who don't bat eyes at $175.

Too short a cable is a new one to me. If I had my druthers I'd have the shortest cables possible to cut down on any loss. I wonder what our Cat Cables guy would say about that?

[Chris D]

Actually, I just bought a custom digital coax cable from Cat Cables, and Doug brought up that same issue.

[jim1961]

It seems like every time I manage to answer one question, another arises I hadnt even thought of. How deep is this rabbit hole anyway?

[wkhanna]

How deep is this rabbit hole anyway?

Oh my.
The surface you have not even yet scratched, dear padowan. :W

[JonMarsh]

Did some googling on the matter (RCA audio cables used for a digital coax connection) and it seems the consensus is that for short runs (< 1.5M), RCA audio cables usually work fine. Most did say also that 75 ohm digital coax should be used though. I am using a 0.5M RCA audio cable and it sounds fine to me.

Anyone agree, disagree?

Well, here's the issue- an RCA cable may be close to 75 ohm characteristic impedance or not. It's a gamble of sorts, and you certainly could get lucky. Now, here's the really weird part- via some TDS studies, it's been established (and discussed on sites like ComputerAudiophile, where I'm a member) that short cables tend to be more of a problem. The reason why is that it's the impedance discontinuity that creates a reflection in the cable. For a short cable, the reflection time is very high in frequency with a short delay, and this is more likely to degrade the leading edge of the digital transition. For a longer cable, (say, 9-12 ft), the reflection takes longer to occur, and appears back at the leading edge in the receiver AFTER the rising transition is completed- and hence, doesn't normally impact the signal.

Another issue with S/PDIF is that compared with AES/EBU, the actual signal level is only about 1/10 the voltage, and of course, the transmission system is single ended, though in high quality systems, transformer coupled at both ends.

The AES/EBU system on paper looks to have problems with cable construction, because of the connector impedance control (or lack of it!). But the circuit characteristic impedance is higher, and the working signal levels much higher. In the end, doing a lot of listening tests, many manufacturers recommend using the AES/EBU as the first choice, and my own listening experiments confirm this. Sometimes it's subtle, other times not so much; obviously, a lot depends on the quality of the source material and the complete associated system. So, if one is primarily a death metal fan, not to worry.... probably anything that completes the circuit connection will be fine! :W But with more demanding acoustic material for which the ear has a fairly solid reference of what the natural instruments sound like, and with wide dynamic range, paying some attention to these details is usually of benefit- and the better the source and DAC and system, the more that's the case, in my experience.

more on this later... I'm starting to work on a posting story for my hate/love relationship with digital.

[JonMarsh]

It seems like every time I manage to answer one question, another arises I hadnt even thought of. How deep is this rabbit hole anyway?

Very, very deep! In the end the results can be quite rewarding, but in the early stages, it's enough to make one consider another turntable purchase, if quality is your main requirement. :W

[jim1961]

Well, here's the issue- an RCA cable may be close to 75 ohm characteristic impedance or not. It's a gamble of sorts, and you certainly could get lucky. Now, here's the really weird part- via some TDS studies, it's been established (and discussed on sites like ComputerAudiophile, where I'm a member) that short cables tend to be more of a problem. The reason why is that it's the impedance discontinuity that creates a reflection in the cable. For a short cable, the reflection time is very high in frequency with a short delay, and this is more likely to degrade the leading edge of the digital transition. For a longer cable, (say, 9-12 ft), the reflection takes longer to occur, and appears back at the leading edge in the receiver AFTER the rising transition is completed- and hence, doesn't normally impact the signal.

Another issue with S/PDIF is that compared with AES/EBU, the actual signal level is only about 1/10 the voltage, and of course, the transmission system is single ended, though in high quality systems, transformer coupled at both ends.

The AES/EBU system on paper looks to have problems with cable construction, because of the connector impedance control (or lack of it!). But the circuit characteristic impedance is higher, and the working signal levels much higher. In the end, doing a lot of listening tests, many manufacturers recommend using the AES/EBU as the first choice, and my own listening experiments confirm this. Sometimes it's subtle, other times not so much; obviously, a lot depends on the quality of the source material and the complete associated system. So, if one is primarily a death metal fan, not to worry.... probably anything that completes the circuit connection will be fine! :W But with more demanding acoustic material for which the ear has a fairly solid reference of what the natural instruments sound like, and with wide dynamic range, paying some attention to these details is usually of benefit- and the better the source and DAC and system, the more that's the case, in my experience.

more on this later... I'm starting to work on a posting story for my hate/love relationship with digital.

Is the short cable issue applicable to true 75 ohm coax or only RCA audio cable used as a digital coax cable?

If it is applicable to 75 ohm coax, is there a prime length?

Do all these issues disappear when using optical, or does that interface have a whole new set of problems of its own?

Do you have a personal preference on filtration of the choices outlined in post 5 ?

I did switch from a 0.5M RCA audio cable to a 1M (38") audio cable and there did seem to be some subtle improvement. I found a 1.32M (52") audio cable around here and tried that one next. Better still I think.

[jim1961]

Oh my.
The surface you have not even yet scratched, dear padowan. :W

Well, I thought the cable length would be a straightforward concept 8O

[jim1961]

http://www.lessloss.com/lessloss-digital-cable-p-195.html

Comparing two same-build digital cables (one 2m long, the other 0.5m long), the levels at which the very first reflections arrive at the cable’s output will be different. The latter cable’s first reflection will have traveled a total of 1m (0.5m x 2), the former a total of 4m (2m x 2).

We can see that the greater the difference in cable length, the greater the difference in reflection level once it has reached the cable’s output. In our example, the longer cable’s first reflection traveled a total of 4m, whereas the shorter cable’s first reflection traveled only 1m. This means that the level of the reflection in the longer cable will have been lowered more. Why? Because it will have traveled farther and therefore will have experienced proportionately more attenuation.

True, the original signal is also attenuated by the longer cable length, but the original signal is attenuated only by traveling the length of the cable once, while the first reflection is attenuated by traveling the cable twice (once back to the input end of the cable and once back to its output end).

If shorter digital cables make digital audio sound worse, it seems logical to simply use a longer cable. (After all, this would reduce the reflection content added to the signal receiving circuitry and lower the amount of jitter). But this won’t do. By lengthening the cable, we allow more external high-frequency noise (from the air) to be induced into the cable. This will just pass through into the circuitry and create intermodulation distortion, also contributing to jitter. Using a longer cable would just change the source and type of jitter, resulting in a different flavor of distortion. So we see that our best efforts are halted by a forced compromise between, on the one hand, lowering internal reflection levels by using longer cables, and on the other, increasing high-frequency noise induction by doing the so.

I suppose this explains the issue.

--- --- ---

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

Lots of technical data here. One conclusion here is that the cable must be 1.5M long.

[jim1961]

Just did a coax vs optical comparison using the Minimum phase apodising filter in both cases. There was a discernible difference. The coax was more open and 3 dimensional compared to the optical that seemed slightly more confined. The subtle room cues of the recording were more apparent with the coax than the optical.

Are these the types of differences anyone else has experienced?

[wkhanna]

Are these the types of differences anyone else has experienced?

Personally, yes.

[jim1961]

So is there a recommended 75 ohm 1.5M digital coax cable for under $30 ?

http://www.bluejeanscable.com/store/digital-audio/

Something from this site maybe? ^^^

Canare LV-77S ?
Belden 1694A ?

[wkhanna]

I think the Bluejeans is a good place to start.
From there I would suggest looking at CatCable link1 link2 next.

[JonMarsh]

Just did a coax vs optical comparison using the Minimum phase apodising filter in both cases. There was a discernible difference. The coax was more open and 3 dimensional compared to the optical that seemed slightly more confined. The subtle room cues of the recording were more apparent with the coax than the optical.

Are these the types of differences anyone else has experienced?

Exactly so. Optical is done by little modules which are purchased as an assembly. Two things you'll notice- often/usually the optical input is only rated for operation to 96 kHz (double rate), not for quad rate signals (176.4 and 192 kHz). This tells you something about the transmission bandwidth. The other problem is the digital precision of the signal, which compared with a good coax system, does't have the same time accuracy- i.e., it has more jitter. Jitter just means you have a slight timing anomaly which is signal level and frequency dependent. But since standard interfaces must recover the clock from the actual transmitted signal, this impacts the clock, if SRC or up sampling isn't used. And while SRC may filter a fair amount of the jitter, it has it's own issues, doing up sampling this way, and will always work better using a low jitter signal to start with than one with more.

What I find in most of my DACs is that the best input is AES/EBU, for dimensionality and spatiality and fine grained detail in playback, followed by FireWire or a well designed async USB, followed by Coax, followed by optical. YMMV, due to other system issues, (common mode noise in an HTPC or music server, for example). In some DAC's, the FireWire input is even better than the AES/EBU; but then, that also depends on how you're getting the data onto the AES/EBU...

[Chris D]

For me, my DACs, and my equipment, I don't have the capability of AES/ABU, and haven't hooked up a DAC to my computer yet over USB, so all mine are running with digital coax cables. No optical.

[jim1961]

Exactly so. Optical is done by little modules which are purchased as an assembly. Two things you'll notice- often/usually the optical input is only rated for operation to 96 kHz (double rate), not for quad rate signals (176.4 and 192 kHz). This tells you something about the transmission bandwidth. The other problem is the digital precision of the signal, which compared with a good coax system, does't have the same time accuracy- i.e., it has more jitter. Jitter just means you have a slight timing anomaly which is signal level and frequency dependent. But since standard interfaces must recover the clock from the actual transmitted signal, this impacts the clock, if SRC or up sampling isn't used. And while SRC may filter a fair amount of the jitter, it has it's own issues, doing up sampling this way, and will always work better using a low jitter signal to start with than one with more.

What I find in most of my DACs is that the best input is AES/EBU, for dimensionality and spatiality and fine grained detail in playback, followed by FireWire or a well designed async USB, followed by Coax, followed by optical. YMMV, due to other system issues, (common mode noise in an HTPC or music server, for example). In some DAC's, the FireWire input is even better than the AES/EBU; but then, that also depends on how you're getting the data onto the AES/EBU...

Thanks for the insights Jon. The Rega DAC has no AES/EBU or Firewire inputs and the USB is limited to 16/48. The other inputs only include coax and optical which are both rated at 24/192. I went ahead and ordered a Bluejeans Beldon 1694a 75 ohm coax digital cable. This thread and my other research has convinced me that a properly specified cable is important.

Although ive owned CD players since 1984, they have always been all-in-one units. So a separated DAC is new for me. Its become obvious I have a lot of catching up to do.

[PewterTA]

I use the bluejeans for my digital transport between my MF V-Link192 and DAC and it's been as good as any other cable I've found to put in there. So I think that should work fine for you!

[jim1961]

I use the bluejeans for my digital transport between my MF V-Link192 and DAC and it's been as good as any other cable I've found to put in there. So I think that should work fine for you!

Thats encouraging, thanks!

[jim1961]

Wanted to thank everyone for steering me in the right direction on the cable. The new cable arrived and several things got markedly improved:

1) Soundstage width and depth as well as image distinction
2) Smoother highs
3) Better room cue resolution and low level detail

[JonMarsh]

Wanted to thank everyone for steering me in the right direction on the cable. The new cable arrived and several things got markedly improved:

1) Soundstage width and depth as well as image distinction
2) Smoother highs
3) Better room cue resolution and low level detail

Excellent! That's what I'd expect, but it's always good when the listener is happy!

[sdl2112]

Good info!...it's on the back burner but a DAC is in my future...thinking NAD M51.

[JonMarsh]

Good info!...it's on the back burner but a DAC is in my future...thinking NAD M51.

I bet you'd really enjoy one of those- it's what I recommend to everyone looking to stay in a sensible price point, and the preamp output volume control works quite well. There are a lot of happy M51 users over at Computeaudiophile, too.

[sdl2112]

I bet you'd really enjoy one of those- it's what I recommend to everyone looking to stay in a sensible price point, and the preamp output volume control works quite well. There are a lot of happy M51 users over at Computeaudiophile, too.

Thanks Jon...I'm close to pulling the trigger...I may quiz you on what's in the future with interfaces...etc

[jim1961]

Question:

When routing the coax, would something like running it under an active piece of gear like the DAC or CD transport cause problems? In other words, does the routing of the cable need to be kept away from equipment, power cables and such?

What kinds of typical influences should one be most on the lookout about? What can most easily pollute the signal?

I bring this up because although things are generally sounding fabulous, it seemed I was detecting occasional artifacts that didnt belong. Initially I had the coax running underneath the CD player and DAC. I moved the cable so its now running along the back of these devices and after a 12 minute listen, didnt detect anything odd this time. Problem solved? Maybe. Need to listen more.

I managed to find an image of the inside of the Rega DAC and the transformer is in the front of the unit, so running the cable along its rear edge puts the cable a good 8"-9" inches away from the transformer, where as before it was maybe 2"-3" away. The Rega Apollo transformer is about 3" from the back.

Here is what my current routing looks like:

[PewterTA]

Great to hear the cable is working good for you!!

I would say it's okay to route underneath, I would just try to keep it away from "known" power connections and internal transformers... to help keep the lowest possible amount of interference from being introduced.

[Chris D]

Wanted to thank everyone for steering me in the right direction on the cable. The new cable arrived and several things got markedly improved:

1) Soundstage width and depth as well as image distinction
2) Smoother highs
3) Better room cue resolution and low level detail

:T I got a good chuckle there. Can't say we didn't tell ya!

As for the cable routing, yes, you want to carefully route ALL audio cables, including coax. The biggest thing is separating audio and power cabling. All my audio cables run down the left side of my racks, and power down the right side. Anywhere that you need to cross audio and power cables, do so at a 90 degree angle to make them perpendicular.

[jim1961]

:T I got a good chuckle there. Can't say we didn't tell ya!

As for the cable routing, yes, you want to carefully route ALL audio cables, including coax. The biggest thing is separating audio and power cabling. All my audio cables run down the left side of my racks, and power down the right side. Anywhere that you need to cross audio and power cables, do so at a 90 degree angle to make them perpendicular.

You did and you were right.

I havent heard anything odd since I got the digital coax away from the DAC and CD player. I agree about cable separation. Now, if I could find some custom length power cables, I could improve that mess in the pic a bit

[wkhanna]

You did and you were right.

.......... Now, if I could find some custom length power cables, I could improve that mess in the pic a bit

Mess!
If you saw the back of my system I hardly think you would call what you have a 'mess'.

You may want to talk to Doug (Lex) at CatCable.
He can usually make custom length cables of many sorts.

[Chris D]

Mess!
If you saw the back of my system I hardly think you would call what you have a 'mess'.

Agreed. I'll see if I can take a picture of the back of my racks sometime. It's... eye opening... and sad.