RF Attenuators = Jitter Reducers

Do you have a SPDIF transformer in your Digital Device

  • Yes

    Votes: 40 71.4%
  • No

    Votes: 16 28.6%

  • Total voters
    56
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I'm posting this on a number of different forums as I believe that it is significant information for those interested in jitter reduction

RF Attenuators have been know about & used for many years but their use on SPDIF cables is not that common, I don't think. I first came across them a long time ago on a thread on DiyHiFi.org that was discussing building a homebrew TDR & Joseph K mentioned using one of these in-line. I forgot about it but recently saw he posted this again in a thread about output measurements of the Hiface. DIYHiFi.org • View topic - PC Audio Jitter - Have a Laugh!

What they are is simply a T-pad resistor divider network built into a BNC adapter. They come in 50 ohm & 75ohm varieties and work by dividing the voltage of the signal (& it's reflections). Hence they can only be used on a cable where the SPDIF signal is higher than normal & can bear some reduction & not drop below SPDIF spec.

They are in-line adaptors that can be connected at the start or end (or both?) of your digital cable. Cable reflections are one of the many sources of added jitter in every digital system. Reducing the strength of these reflections should result in a reduction in the jitter generated by the transport to DAC connection.

There are many situations where this should result in better sound. I would think that they will improve any SPDIF connection as I don't believe there is one made that is reflection free? They may also be an effective way of reducing the sonic penalty usually incurred in using a BNC to RCA adaptor. So this could mean that by using these attenuators an RCA input on your DAC should now be about the same sonically as a BNC input.

These are premises based on how the attenuators work - real world results may be different although I have tested a BNC to RCA adaptor with & without one of these attenuators & the sound was noticeably smoother & without edge with the attenuator in-line. If you go to the DiyHifi thread I mentioned you will see shots of TDR plots showing their effectiveness - thnaks to Joseph K for this!

Here is the cheapest place I have found on the internet for these attenuators at $12 each - there are other places that will charge you >$30 for eactly the same thing - I've even seen them for >€100 in RS which I thought must be a mistake! I believe this is cheap enough to allow experimenting with the range of values available. Attenuators, Plug-In & Coaxial, Fixed

Scroll down to the bottom of the page - it's the BNC 75ohm variety that you want. They come in 3, 6, 10, 15, 20dB versions - the 3 & 20dB ones could probably be ruled out as too low & maybe too high an attenuation. What we want to aim for is enough attenuation to suppress signal reflections BUT not too much to reduce the signal voltage to too low. You will know if you have done this as the DAC will not lock to the SPDIF signal.
 
Obviously if you reduce the signal level too much & drop outside of the level at which your receiver will lock, it isn't of much use. If your DAC receiver chip is one of the CS84** ones - these are known to like higher input voltages then the SPDIF spec of 500mVpp but I don't know how the reduction in reflections versus the overall reduction in signal voltage will work out. All I can say is that these same receivers are used as DACs to transports that output the SPDIF standard of 500mV & that's all I'm suggesting the attenuators should reduce the signal to.

I have used it & substantially improved the sound from both a stock Hiface & a modified Hiface. This was using an RCA SPDIF cable into a DAC with RCA input. First I used a BNC Hiface with a BNC to RCA adapter. Then I put the attenuator in-line & noticed a complete reduction in edginess, sibilance & a better focus to instrument positions.

Why buy expensive digital cables when these simple & cheap devices can be used to achieve a better solution in certain circumstances noted above. It's better solution because of the fact that a good digital cable if well made (expensive!) will have controlled impedance from end to end & will be a close match to the desired 75ohm. Outside of the cable at the DAC end or the transport end the cable cannot ameliorate any impedance anomalies that are encountered by the digital signal so these will give rise to reflections travelling down the cable. The attenuators, attenuate these signals!
 
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It was your own choice hanging out there. Not that diyaudio.com is without faults but the level of moderators there is unacceptable. I don't even want to know what the issue is but very probably it is not even worth mentioning. Maybe you are asking for ideas and even sell products with that knowledge but even then I would not accept the level of discussion. Only for that reason I won't register at diyhifi.org.

Jocko Homo thinks he is the only one with knowledge and that everyone wants to steal his ideas and commercialize them. IMO there are too many guys over there with mental/megalomaniac issues or non grown up behavior. I recall one of them posting here with at least 4 different personalities. All bragging about knowledge they only seem to have, bitter fools.

Again, it is your own choice. There are some things to be critiqued everywhere but I think this website is far more friendly and forgiving. Life is way too short to be busy with such nonsense.

Regarding the subject: maybe it is time to say goodbye to SPDIF and build a good DAC straight in the device. Less hassle, less problems, less discussion, less time loss. SPDIF was born with defects and all gurus have been busy with it and it still is not optimal. The best solution is to avoid it completely.
 
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I would think that they will improve any SPDIF connection as I don't believe there is one made that is reflection free?

Where's your evidence for this?

I very much doubt that the engineers responsible for designing these devices are failing to terminate them correctly. They certainly know more about engineering them than the average audio buff. It's simply a matter of arranging for the receiving end to have a resistance of 75 ohms. Hardly rocket science is it? It's also the way to correct the situation if there is a problem, not add attenuation to the interface at whatever point.

You can measure this at the receiving end with a DMM. It'll have to be very low for there to be a problem, and it won't be if the receiver is working at all. If it's too high, just add a parallel resistor to bring it down to 75R.

w
 
It was your own choice hanging out there. Not that diyaudio.com is without faults but the level of moderators there is unacceptable. I don't even want to know what the issue is but very probably it is not even worth mentioning. Maybe you are asking for ideas and even sell products with that knowledge but even then I would not accept the level of discussion. Only for that reason I won't register at diyhifi.org.
I'm not asking for ideas & using it to develop products but that is his contention except he remains vague when challenged to give me one idea of his used by me.

Jocko Homo thinks he is the only one with knowledge and that everyone wants to steal his ideas and commercialize them. IMO there are too many guys over there with mental/megalomaniac issues or non grown up behavior. I recall one of them posting here with at least 4 different personalities. All bragging about knowledge they only seem to have, bitter fools.

Again, it is your own choice. There are some things to be critiqued everywhere but I think this website is far more friendly and forgiving. Life is way too short to be busy with such nonsense.

Regarding the subject: maybe it is time to say goodbye to SPDIF and build a good DAC straight in the device. Less hassle, less problems, less discussion, less time loss. SPDIF was born with defects and all gurus have been busy with it and it still is not optimal. The best solution is to avoid it completely.
SPDIF is not as bad as is made out based on my listening tests & reports from others I know. Lots of people have already invested in their DAC of choice & getting a clean, low jitter SPDIF feed into these DACs is still a worthwhile endeavour, I believe.

These attenuators work both in theory & practise - well worth an experiment for $12
 
Where's your evidence for this?

I very much doubt that the engineers responsible for designing these devices are failing to terminate them correctly. They certainly know more about engineering them than the average audio buff. It's simply a matter of arranging for the receiving end to have a resistance of 75 ohms. Hardly rocket science is it? It's also the way to correct the situation if there is a problem, not add attenuation to the interface at whatever point.
Ah, it's not this simple now! The zout of the SPDIF driver needs to be taken into account when designing the SPDIF output stage , including SPDIF transformer - this isn't simply a case of putting a 75ohm resistor in. The 75ohm impedance required of the circuit is not a static resistance measurable with a DMM!

You can measure this at the receiving end with a DMM. It'll have to be very low for there to be a problem, and it won't be if the receiver is working at all. If it's too high, just add a parallel resistor to bring it down to 75R.

w
Again, you are way off about measuring this with a DMM!
 
Yes. You're mistaken in what you hear.

We had to go to considerable lengths with very fast rise times, a very expensive oscilloscope and long, long cables for a university lab demonstration of reflection in digital systems, so I don't believe you've got plots. Not in an SPDIF system.

w
 
Yes. You're mistaken in what you hear.

We had to go to considerable lengths with very fast rise times, a very expensive oscilloscope and long, long cables for a university lab demonstration of reflection in digital systems, so I don't believe you've got plots. Not in an SPDIF system.

w

I didn't do the plots but Joseph K, a member here did & posted them on DiYHiFi.org - maybe he wants to say something about his measuring technique.

He used a DIY TDR which is a piece of equipment designed to show reflections (Time-Domain Reflectometer). http://en.wikipedia.org/wiki/Time-domain_reflectometer How did you use a scope for your test which showed reflection & what signal rise time was being measured?
 
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Only for that reason I won't register at diyhifi.org.

I applied to register there but seems they don't want me:D I agree there's quite a high level of paranoia - the fact that images posted can only be seen by people who've registered just as one example.

Jocko Homo thinks he is the only one with knowledge and that everyone wants to steal his ideas and commercialize them.

Agreed with your statement about his paranoia, but really he's technically excellent and sometimes very helpful. You just have to know how to ask. He's not the only one with knowledge, he'll mention phred knows various things as well as he does for instance.

IMO there are too many guys over there with mental/megalomaniac issues or non grown up behavior. I recall one of them posting here with at least 4 different personalities. All bragging about knowledge they only seem to have, bitter fools.

Well some of them clearly do have knowledge from their long experience. I've learned a few useful things there.

Again, it is your own choice. There are some things to be critiqued everywhere but I think this website is far more friendly and forgiving. Life is way too short to be busy with such nonsense.

I agree this place is far more welcoming. Doesn't mean there's nothing to learn at 'the other place' though, so I don't blame John for going there.

Regarding the subject: maybe it is time to say goodbye to SPDIF and build a good DAC straight in the device. Less hassle, less problems, less discussion, less time loss. SPDIF was born with defects and all gurus have been busy with it and it still is not optimal. The best solution is to avoid it completely.

The best solution is to integrate everything but people have CD players with SPDIF outputs and enjoy playing CDs. Its still the cheapest way to get a digital source with DVD players going for well under $30. So SPDIF will be around for a while yet, just have to make the best of it.
 
Ah, it's not this simple now! The zout of the SPDIF driver needs to be taken into account when designing the SPDIF output stage , including SPDIF transformer - this isn't simply a case of putting a 75ohm resistor in. The 75ohm impedance required of the circuit is not a static resistance measurable with a DMM!

Again, you are way off about measuring this with a DMM!

This is simply not true. I have experience of the design of RF circuits up to 2.4GHz.

If there is a mismatch at the driving end, the reflection goes back into the driving device, and the signal entering the cable is reduced. Once the signal has entered the cable, if it encounters a 75R termination at the receiving end there will be no reflection.

I want to take exception to the implied invidious and insulting slur that professional design engineers are routinely and persistently negligent in the pursuit of their chosen careers.

While it may be true in the design of audio amplifiers and baseband hi-fi systems in general that the work of fools and incompetents can be foisted on an ill-educated and gullible public, this is the only area of electronics where this holds true.

The design of SPDIF and other digital audio and computer interfaces is not subject to the vagaries of a public basing their buying decisions on little more than superstition.

The correct termination of transmission lines is a subject which has been well understood now for a period approaching a century.

No-one suggests that it would be a good idea to put an attenuator in the cable connecting their TV to the antenna because there's probably a mismatch at the TV and it will cut down reflections. No, we expect that the TV designer will have done his job to that extent.

Why, then, are suggestions such as these rife in the field of home audio?

It's because there are numerous individuals who, armed with a few incompletely understood facts, unashamedly seek to elevate themselves above their even-less-well-educated fellows for no better reason than the gratification of their egos, regardless of the financial cost to others or the obfuscation of the truth.

Please try to exercise some self control before indulging in the propagation of these totally unfounded assertions.

w
 
Hey Wakibaki, no slur intended - it's surprising you took it up that way.

We are talking about audio devices here - are you suggesting that SPDIF & digital audio devices are well designed & not "the work of fools and incompetents" as you suggested were behind audio amplifiers? Talk about slurs :nownow:

These attenuators are routinely sold & used in TV & Sat systems to prevent signal reflections & knock down a too high signal.

Ah come on - you need to stop being so self righteously dogmatic & start with a more scientific mind, open to listening to new information.

What I'm suggesting here is an experiment for people to do that own a Hiface & similar transports that output a SPDIF significantly > 0.5mVpp. It costs $12 & at least 4 people so far (me, Joseph K, Fmak & another) have said yes, it sounds better & there are TDR plots which show it's effect if you want to bother actually looking into it.

Otherwise, you have your opinion & that's fine - just don't try to personalise this thread it into an attack or slur on anybody - that, I'm afraid, is in your own imaginings!

There seems to be a lot of this about - is there some contagious disease doing the rounds?
 
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AX tech editor
Joined 2002
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[snip]These attenuators are routinely sold & used in TV & Sat systems to prevent signal reflections & knock down a too high signal.[snip]?

Yes they are sold to cut down signal levels - but not to cut down reflections; this is a physical impossibility. Reflections do not depend on signal levels, only on impedance levels. Make sure you terminate properly and there are NO reflections.
BTW Do you realise that lowering signal levels increases jitter? The slew rate of the signal at the receiving end gets lower so the uncertainty of the rise- and fall time switching point increases, leading to greater jitter. Maybe you heard that and thought 'it must be better' ;)

jd
 
BTW Do you realise that lowering signal levels increases jitter? The slew rate of the signal at the receiving end gets lower so the uncertainty of the rise- and fall time switching point increases, leading to greater jitter. Maybe you heard that and thought 'it must be better' ;)

jd

I'd say you are right under certain conditions.


Though you'd checkout the EC-Designs I2S attenuator story.
It'll all depend on the quality of the incoming signal, the environmental conditions and the receivers jitter sensitivity how far down you can go.

Too much energy into the substrate of the receiver might cause different issues on the receiver side according to John.

EC-Designs at least reports pretty good results after attenuation the I2S signals.

Cheers
 
Yes they are sold to cut down signal levels - but not to cut down reflections; this is a physical impossibility. Reflections do not depend on signal levels, only on impedance levels. Make sure you terminate properly and there are NO reflections.

Cutting down the signal level will also reduce reflection level if it is already on the line with the signal, no?
Yes, if you can adjust the impedances to their correct values at both transmitter & receiver ends - how many people can do this? It's not a matter of putting in a 75R as wakibaki says. Also some people don't wan to open up their transport and DAC to adjust the impedance.

BTW Do you realise that lowering signal levels increases jitter? The slew rate of the signal at the receiving end gets lower so the uncertainty of the rise- and fall time switching point increases, leading to greater jitter. Maybe you heard that and thought 'it must be better' ;)

jd
I'm only suggesting this might be useful for transports that have a high signal level to begin with & reducing it to bring it into SPDIF spec range. will also reduce reflection signal levels - the equivalent of high return loss

See this paper: http://www.wenzel.com/documents/swr.html
 
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jkeny

You simply don't know what you're talking about.

Anyone who imagines that you do should look at R1, the termination resistor in this SPDIF receiver on Rod Elliott's ESP site:-

S/PDIF Digital to Analogue Converter

Alternatively ask any qualified radio amateur. Or buy a text with a treatment of transmission lines.

ALL that is required is a 75R resistor across the line at the receiving end. You CAN just measure it with an ohmmeter.

Who in their right mind would imagine that anybody with the capability of designing a SPDIF receiver would deliberately or negligently leave it out? It's beyond the bounds of credibility, but you want us to believe it happens all the time.

You suggest that what I am presenting is an 'opinion'. I am sure you would like it to be, because then it would be open to argument, but it is not. It's not an opinion, it's a FACT. Just like the many, many scientific facts that the majority of us who come on here expect to get a straight answer about.

Does a capacitor pass AC but not DC? It's a FACT. Does an inductor tend to resist changes in the current through it? It's a FACT. Do radio waves propagate at 300 million metres per second? It's a FACT. It's got nothing to do with 'closed mind' or 'more scientific attitude'.

You evidently imagine that your behaviour is without consequence. The attitude of mind that permits you to mislead people in this way will, however, lead to you in turn being misled, if it hasn't already. This is called karma, the law of moral causation. Oh, and it's a fact too, whether you know it or not. If you don't mend your ways, one day you will look round and realise, 'Oh, I've been taken for a sucker.'

w

Perhaps you won't even realise...
 
I know what jitter sounds like

Curious - how do you know what jitter sounds like? Have you managed to isolate the effects of jitter so you're sure you're hearing it on its own? If so, I'd like to know how you did this.

what I heard was a reduction in jitter from a slight edge & sibilance to the sound to a smoother sound with a better definition of instruments position in the sound field. Others have reported similar!

My guess here is its also affecting the RF intermodulation, not just the jitter. I get 'slight edge and sibilance' from not paying attention to RF issues. I do agree that jitter affects the definition of soundstage images. So I hypothesise that you're reducing two effects here, not just one.
 
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