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-   -   RF Attenuators = Jitter Reducers (http://www.diyaudio.com/forums/digital-source/168901-rf-attenuators-jitter-reducers.html)

jkeny 19th June 2010 09:07 PM

RF Attenuators = Jitter Reducers
 
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.

jkeny 19th June 2010 09:08 PM

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!

jkeny 19th June 2010 09:41 PM

If you can't find the Diyhifi thread let me give you directions: PC Audio Jitter - Have a Laugh!

While there look in on the thread where I'm being abused & accused of Trolling & trying to make money out of this - I'm the schmuck & Troll referred to http://www.diyhifi.org/forums/viewto...rs" at Head-FI

Perhaps I own minicircuits? I'm sure they'll drum up some other accusation to satisfy whatever their need is!!

jean-paul 19th June 2010 09:59 PM

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.

wakibaki 19th June 2010 10:23 PM

Quote:

Originally Posted by jkeny (Post 2221393)
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

jkeny 19th June 2010 10:24 PM

Quote:

Originally Posted by jean-paul (Post 2221422)
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.

Quote:

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

wakibaki 19th June 2010 10:26 PM

Quote:

Originally Posted by jkeny (Post 2221445)
These attenuators work both in theory & practise - well worth an experiment for $12

Just $12 down the drain...

w

jkeny 19th June 2010 10:32 PM

Quote:

Originally Posted by wakibaki (Post 2221444)
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!

Quote:

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!

jkeny 19th June 2010 10:35 PM

Quote:

Originally Posted by wakibaki (Post 2221448)
Just $12 down the drain...

w

Are you telling me that I'm mistaken in what I & others hear or that the plots of reflections & their reduction using these attenuators are false?

wakibaki 19th June 2010 10:43 PM

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


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