Synergy?

[amirm]

This is the key. All sorts of properties that exist at radio frequencies, such as skin effect and VSWR, are irrelevant at audio frequencies. Yes, an impedance mismatch at connection points causes reflections and electrical standing waves. At 100 MHz this is an important consideration for maximizing power transfer. But it doesn't matter at audio frequencies, or even at the 2x audio frequencies used for digital signals. I've connected audio gear via S/PDIF many times using whatever random RCA cables I had lying around, and it never made any difference.

--Ethan

Funny that this discussion has come up as for the last couple of days, I have been working on characterizing S/PDIF cables. Here are the results of using a generic RCA cable, vs a Transparent audio interconnect cable (NORMAL audio, NOT S/PDIF) which has a black box that filters high frequencies. Here is the impact on the S/PDIF waveform:

The nice looking square wave in blue is the normal coax, and the green, reduced bandwidth audio cable. Clearly we see the high frequencies taken away. Here is the numerical difference in jitter:

Normal coax: 435 picoseconds
Reduced bandwidth Transparent cable: 5,000 picoseconds

So we have a 10:1 increase in jitter when the cable is designed around audio frequencies. For 16 bit audio, we like to see 500 psec by the way.

We can see why in this frequency response measurement done two ways: one with my generator set to low impedance of 20 ohms and another, to a high of 600:

We see that the response is flat with 20 ohms for both cables. But at 600, Transparent cable shows a drop of about 0.2 dB at 40 Khz which matches your criteria of 2X audio bandwidth. Yet we see that by not maintaining that level at higher frequencies, jitter is sharply increased.

So no, 2X audio bandwidth is not even close to being enough for S/PDIF cable. Yes, your receiver is resilient against jitter and will extract audio samples and you hear the sound. But the induced cable jitter is very high and likely will show up in the analog output of the DAC.

 

[NorthStar]

That's a great post just above Amir. :b

 

[Gregadd]

The Joker commenting on Batman: "Where does he get those gret toys?"

 

[jkeny]

Thanks Amir,
Depending on the risetime of the SPDIF square waves, the bandwidth required for transmission of reasonably intact waveforms is in the Mhz region.
See Here http://www.epanorama.net/documents/audio/spdif.html

Bandwidth occupation : 100kHz up to 6Mhz (no DC!)
Signal bitrate is 2.8Mhz (Fs=44.1kHz), 2Mhz (Fs=32kHz) and 3.1Mhz (Fs=48kHz).

Amir, have you ever tried an RF attenuator on one of these RCA cables?

 

[andromedaaudio]

Great post amirim .
0,2 db FR drop on 40 khz will not be audible ,...... well there will be some off course that can hear it i am afraid
I wonder what their Speaker cable will test like , if treble is reduced bass is more emphasized .
The mit i looked at had a coil (with iron center )in series with the signal , i dont now the value , but that will certainly affect the speakerscrossover behavior .

 

[FrantzM]

Funny that this discussion has come up as for the last couple of days, I have been working on characterizing S/PDIF cables. Here are the results of using a generic RCA cable, vs a Transparent audio interconnect cable (NORMAL audio, NOT S/PDIF) which has a black box that filters high frequencies. Here is the impact on the S/PDIF waveform:

The nice looking square wave in blue is the normal coax, and the green, reduced bandwidth audio cable. Clearly we see the high frequencies taken away. Here is the numerical difference in jitter:

Normal coax: 435 picoseconds
Reduced bandwidth Transparent cable: 5,000 picoseconds

So we have a 10:1 increase in jitter when the cable is designed around audio frequencies. For 16 bit audio, we like to see 500 psec by the way.

We can see why in this frequency response measurement done two ways: one with my generator set to low impedance of 20 ohms and another, to a high of 600:

We see that the response is flat with 20 ohms for both cables. But at 600, Transparent cable shows a drop of about 0.2 dB at 40 Khz which matches your criteria of 2X audio bandwidth. Yet we see that by not maintaining that level at higher frequencies, jitter is sharply increased.

So no, 2X audio bandwidth is not even close to being enough for S/PDIF cable. Yes, your receiver is resilient against jitter and will extract audio samples and you hear the sound. But the induced cable jitter is very high and likely will show up in the analog output of the DAC.

 

[Ethan Winer]

Here are the results of using a generic RCA cable, vs a Transparent audio interconnect cable (NORMAL audio, NOT S/PDIF) which has a black box that filters high frequencies.

Amir, you are the master and I bow down to you.

How long were the wires? How much additional capacitance did you add? What was the capacitance of the normal RCA wire?

Yes, I understand the difference between sine waves and square waves. No, I don't see how you proved anything relevant other than, sure, with a contrived test that adds [who knows how much] capacitance you can screw up any signal path.

Now, if you really want to prove something useful, please show the degradation that occurs with a normal RCA cable no longer than 6 feet versus any high-end RCA cable of the same length.

--Ethan

 

[microstrip]

(...) So no, 2X audio bandwidth is not even close to being enough for S/PDIF cable. Yes, your receiver is resilient against jitter and will extract audio samples and you hear the sound. But the induced cable jitter is very high and likely will show up in the analog output of the DAC.

Amir,

Interesting graphs. But you conclusion seems to be double-edged : do you admit that no DAC receiver can reject the small amount of jitter your are showing?

 

[amirm]

Great post amirim .
0,2 db FR drop on 40 khz will not be audible ,...... well there will be some off course that can hear it i am afraid
I wonder what their Speaker cable will test like , if treble is reduced bass is more emphasized .
The mit i looked at had a coil (with iron center )in series with the signal , i dont now the value , but that will certainly affect the speakerscrossover behavior .

When I get some time I will measure them too. I have to figure out a way to connect them to the terminals of the test gear

 

[amirm]

Amir,

Interesting graphs. But you conclusion seems to be double-edged : do you admit that no DAC receiver can reject the small amount of jitter your are showing?

My only conclusion from those graphs is that cables do induce jitter. Copper is not copper. Wire is not wire. There are differences here. Adding jitter by the cable makes the job of the downstream device harder. How well each DAC deals with this is unknown. Jitter reduction is not a standardized circuit with predictable performance. I do plan to test further though and see what I can discover.

 

[microstrip]

My only conclusion from those graphs is that cables do induce jitter. Copper is not copper. Wire is not wire. There are differences here. Adding jitter by the cable makes the job of the downstream device harder. How well each DAC deals with this is unknown. Jitter reduction is not a standardized circuit with predictable performance. I do plan to test further though and see what I can discover.

This is exactly my point. If there is no known scientific correlation between jitter in a cable leaving the CD transport and the final sound quality at the output of the DAC, we have either to rely on our listening or wait for your complete studies. Or oversimplify the problem and just minimize using the usual single value parameter known as time jitter.

Coming back to the thread, I can not see where there is any fundamental difference between this current situation and any other high-end situation where we also do not have predictable performance, such as many cases of synergy between components that were referred in this thread.

 

[amirm]

This is exactly my point. If there is no known scientific correlation between jitter in a cable leaving the CD transport and the final sound quality at the output of the DAC, we have either to rely on our listening or wait for your complete studies. Or oversimplify the problem and just minimize using the usual single value parameter known as time jitter.

But we can measure it easily. No "study" is necessary. Where it gets fuzzy is if we lack the measurement in which case the advice is simple: use the rated cable so that you don't have to worry about it.

 

[microstrip]

But we can measure it easily. No "study" is necessary. Where it gets fuzzy is if we lack the measurement in which case the advice is simple: use the rated cable so that you don't have to worry about it.

Excellent solution. But how should we rate the cable? By price? Unhappily the classical measurements of impedance and cable loss are not enough.

Do you have experience with the old Madrigal purple MDC1 and MDC2 digital cables? I still keep them - they could help a lot to create an wonderful full bodied sound in many systems. We called it the "little purple" !

 

[jkeny]

If anybody is really interested & wants to put in some time studying a really deep analysis & some really interesting ways of jitter measurement using a $40,000 (I think) LeCroy & resultant graphs & a technical discussion about the results, you should have a look here http://www.diyhifi.org/forums/viewtopic.php?p=45228#p45228. The test set-up & measurement technique are described in detail & very interesting

Here's the best version using all BNC connectors & good 75ohm cable
"Here it is my best effort, with a ~similar length, 40cm, Belden RG59 cable assembled with good 75ohm BNC connectors." http://postimage.org/image/23drcn5ms/

And one with 50ohm RCA connectors
"Now I had changed the scope termination to 50ohm. This creates -20% reflection at the "far", oscilloscope end." http://postimage.org/image/9qzw6tus/

And his remarks about these plots

As it was shown earlier, data dependent jitter, DDj, always appears in the FFT spectra as spurs.
The smoother the floor is, the less DDj is present.
One effect of the attenuator is that the DDj spurs are getting visibly lower.
There are a lot present, in the low audio range, in the shot with the RCA connector.
It gets a bit better with full BNC connectors, though the 50ohm mismatch still remains and causes problems.
Finally the attenuator helps to restore a cleaner situation.
And this hash visible here is in the audio band!! That spur at the upper end of the spectra, not diminishing, is correlated to the 44.1kHz sampling frequency.

Just as an aside - those measurements are actually about the conditioning effects that the RF attenuator I am recommending has on the SPDIF signal - so it is well documented!!

Might as well include the graph showing the RF attenuator effect

 

[NorthStar]

Wow, great post John!

 

[cooljazz]

If anybody is really interested & wants to put in some time studying a really deep analysis & some really interesting ways of jitter measurement using a $40,000 (I think) LeCroy & resultant graphs & a technical discussion about the results, you should have a look here http://www.diyhifi.org/forums/viewtopic.php?p=45228#p45228. The test set-up & measurement technique are described in detail & very interesting

Here's the best version using all BNC connectors & good 75ohm cable
"Here it is my best effort, with a ~similar length, 40cm, Belden RG59 cable assembled with good 75ohm BNC connectors." http://postimage.org/image/23drcn5ms/

And one with 50ohm RCA connectors
"Now I had changed the scope termination to 50ohm. This creates -20% reflection at the "far", oscilloscope end." http://postimage.org/image/9qzw6tus/

And his remarks about these plots

Just as an aside - those measurements are actually about the conditioning effects that the RF attenuator I am recommending has on the SPDIF signal - so it is well documented!!

Might as well include the graph showing the RF attenuator effect

Is this one of the four measurements that reveals everything we've ever needed to know, to explain everything to do with audio??

CJ

PS...nice toy!!

 

[jkeny]

Is this one of the four measurements that reveals everything we've ever needed to know, to explain everything to do with audio??

No, simply some reference points to analyse the difference between RCA & BNC connectors on SPDIF.

 

[amirm]

No, simply some reference points to analyse the difference between RCA & BNC connectors on SPDIF.

Not to nitpick but that is not what he is measuring . His scope has a BNC connector so he can't change that. So what he is doing is changing the internal impedance of the scope to be 50 vs 75 ohm as to simulate what an RCA connector would do assuming it nets out to 50 ohms.

 

[jkeny]

Not to nitpick but that is not what he is measuring . His scope has a BNC connector so he can't change that. So what he is doing is changing the internal impedance of the scope to be 50 vs 75 ohm as to simulate what an RCA connector would do assuming it nets out to 50 ohms.

AFAIK, he is putting a BNC/RCA adapter on the scope's BNC connector - is this not what you understood he was doing?

 

[amirm]

AFAIK, he is putting a BNC/RCA adapter on the scope's BNC connector - is this not what you understood he was doing?

He has no choice to do that because again his scope only has a BNC connector. But he doesn't just test that. He is changing the input impedance of the scope. I don't understand why he is doing that. If RCA is harmful, then using such a cable with an adapter should also show the problem. In other words let's assume both the source and sink device have the proper design with 75 ohms and then let's see what happens when we use the wrong cable connector.