75ohm digital cable

[bumpy]

Can someone explain what this means please. Its certainly not the internal resistance of the cable. :(

[Firebottle]

At very high frequencies (much higher than audio) the capacitance and the inductance of the cable come into effect.

This capacitance and inductance is 'distributed' capacitance and inductance and can't be thought of as 'lumped' values, eg the total value of the capacitance for the length of cable.
These distributed values form an impedance that is directly related to the ratio of the diameter of the inner and outer conductors when considering a coaxial cable.

[bumpy]

I think I understand that :)

In terms of just carrying a signal, any interconnect seems to do the job, but what are the implications on SQ when using such a phono interconnect?

[Firebottle]

Put simply with mismatch reflections of the signal occur so the waveform gets ragged. This can cause issues associated with jitter dependent upon the tolerance of the receiving equipment.

[bumpy]

Thanks

[Mr. C]

Sorry for the amount of posts here but its an important question and one I feel needs some in-depth answers.

These posts are part of a reply I produced for a technical forum a few months back, I am just so busy at the moment my time is really finite I will produce some interesting experiments over Christmas which will dig deeper into a broader range of cable impedance issues for you but for now some basic get you going stuff!



It is accepted that 50 Ohms is the T & E standard, the ideal transmission vector is calculated around 43ish which is why 50Ohm is used in pretty much all test equipment (except when looking at certain voltage rails we use 1M ohm test probes)

Coax impedance is anywhere between 20 and 120 Ohms depending on application required. Below is the formula for working out cable impedance's. However I use a Vector Network analyzer with Smith chart ability and also VSWR measurements to look at impedance characteristics of different cables AND connectors (materials also). For those VNA#s that do not have a 75 Ohms facility we use 50<>75 matched transformer Baluns.



Characteristic impedance (Zo) is the most important parameter for any transmission line. It is a function of geometry as well as materials and it is a dynamic value independent of line length; you can't measure it with a multimeter. It is related to the conventional distributed circuit parameters of the cable or conductors by:
Zo=√[(R+jωL)/(G+jωC)]

where: R is the series resistance per unit length (Ω/m)

L is the series inductance (H/m)

G is the shunt conductance (mho/m)

C is the shunt capacitance (F/m).

L and C are related to the velocity factor by:

velocity of propagation = 1/√LC = 3 × 108/√ɛr

For an ideal, lossless line R = G = 0 and Zo reduces to √(L/C). Practical lines have some losses which attenuate the signal, and these are quantified as an attenuation factor for a specified length and frequency (Table 1.8 on page 29 shows these for coaxial cables). Table 1.9 on page 41 summarizes the approximate characteristic impedance's for various geometries, along with velocity factors of some common dielectric materials. The value 377 (120π) crops up several times: it is a significant number in electromagnetism, being the impedance of free space (in ohms), which relates electric and magnetic fields in free-field conditions.

Driving a signal down a transmission line provides an important exception to the general rule of circuit theory (for voltage drives) that the driving source impedance should be low while the receiving load impedance should be high. When sent down a transmission line, the signal is only received un-distorted if both source and load impedance's are the same as the line's characteristic impedance. This is said to be the matched condition. It is easiest to consider the effects of matching and mismatching in two parts: in the time domain for digital applications and in the frequency domain for analog radio frequency applications.


A great many DIY electronics enthusiasts tend to use these impedance converting devices, its not really a complete solution if you are looking for a true impedance match. Ideally all things being equal keeping the pathway the same Z all the way through from Tx to Rx.

I appreciate however that most of the after-market clocks are specific 50 Ohms.

When we measure a 75 ohm impedance path ways and cables we use a set of true 75 Ohm calibration tools and have our VNA or TDR devices set to 75Ohm. Totally understand 99.7% of audio users are not going to have this facility.

What I can do is set up an example of a 75 ohm cable being used with 50 Ohm measurement parameters, this will give you an understanding of how the reflections can be seen on the waveform.

Think of it like a converter from SPDIF<>AES or single end to balanced always something lost in translation I feel.

Will it be better than a total mis match 50<>75? possibly yes, possibly no, sorry for not being more definitive.

[Mr. C]

As promised some results from using impedance mis matched cables and adapters.

I used the lab's Mutec Ref10 SE-120 master clock as the signal generator, scope has been calibrated, all test cables 18Ghz reference ultra low loss 50 ohm, audio cables were 75 ohms matched. 50 Ohm adapter is an R&S reference 8Ghz model.

First images are a base line set, 50 ohm output, 50 ohm feed 50 ohm receiver, also I enhanced the waveform to highlight the very slight overshoot to give you an idea of of a close up of the wave form top edge definition.


https://audiophilestyle.com/uploads/monthly_2020_07/1185919992_50ohncable50ohmfeedmeasrements.jpg.ffbf f7346c3eeafbb630d8ddba653ffb.jpg

https://audiophilestyle.com/uploads/monthly_2020_07/1185919992_50ohncable50ohmfeedmeasrements.jpg.ffbf f7346c3eeafbb630d8ddba653ffb.jpg

https://audiophilestyle.com/uploads/monthly_2020_07/1185919992_50ohncable50ohmfeedmeasrements.jpg.ffbf f7346c3eeafbb630d8ddba653ffb.jpg


https://audiophilestyle.com/uploads/monthly_2020_07/2073759707_50ohmzoomonovershoot.thumb.jpg.277a16cc 3418670a9409006ce9f1f456.jpg

[Mr. C]

Next set of images show a 50 ohm cable being fed a true 75 ohm signal to a 50 ohm receiver note the reduced PK<>PK voltage and difference in the top edge wave form.

https://audiophilestyle.com/uploads/monthly_2020_07/772132369_50ohmcable75ohmfeed.thumb.jpg.e5aae4baf9 2dff42378fa89ba248f52c.jpg

https://audiophilestyle.com/uploads/monthly_2020_07/830774067_75ohmfedtopedgewaveform.thumb.jpg.dbd71a b1f8cbda1b97b7318d92d9d8f9.jpg

[Mr. C]

Now a genuine 75 ohm audio clock cable using the 50 feed into a 50 ohm receiver, as you can see plus the top edge definition again is greatly effected.

https://audiophilestyle.com/uploads/monthly_2020_07/1787058245_75ohmcable50ohmte0.thumb.jpg.396bce502e 1dd835ef28ab9d4fac9811.jpg

https://audiophilestyle.com/uploads/monthly_2020_07/232985555_75ohmcable50ohmte1.thumb.jpg.bebbacb72d6 850771dcab592f040e157.jpg

[Mr. C]

Last up 75ohm cable, 75 ohm feed into a 50 Ohm receiver, the lastly adapter added in to the chain for comparison.

As you can see the degradation of the wave form is pretty obvious, and especially with word clocks the wave form actually looks like a square wave form with very little deviations is the more ideal the situtaion.

https://audiophilestyle.com/uploads/monthly_2020_07/1542277756_75cab75line0.thumb.jpg.8a382df2d8759105 e69bc67b8b4b733d.jpg

https://audiophilestyle.com/uploads/monthly_2020_07/1988108974_75cab75line1.thumb.jpg.968835842a7697b0 335eed0e9e3dddf1.jpg

[Mr. C]

For the incoming data stream not to trigger either the wave form would have to be very badly distorted on the leading edge or the overshoot would need to be in the high double figures. another possible option could be the clock recovery circuit being of mediocre design with fairly high corner stone frequency and very average PLL ability this would lead to issues and a large increase in distorted sounds if not drop outs so pretty rare for this situation to happen



Yes the alignment of the audio data to word bit data would not be ideal at all, simply as the main timing (derived from the word clocking circuitry) would be sub optimal and therefore an increase in time interval error (TIE) would occur with out question



Any wave form deviation from the normal incoming wave form that point the circuit would usually see during normal operations can result in an in jitter.



These deviations are cause by the following:-



TRANSMISSION LINE problems, impedance mis-matching all the whole line no just the cables or board connectors, circuit board embedded pathways, components in that circuit where inductors capacitors or resistors.



BANDWIDTH data line not being sufficiently capable of handling the full amount of transmitted data remember audio word clocks are a staple 10Mhz so a pretty low level data stream required, however if all of the those components that make that pathway are not up to the task then jitter will occur. We use a tool called an eye diagram to look at serial data transmission lanes and can determine by observing the decoded patterns where a great many issues actually line in the circuit.



CROSSTALK pretty obvious where other signals which should be totally isolated from the clocking sections of device actually interferes with the clock for data circuit that is connected to the clock causing introduced noise which leads to unwanted induced jitter





Data generated jitter which is caused by bad programming at the core cpu or PFGA etc this can generate ISI (intersymbal interference) induced jitter and duty cycle distortion this is quite common in either highly complex devices where a lot of processing power is required to run a device and it takes a long time for total debug or in a simple device with novice programmer skills



We have special seial data analysis tools to look at all these types of jitter problems more on that later



RANDOM NOISE



which is cause by thermal flunctations the higher the temperature the more jitter.



This next one can be difficult to understand ,shot noise which is the random movement of electrons within the circuit like black holes and large celestial bodies mbe moved by dark matter not a lot you can do to take into account for this issue!



Lastly in this section frequency noise or pink noise the lower the frequency the more noise is introduced into the design all if these items designs take into account when coming up with a new product so tradeoffs are going to happen



All of the above cited problems can cause data line data corruption problems which will lead to the Rx end of your data stream incorrectly reading the edge crossing if the data which will result in jitter factor of electronics life.



There are more items that need addressing with jitter we have just touched on the surface however that is for a later post.

[Mr. C]

Thinking about at the above posts I should have constructed a finial post to condense this information into bullet points.



Ideal conditions:-



Zero compromise:-

Transmission 50 Ohm output <> 50 Ohm Cable <> 50 Ohm receive end terminal

Transmission 75 Ohm output <> 75 Ohm Cable <> 75 Ohm receive end terminal



Not so good:-

Transmission 50 Ohm output <> 75 Ohm Cable <> 50 Ohm receive end terminal

Transmission 75 Ohm output <> 50 Ohm Cable <> 75 Ohm receive end terminal

Transmission 75 Ohm output <> 75 Ohm Cable <> 50 Ohm receive end terminal

Transmission 50 Ohm output <> 75 Ohm Cable <> 75 Ohm receive end terminal



Least desirable:-

Transmission 50 Ohm output <> 75 Ohm Cable <> 50 Ohm <> 75 Ohm adapter at receive end terminal

Transmission 75 Ohm output <> 75 Ohm Cable <> 75 Ohm <> 50 Ohm adapter at receive end terminal



What you are attempting to achieve is removing as many of the reflection pathways as possible in the same way in your audio system you are trying to remove as much, electrical, mechanical, vibrational and radio reference noise as possible.

You make a comment on one of the results where the using a 75 Ohm cable on the 50 Ohm Tx & Rx positions resulted in a not too dissimilar wave form than the ideal impedance matched example.

Later on I will delve onto some jitter analysis with eye diagrams to illustrate that even small amounts of deviation will not yield the desired outcome. Also show how an ok looking square wave form can in fact had a multitude of sins.



Having spent a lot of years with this subject and many digital audio projects I can safely and happily share my thoughts, you can quite easily determine an impedance mismatch digital audio cable against the correctly matched item.



With clock cables this can manifest itself as less than ideal sound staging, more diffuse vocal placement, the timing is less coherent with an overall sense if it's not quite correct.



For those of you that use a a word clock and have genuine impedance matched cables, just try changing it for say a normal analogue cable of the same termination or a 50Ohm lab cable. Let us know what you feel.



Sorry back to the subject of adapters an issue personally I will not compromise on in any form in my professional and audio life.



Yesterday I received a large delivery of supplies for a new project I am working on and along with the components with another small box inside was pair of matching 50<>75 balun's BNC terminated for another project which has been on the back burner due to myself needing to construct a test rig for characterizing insertion loss for a/c mains common mode filters.



This means I have how some rather decent 50<>75 Ohm isolation transformers so this may work out to an acceptable compromise for some individuals.



These are NOT audio products but genuine lab test standard isolation transformers with impedance transitions by a company called North Hills if I have time today I will test it for you.



Below is wave form analysis of a Mutec MC3+ in word clock output mode using 176.4Khz files this will give you an idea of how some smaller companies and individuals actually go into deeper investigations with there research and designing stages for their products I suspect Uptone have this philosophy as well.


https://audiophilestyle.com/uploads/monthly_2020_07/243814185_Snosampling@176.4Khzwordclockseries--00006.thumb.jpg.440870ce59851e381c86769e7b9712c0.j pg

[Mr. C]

Also as well as wave form ringing and reflections note the difference in amplitude and positive and negative waveform widths. :)