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Thread: 75ohm digital cable

  1. #11
    Join Date: Jan 2008

    Location: Gerrards Cross

    Posts: 2,696
    I'm Tony.

    Default Impedance Importance with regard to cable matching P VI

    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.
    Coherent Systems
    Real high end sound with musicality not hifi

  2. #12
    Join Date: Jan 2008

    Location: Gerrards Cross

    Posts: 2,696
    I'm Tony.

    Default

    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.


    Coherent Systems
    Real high end sound with musicality not hifi

  3. #13
    Join Date: Jan 2008

    Location: Gerrards Cross

    Posts: 2,696
    I'm Tony.

    Default

    Also as well as wave form ringing and reflections note the difference in amplitude and positive and negative waveform widths.
    Coherent Systems
    Real high end sound with musicality not hifi

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