Discussion on digital Cables
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reignofchaos
Well-Known Member
Impedance. Analog cables are 110 ohm, coaxial digital are 75 ohm. The connectors are different in each.
Also digital ICs are typically 0.75m or 1.5m to avoid internal reflections.
T
That’s very clear differences between them in electrical resistance and internal reflections.
A few questions come to mind:
What are internal reflections and how to the affect SQ?
What is the role of wire resistance in analog and digital signals?
Can “digital” and “analog” IC cable be used interchangeably (with appropriate connectors)?
Thank you @reignofchaos for the succinct description.
That’s very clear differences between them in electrical resistance and internal reflections.
A few questions come to mind:
What are internal reflections and how to the affect SQ?
What is the role of wire resistance in analog and digital signals?
Can “digital” and “analog” IC cable be used interchangeably (with appropriate connectors)?
digital coaxial IC terminated with RCA type connectors (and not BNC) are very good as analogue interconnects.
For digital transmission (more so for video than audio), the connectors need to be 75ohm spec which is usually limited with RCA type connectors, but the connectors should preferably be of coxial type build and not the typical fork type RCA connectors.
For digital coax transmission. 1.5 meter or feet, I dont remember is the recommended length as per a research, but I have not heard any audible difference in shorter or longer lengths.
Ultimately, cables will remain a very subjective topic with intermix of science and woodoo.
reignofchaos
Well-Known Member
Apparently internal reflections can cause the crests and troughs of the signal to become less distinct potentially causing bit inversions.
Digital signal transmission requires 75ohm connectors as part of the spec to maintain signal integrity. Why this is the case? Well I honestly do not know. Ideally BNC connectors must be used as 99% of RCA connectors don't have 75 ohms resistance.
You can use a digital ic for analog transmission.
I was always unsure about this 75 ohms thing. I used to think how can impedance of a cable remain 75 ohms when the length of the cable will be variable? How can you say that a tv dish cable running from the rooftop to the ground floor maintain 75 ohms? Whatever cable I had measured around 0 ohms using a multimeter. So I did some good amount of reading on this 75 ohms things and found out the actual cables are not 75 ohms. It is the connector that are 75 ohms for whatever that is required for digital transmission. So it is the connector the most important thing rather than the cable. This video pretty much cleared the doubts about 75 ohms impedance.
In reality one uses 75 ohms resistors on terminations. Even ethernet requires to be 75 ohms. Just like the so called 'made for digital' cables, the resistance of the twisted pair will be miniscule. The 75 ohms comes from 75 ohms resistors added in the circult. The 75 ohms cable is now just a marketing term. Don't ever take it at face value.
Why is this 75 ohm resistor present in the Ethernet connector? (10/100Base-TX)
In this datasheet (and in other similar) pins J4, J5, J7, J8 are connected to the ground with a resistor and cap. For 10/100Base-TX pinout it seems they are not used (at least for modern of the sta...
So a big lesson learnt. It's not the damn cable. It is always the termination. https://tekron.com/news/release/when-to-use-a-terminating-resistor/
Another lesson learnt which makes all claims of 75 ohms is snake oil. This is just a compromise that was adopted regarding cable flexibility. In fact 50 ohms is better.
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you are correct, the critical part is the termination as i have mentioned in the earlier post.I was always unsure about this 75 ohms thing. I used to think how can impedance of a cable remain 75 ohms when the length of the cable will be variable? How can you say that a tv dish cable running from the rooftop to the ground floor maintain 75 ohms? Whatever cable I had measured around 0 ohms using a multimeter. So I did some good amount of reading on this 75 ohms things and found out the actual cables are not 75 ohms. It is the connector that are 75 ohms for whatever that is required for digital transmission. So it is the connector the most important thing rather than the cable. This video pretty much cleared the doubts about 75 ohms impedance.
In reality one uses 75 ohms resistors on terminations. Even ethernet requires to be 75 ohms. Just like the so called 'made for digital' cables, the resistance of the twisted pair will be miniscule. The 75 ohms comes from 75 ohms resistors added in the circult. The 75 ohms cable is now just a marketing term. Don't ever take it at face value.
Why is this 75 ohm resistor present in the Ethernet connector? (10/100Base-TX)
In this datasheet (and in other similar) pins J4, J5, J7, J8 are connected to the ground with a resistor and cap. For 10/100Base-TX pinout it seems they are not used (at least for modern of the sta...electronics.stackexchange.com
So a big lesson learnt. It's not the damn cable. It is always the termination. https://tekron.com/news/release/when-to-use-a-terminating-resistor/
Another lesson learnt which makes all claims of 75 ohms is snake oil. This is just a compromise that was adopted regarding cable flexibility. In fact 50 ohms is better.
actually for long range transmission (as confirmed by a friend in the army) the preferred termination is 50 ohms, it is only for AV that it is 75 ohms , as it is the industry standard.
there is also 110 ohms i guess which is used in critical applications on short lengths.
The 75 ohms impedance of a typical coax cable is termed as characterestic impedance and not the actual resistance offered by the cable.
For example the Belden 75OHM RG6 cable has a resitance of around 7 ohms per 1000 feet.
The characteresic impedance is defined as follows, not that I understand it much.
The last line summarizes that the cable's characterestic impedance should match that of the output and input which is 75ohms for audio and video transmissions as per the industry standard for digital coax.
The characteristic impedance or surge impedance (usually written Z0) of a uniform transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in the absence of reflections in the other direction. Alternatively, and equivalently, it can be defined as the input impedance of a transmission line when its length is infinite. Characteristic impedance is determined by the geometry and materials of the transmission line and, for a uniform line, is not dependent on its length. The SI unit of characteristic impedance is the ohm.
The characteristic impedance of a lossless transmission line is purely real, with no reactive component. Energy supplied by a source at one end of such a line is transmitted through the line without being dissipated in the line itself. A transmission line of finite length (lossless or lossy) that is terminated at one end with an impedance equal to the characteristic impedance appears to the source like an infinitely long transmission line and produces no reflections.
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Characteristic impedance, not impedance. The characteristic impedance depends on the diameter of the center conductor, the diameter of the outer conductor and the dielectric constant of the insulator between the two conductors.
The two conductors must remain concentric and their separation must be constant to maintain the specific characteristic impedance. The moment the concentricity is broken by using a wrong connector/plug (e.g. a regular RCA plug), the distance between the core and outer conductor no longer remains as intended, and its characteristic impedance of the cable assembly (actual cable + connectors) will change. Hence the use of BNC or RCA plugs that maintain this "concentric" geometry.
Though characteristic impedance is measured in Ohms, one cannot measure characteristic impedance with a multimeter. A multimeter will only measure the resistance (in Ohms) of the core or outer conductor, or a combination of both.
Let us not confuseb the terms resistance, impedance and characteristic impedance. They are not interchangeable.
Characteristic impedance is independent of cable length whereas resistance and impedance are directly proportional to length.
and I think you require something known as TDR to measure characteristic impedance
Time-domain reflectometer - Wikipedia
for AV transmission it is a critical value as per the industry standards for digital coax.
But now with HDMI being the primary way to transmit AV signals and multi-channel audio, digital coax is relegated to only 2-channel audio transmission. That too with most modern sources, it is now mostly USB (due to higher bandwidth to support DSD). Coax and toslink are nowadays mainly used with streamers.
keith_correa
Well-Known Member
There is a school of thought that says it is this characteristic impedance that causes different interconnects to cause sonic differences and also a given interconnect to cause varied sonic differences in different equipment.
So now we have one more thing to obsess on - for maximum signal transmission [with minimal reflections], the characteristic impedance from the point the signal leaves the board [if applicable] of a source needs to be the same for the following:
Wire leading the signal to the output RCA jack
Output RCA jack
Wire leading the signal from the output RCA jack to the input RCA jack of the next stage
Input RCA jack
Wire leading the signal from the input RCA jack to the board [if applicable]
...
My head hurts!
So now we have one more thing to obsess on - for maximum signal transmission [with minimal reflections], the characteristic impedance from the point the signal leaves the board [if applicable] of a source needs to be the same for the following:
Wire leading the signal to the output RCA jack
Output RCA jack
Wire leading the signal from the output RCA jack to the input RCA jack of the next stage
Input RCA jack
Wire leading the signal from the input RCA jack to the board [if applicable]
...
My head hurts!
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Lesson: buy products that have the connectors directly solderd on the board
If I am not mistaken, isnt this one of the things what the proprietary quad link cables and connections try to do ?
This OT discussion on digital Cables on Mega IC shootout has been branched off into this particular thread. Please carry on the discussion here.
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