If we are talking about coax cable for SPDIF than everything is being transferred at the same speed. Due to limited bandwidth only DSD-64 can be transferred over SPDIF in a DOP format but very few DACs support this feature.
-
WANTED: Happy members who like to discuss audio and other topics related to our interest. Desire to learn and share knowledge of science required. There are many reviews of audio hardware and expert members to help answer your questions. Click here to have your audio equipment measured for free!
50 ohm vs 75 ohm vs 125 ohm coaxial cable
- Thread starter Chrise36
- Start date
If we are talking about coax cable for SPDIF than everything is being transferred at the same speed. Due to limited bandwidth only DSD-64 can be transferred over SPDIF in a DOP format but very few DACs support this feature.
- Thread Starter
- #22
That is what i thought. I also read this:Capacitance in cable is usually measured as picofarads per foot (pf/ft). It indicates how much charge the cable can store within itself. If a voltage signal is being transmitted by a twisted pair or coaxial cable, the insulation on the individual wires becomes charged by the voltage within the circuit. Since it takes a certain amount of time for the cable to reach its charged level, this slows down and interferes with the signal being transmitted. Digital data pulses are a string of voltage variations that can be represented by square waves with near-vertical rise and fall transitions. A cable with a high capacitance slows down these voltage transitions so that they come out of the cable looking more like “saw-teeth”, rather than square waves, and the circuitry may not recognize the pulse. The lower the capacitance of the cable, the better it performs at higher frequencies which explains why the newer standards have higher impedance:
Standard Impedance (Ω) Tolerance Ethernet Cat.5100±5 Ω[4]USB90±15%[5]HDMI95±15%[6]IEEE 1394108+3
−2%[7]VGA75±5%[8]DisplayPort100±20%[6]DVI95±15%[6]PCIe85±15%[6]
Standard Impedance (Ω) Tolerance Ethernet Cat.5100±5 Ω[4]USB90±15%[5]HDMI95±15%[6]IEEE 1394108+3
−2%[7]VGA75±5%[8]DisplayPort100±20%[6]DVI95±15%[6]PCIe85±15%[6]
No. Without trying to explain the physics of transmission line theory, I'll note that many if not all (I am not sure about the video standards VGA, Display Port, and DVI) of those standards are for differential lines, not single-ended, and thus twice the impedance of a single line. That is, Ethernet is the same as two 50-ohm lines with one in reverse polarity ("out of phase") relative to the other.
The bandwidth limitation @Krunok mentioned is not due to the coax nor the capacitance in it.
Ideal capacitors and inductors do not lose energy though do cause shift phase. In a transmission line the capacitance and inductance is chosen to provide the target impedance which should match the source (driver) and load (receiver). You can think of the inductance as "cancelling" the capacitance (not really the way it works at the EM level but for hand-waving...) Look again at the basic transmission-line equations: If R = 0 (no loss) then frequency (w) drops out of the impedance equation. If everything is perfectly matched energy transfer occurs without loss at any frequency (no loss in bandwidth) and only a delay to pass through the line. Real resistance in the line causes loss and bandwidth reduction, and there are other real-world effects at RF and mW frequencies that do not really matter at very low frequencies like S/PDIF, as stated earlier.
The bandwidth limitation @Krunok mentioned is not due to the coax nor the capacitance in it.
Ideal capacitors and inductors do not lose energy though do cause shift phase. In a transmission line the capacitance and inductance is chosen to provide the target impedance which should match the source (driver) and load (receiver). You can think of the inductance as "cancelling" the capacitance (not really the way it works at the EM level but for hand-waving...) Look again at the basic transmission-line equations: If R = 0 (no loss) then frequency (w) drops out of the impedance equation. If everything is perfectly matched energy transfer occurs without loss at any frequency (no loss in bandwidth) and only a delay to pass through the line. Real resistance in the line causes loss and bandwidth reduction, and there are other real-world effects at RF and mW frequencies that do not really matter at very low frequencies like S/PDIF, as stated earlier.
Last edited:
- Thread Starter
- #24
So you are saying even for dsd which operates in the mhz section and even if we had a single ended coaxial system source cable receiver with higher impedance we would measure and get the same square waves.
If it is S/PDIF and you used a 93-ohm coax instead of 75-ohm the square waves would not be as clean but I doubt performance would be affected for a 1 m cable.
Here is an example from a different world but same idea. The top is the received signal with the right coax; the bottom is what happens when the coax is higher in impedance than the source/receiver impedances. Ignore the little glitches at the very start of the signals (launch discontinuities). Note the extra ripples in the signals on the bottom compared to the top. You would probably recover the data just fine in either case but the signal is better when everything is matched.
Here is an example from a different world but same idea. The top is the received signal with the right coax; the bottom is what happens when the coax is higher in impedance than the source/receiver impedances. Ignore the little glitches at the very start of the signals (launch discontinuities). Note the extra ripples in the signals on the bottom compared to the top. You would probably recover the data just fine in either case but the signal is better when everything is matched.
Last edited:
Max bandwidth SPDIF can carry is 2 channels with audio bitrates of 192kHz which corresponds to a rate of 12.288 MHz. This limitation is by the standard itself- cables can carry much higher bitrates/bandwidth.
If source, cable, and receiver were all 93 ohms it would be a perfect square wave everywhere. Same as if they were all 50 ohms or 100 ohms. Lower impedances are preferred in the RF world because they lead to wider traces on boards and those have lower loss. Higher impedances will have lower bandwidth in the presence of real-world mismatches. None of this is really going to matter for a 1 m cable carrying DSD.
SPDIF has been designed in such a way that cables and their impedance is simply not an issue for home use. For professional usage there is AES3 on which SPDIF is based.
Everything you would like to know about SPDIF (and much more) can be found here.
- Thread Starter
- #31
T
I see now i understand a bit why 75 ohm is chosen
Eirikur
Senior Member
- Joined
- Jul 9, 2019
- Messages
- 318
- Likes
- 510
- Joined
- Apr 24, 2019
- Messages
- 1,501
- Likes
- 2,822
I have a spool of 93 Ohm coax IIRC they used it in early computers for the low capacitance. 50 Ohms came from a trade off between power and attenuation, 50 being a nice round number near the geometric mean of the optimum for each. 75 Ohms came from the 2:1 balun match for a 300 Ohm dipole antenna. For short cables most of this is much adieu about nothing.
Yah, IBM used 93-ohm coax to interconnect their early mainframes and terminal clusters. I don't remember where 75 ohms came from; 77 ohms was what Bell Labs determined was the optimum for an air dielectric IIRC. And didn't Bell Labs also determine 30 ohms was another optimal impedance for low attenuation? Then the geometric mean is 48 ohms and average is about 53 ohms so they picked 50 ohms (or was it 52?) as a compromise? My memory is foggy (before my time) but I seem to remember the 30 and 77 ohm numbers. I used to know the history from my (long ago) ham radio days... Senility sucks.
Our college was updating their mainframes in the late 70's/early 80's and there was tons of 93-ohm cable being tossed. Wish I'd kept more of it and sold it for the copper!
- Thread Starter
- #36
A hifi magazine i was reading 10 years ago was suggesting that for short lenghts we could use 50 ohm cable which is wrong. Anyway i connected my dvd with a normal 50 ohm rca and it worked fine.
That's not something that needs measuring. This isn't analog- it either works perfectly, or it doesn't work at all. Think of hooking up a printer to your computer; if the cable is good enough, it prints your document. If it isn't, the document doesn't print. You don't get a document where the words are blurry or Shakespeare reads like Tom Robbins.
- Thread Starter
- #39
So how do you feel about this: http://archimago.blogspot.com/2013/04/guest-review-measurements-dr-franks.html?m=1 his measurements show that the longer 50 ohm subwoofer cable measures just a bit better.
"As you can see, the objective results were utterly pointless and do not explain the sunny vs. total eclipse audible difference whatsoever. Anyone who listens without noticing a difference must have sensorineural hearing loss. "
Measurements of these cables are the same. Considering measurement precision small variations are normal and expected.
Similar threads
- Poll