Choosing Blu ray player

[Smokey]

It's apparently more complicated than that. The issue is signal echo/bounce that occurs when the source and target impedances are mismatched; it hasn't to do with the impedance of the cable itself.

But there is no impedance mismatch between the source and target for coaxial digital protocol. Like the video connection, the source will have 75 ohm output and the target will have 75 ohm input impedance. And to keep the effects of cable minimum, the cable should have 75 ohm impedance.

If cable is not 75 ohm for these type of connections, then you will run into problems like signal echos and noise pick up.

Most manufacturers advise to keep the coaxial cable run short, in fact, Audio Note recommended a .75 meter as optimum.

Whether we like it or not, the cable will have an effect on the signal it is carrying. The longer the cable, the more cable resistance, capacitance, inductance and noise ratio effects will have on the signal. So keep it short

[Feanor]

But there is no impedance mismatch between the source and target for coaxial digital protocol. Like the video connection, the source will have 75 ohm output and the target will have 75 ohm input impedance. And to keep the effects of cable minimum, the cable should have 75 ohm impedance.

If cable is not 75 ohm for these type of connections, then you will run into problems like signal echos and noise pick up. ...

I suggest your seek & read the discussion at Audio Asylum; I'm not an electrical engineer and can't provide you with a full discussion. There are people at AA who are and can. One thing, though, the actual impedance of the cable itself is far less than 75 ohms; the 75 ohm specification pertains to the source and receiving circuits.

...
Whether we like it or not, the cable will have an effect on the signal it is carrying. The longer the cable, the more cable resistance, capacitance, inductance and noise ratio effects will have on the signal. So keep it short

There's no question of my likes & dislikes: I agree that cables can effect the signal. However I use a 30 foot coax cable myself with excellent results.

BTW, Mr P, I wouldn't take any comment pertaining to things digital coming from Audio Note as authoritative.

[Smokey]

I suggest your seek & read the discussion at Audio Asylum; I'm not an electrical engineer and can't provide you with a full discussion. There are people at AA who are and can.

There are other sources of information beside AA

One thing, though, the actual impedance of the cable itself is far less than 75 ohms.

And why would that be?

One specifcation of [quality] digital coaxial cable is is having 75 ohm impedance (same with video cables too).

[Feanor]

...
And why would that be? {coax cable actual impedance much less that 75 ohms}

One specifcation of [quality] digital coaxial cable is is having 75 ohm impedance (same with video cables too).

No need to take my word for it. Grab yourself a length of coax and measure the resistance with you handy dandy multimeter. I just now took a 6' length and measured: signal (core) = 0.3 ohms; ground (shield) = 0.7 ohms.

[Smokey]

No need to take my word for it. Grab yourself a length of coax and measure the resistance with you handy dandy multimeter. I just now took a 6' length and measured: signal (core) = 0.3 ohms; ground (shield) = 0.7 ohms.

What you are measuring is "DC" resistance of cable as cable impedance can not be measured with a multimeter. Special instrument is needed to measure cable impedance since we will be dealing with AC signal, cables inductance and capacitance. AC signal will see the cable impedance, but DC willl not

[Feanor]

What you are measuring is "DC" resistance of cable as cable impedance can not be measured with a multimeter. Special instrument is needed to measure cable impedance since we will be dealing with AC signal, cables inductance and capacitance. AC signal will see the cable impedance, but DC willl not

I stand corrected in as much as there is a difference between AC impedance and DC resistance. However as between one purpose-made coax cable and another versus other components in the source/cable/receiver system, I suggest differences are slight. I note that specified DCR's vary and are specified by unit of length, where as nominal impedance is simply specified as 75 ohms regardless of the length.

HERE is an article by Steve Nugent wherein he argues the coax S/PDIF ought to be at least 1.5 meters (5 feet) in order to minimized jitter resulting from signal echos caused by source/receiver impedance mismatches.

Excerpt ...

When a transition is launched into the transmission line, it takes a period of time to propagate or transit to the other end. This propagation time is somewhat slower than the speed of light, usually around 2 nanoseconds per foot, but can be longer depending on the dielectrics used in the digital cable. When the transition reaches the end of the transmission line (in the DAC), a reflection can occur that propagates back to the driver in the Transport. Small reflections can occur in even well matched systems. When the reflection reaches the driver, it can again be reflected back towards the DAC. This ping-pong effect can sustain itself for several bounces depending on the losses in the cable. It is not unusual to see 3-5 of these reflections before they finally decay away, particularly when using the best digital cables, which are usually low-loss.

So, how does this affect the jitter? When the first reflection comes back to the DAC, if the transition already in process at the receiver has not completed, the reflection voltage will superimpose itself on the transition voltage, causing the transition to shift in time. The DAC will sample the transition in this time-shifted state and there you have jitter. Let’s look at a numerical example: ...

Elsewhere Nugent suggest a 10 foot maximum, although I've hear others argue for 30 feet of more. Personally I'm using Blue Jeans Cable coax, Belden 1694A cable with Canare RCAP compression-fit connectors, HERE, with excellent results. (In my case the longer length is partly because of my own computer and DAC placement requirements.)

[Smokey]

[I]HERE is an article by Steve Nugent wherein he argues the coax S/PDIF ought to be at least 1.5 meters (5 feet) in order to minimized jitter resulting from signal echos caused by source/receiver impedance mismatches.

I read his article and he does make sense. I agree with him that RCA connectors and its wiring may cause reflection due to not being 75 ohm. That is something we don't have control over unless doind some serious DYI wiring

But there might be a flaw in his assumption of timing reflection. He assume that that space timing of digital signal is constant as for rising and falling time. But if one look at the coaxial signal itself, we can see that timing between rising and falling signal is not constant:



The signal is encoded in Manchester coding and and as you can see the timing vary between when signal fall and rises. So when we do have a reflection as Steve Nugent mentioned, one will never know whether the reflected signal will arrive at rise/fall time of source signal, or when the souce signal is at steady state.

Also he mentioned that we may have reflection at these point along transmission-line components:

The traces on the Transport circuit board that connect to the driver chip
The wiring to the output connector
The output connector jack and plug (BNC or RCA)
The digital cable
The input connector jack and plug at the DAC input (BNC or RCA)
The wiring to the circuit board
The traces on the DAC circuit board that connect to the receiver chip

So how do we accont for the timing of all these reflected signal?