Scope Classic SPDIF impedance

[quantum]

So apparently true 75ohm RCA connectors do not exist. BNC are 75 ohm. But since RCA is less than 75ohm, that means that Scope cards (except Plus versions) are not capable of true 75ohm digital output. I wonder is there's a DIY or any other way to change the Classic version to the Plus to gain 110ohm AES outs.

On the other hand, would replacing the RCA connectors with BNC give me true 75ohm digital output?

And by the way, does Scope even transmit a true 75ohm signal in the first place?

[garyb]

this is a non issue.

you are not going to be able to easily diy switch to balanced lines. use a proper rca digital 75ohm cable. there are tons of them out there, although it's much easier to find optical than coax cables, the coax rca cables are more than available and they definitely work. http://en.wikipedia.org/?title=S/PDIF

[quantum]

Thanks but I have done that already. RCA works better than optical almost always but the problem is that RCA connectors can never be true 75 ohm due to their dimensions, only BNC can be true 75 ohm. So I guess the only way to get a true 75 ohm connection from Pulsar cards is to replace the RCA connectors with BNC, that is if Scope even transmits such a connection. RCA connectors are only 50ohms.

[garyb]

sp/dif is already a consumer spec. it's unbalanced. i doubt if putting BNC connectors on the cable whip will make an extraordinary difference, but....

even if the jack is 75ohms, what about the connection inside each unit? i think there are plenty of other things to worry about before this unless you're running very long lengths or using it for HD Video. sp/dif isn't for lab work.

i would say the cable is far more important than the connector, even if the connector has an impact.

[quantum]

Is there a way to find out if Pulsar 1 or 2 Classic cards even transmit 75ohm signals or a way to measure this on my own?

[garyb]

no real practical way, but yes, they transmit and look to receive 75ohm signals on the sp/dif connectors.

[quantum]

thanks. so if that's true then replacing the breakout rca jacks with bnc would give at least 25ohm higher reading, according to:

http://www.diyaudio.com/forums/digital- ... sible.html
and
http://www.bluejeanscable.com/articles/75ohmrca.htm.

right now i have recording and mastering in one setup routed through scope on a pc. eventually for mastering i'm switching to soundblade and mac with yellowtec puc2 lite. unless someone has a pulsar plus to sell.

on the other hand i wonder if the yellowtec card will install and run ok next to pulsar 1 or 2. it's an external usb card made in germany with only 1 aes in and 1 aes out, up to 192khz. that way i wouldn't really need soundblade and could run those 192khz signal for people who are scared of anything lower

[jksuperstar]

Do you see a problem or are you cutting off your legs before trying to walk?

I have only seen spdif issues on one piece of gear, an E-mu Command Station. But by adding a redundant audio cable ( just to get proper ground) the issue was fixed. I never bothered to modify the unit.

[quantum]

[quote="jksuperstar"]Do you see a problem or are you cutting off your legs before trying to walk?

problem is i don't have a pulsar plus with AES or the Yellowtec AES interface yet. but I guess I should've said how does Scope work with other sound cards.

[garyb]

fine, if the other card works properly.

i doubt if you'l notice a lot of difference. most people obsessing over these impedance issues regarding sp/dif are concerned about HD video.

agian, sp/dif is a consumer standard. it far surpasses the needs of it's actual function even using rca plugs. it should be more than fine as is, unless the other card is a piece of sh@$. you're not using super long cable runs, right? long cable runs would be unwise with an unbalanced cable anyway, even with perfectly matched impedance. shorter runs won't be affected by a slight impedance mismatch. this is digital not analog. the info is packets of data with a timestamp. if data is missing, you get an error indicator, if the data is intact it's intact. are you having a problem with heavy aliasing or something? is data being rewritten in the cable?

[quantum]

the yellowtec is usb so it should be fine. my digital cable runs are all under 4ft. i have no sound quality issues or dropouts. but since all of my digital chain is aes i'll just get the yellowtec to have a piece of mind, do 192khz, and have the option for soundblade.

[quantum]

shorter runs won't be affected by a slight impedance mismatch.

Just read a post on Lavry's site saying the same thing, that for cables under 2 meters there's no difference between rca/bnc 50ohm and bnc 75ohm connections assuming the cable is thick enough. And this does make sense since a 50ohm resistance would give louder sound than 75ohm. I also tested this with Canare 75ohm to 110ohm trafo adapters. 110ohm resistance basically reduces the signal to a pro line level so that it can be turned up on the DAC or pure analog side. a mastering chain is a perfect example of where 110ohms is most useful so that the playback dac's outs are not at their lowest position.

[jksuperstar]

Wait, are you saying that a 110ohm cable on a digital signal attenuates the *volume*? huh?

Comparing AES3 balanced, unbalanced, and SPDIF: If you use the specified cable, within the specified length, you will get the EXACT SAME DATA across all three interfaces. One will not sound better than another. One might be more noise tolerant, but that's up to you to keep your studio fairly "clean" anyway, and the cables will do their jobs correctly.

A digital signal will not be attenuated in volume if you use the incorrect cable impedance...it *can* give sloppy edge transitions on those signals, meaning it's harder for a receiver to extract the timing/clock information, and get your samples back accurately without glitches occurring. Adding a word clock to this scenario may help some, depending on the receiver, but you are still open to bad data detected by the DAC.

Last, the RCA connector is PART of the AES spec. It's not a cheap version. BNC was added to the spec for industry use, such as in broadcast. But the electrical spec accounts for the RCA connector, and since the only difference between unbalanced AES3 and SPDIF is how the bits are defined, you get to select which one you are transmitting from within SCOPE. The electrical part transmission, however, is the same.

So, use the RCA connector 75ohm cable. It was designed for the use you want. IF you need longer runs, you can use an AES/SPDIF converter, because you'll need the electrical conversion to the different voltage levels, that just changing the connector won't do for you.

[garyb]

not to mention balancing/unbalancing the connections...
there's no difference between XLR and rca if both are 2 conductor. both use the same type of physical connection. i haven't done the math, but i suspect that the rca connectors offer more surface area in their connections than XLR connectors do, so for a 2 conductor connection(what an unbalanced sp/dif connection will be no matter what connector is used), rca may be superior.

[quantum]

A digital signal will not be attenuated in volume if you use the incorrect cable impedance...

I tested this by passing a signal through an RCA 50ohm to BNC 75ohm cable and then into the Canare 75ohm BNC to 110ohm AES adapter. The sound is much lower in volume rather than when using just a regular RCA cable. I realize it's just a digital connection, but it's also electrical, not optical, so when you increase ohms across the connection, apparently it reduces the volume. Scope and the unit after were at 0db. And you are right when you say that the signal is not degraded.

[jksuperstar]

Are you transmitting from SCOPE as SPDIF, or AES in both scenarios? It seems you are receiving AES, so you should make sure scope is always transmitting AES.

[quantum]

Are you transmitting from SCOPE as SPDIF, or AES in both scenarios? It seems you are receiving AES, so you should make sure scope is always transmitting AES.

Transmitting as SPDIF via Scope 4.5 Classic. I don't have the option for AES since it's not the plus version or XITE. But even if you think about this electrically, a connection going from lower ohms to higher ohms should lower the volume on the output end, or maybe it's just the Canare bnc to aes adapter. Either way at least I am not losing quality, or enough of it to notice

[garyb]

the signal level will be different if you connect sp/dif to aes/ebu, but the volume is digital. of course, the sp/dif level will be low going into aes/ebu because the scale is different.

[quantum]

the signal level will be different if you connect sp/dif to aes/ebu, but the volume is digital. of course, the sp/dif level will be low going into aes/ebu because the scale is different.

Thanks for confirming this Gary.

[garyb]

again, the volume of the audio will not be affected.