[Porksoda]

So I am making a DAC, and figured why not make the spdif cable too? Silly me, I thought it would be the same as building a standard RCA interconnect, but came to find out there are some special requirements for SPDIF.

I notice that the cables (more specifically the terminations) are all supposed to be 75 ohm. What does this mean, exactly? Is this 75 ohms from the solder point to the end of the connector, or 75 ohms from signal to ground? Is this on both ends? If that were the case each one would be 150 ohms, right? (assuming the resistance is signal to ground as I suspect)

Also, in some DAC designs (the audionote tube DAC being the one I saw), they include a resistor from signal to ground on the input. It is frequently, you guessed it, 75 ohms. What does this mean for cable terminations? In this case would standard (no built in resistance) terminations work since the impedence is taken care of internally, or does this need to be done in addition to the 75 ohm terminations?

Lastly, what is a good place to buy good quality bulk coax cable in relatively small quantities, like 10 ft or so, and since the cable is coax, what special stuff needs to be done to terminate it with regards to proper terminators and whatnot?

 

[Garbz]

I wont answer most of this because i'm not 100% sure myself. But i can say that when you terminate the cable there's only one way to do it and retain the 75 ohm impednace, and that is BNC connectors which incidently make by far the best cable bar none. Copper/Silver coax with BNCs will outperform any $gazillion "Digital Interconnect" which is terminated with RCAs or Toslinks.

 

[bryman79]

Porksoda,

Yes, you do want S/PDIF passed through a properly terminated coax as Garbz mentioned. Proper termination maintains the 75-Ohm impedance between circuits in the different pieces of equipment, which reduces jitter. 75 Ohm refers to the load that the alternating signal sees.

DO NOT use RCA... there is no true 75 Ohm RCA. Those with a TDR will tell you they measure around 32-33 Ohms. See here: http://www.diyaudio.com/forums/showt...threadid=61569. See page 3 for equations. And yes, Jocko owns a TDR and has tested these components.

I have used Trompeter wrench-crimp conductors at the suggestion of Jocko and other experienced builders. They claim that they maintain impedance better than others, and from using them myself, I can attest to their build quality.

Note that cable type and length do affect sound quality. Reflections are reduced by increasing cable length. I'm not sure if reflections are the cause of sound difference among different types of cable though. Nonetheless, I'm currently using Belden 1505a with my sources and DACs. With all combinations of my equipment, 3m sounds better than 2m. I still have to make up a 4m and give it a listen. Don't go over 10m though.

Note that the input and output buffer circuits do affect jitter and sound quality. Simply putting a 75 Ohm resistor does not make a 75 Ohm impedance load, nor will it sound the best. I can post some of the recommended circuits that I have compiled based on sound and TDR performance if you'd like. You might also want to do a search at DIYAudio and DIYHifi.

Regards,
Bryan

 

[Garbz]

bryman in the post stages of my DAC design i'm interested in these circuits for last minute changes. So please post away.

 

[tangent]

Quote:

Originally Posted by Porksoda I notice that the cables (more specifically the terminations) are all supposed to be 75 ohm. What does this mean, exactly?

When you have two conductors in parallel with each other (e.g. coax cable) you have parallel capacitance between them (a cap is just two conductors separated by an insulator, after all) and series inductance (all wire has inductance). How much capacitance and inductance depends on the geometry of the thing.

If you imagine such a cable to be infinitely long, it will have an AC impedance fixed by this geometry. This is called the cable's characteristic impedance.

Why do you care? Because when a transmission line is terminated in its characteristic impedance, you get the cleanest transfer of signal. If the line is imbalanced, you get reflections, standing waves, etc. which futz with your signal quality.

The anecdote above of a longer cable working better than a shorter cable is also true. Consider IDE disk drive cables. Have you ever wondered why they place the connectors 4 inches apart or so, when they're virtually always used to connect drives that are maybe 2 inches apart? Answer: if the connectors were closer together, you'd get reflections, because the cable between drives isn't long enough for characteristic impedance effect to take over. You see the same effect, only more pronounced, in LVD SCSI cables; there, you'll never see connectors closer than about 6-8 inches apart. Different frequencies and different geometries = different rules about minimum length.

 

[Jam_Master_J]

Thanks for the explanation Tangent. Is that why when looking at coaxial cable online they recommend what connector to go with it? Is that impedance matching or strictly a fit issue?

On a related note, I'm trying to find an RCA connector suitable for S/PDIF duty. I'd prefer to buy from this site but none of the connectors indicate an impedance or are noted as being specifically for video/digital duty. Are none of these connectors compatible with digital at all?

-JMJ

 

[Porksoda]

Ugh, of COURSE it's never a simple thing with hi-fi audio

 

[Garbz]

The impednace of a connector is detmined by it's physical properties. RCA connectors have 20-30ohm impedance when terminated. There's no way around this because of the design of the plug. Sure you can buy WBT's nextgen RCA plug which is 75 ohm because of a reduced ground coverage, but as soon as it's connected to a standard RCA socket it's back down to 20-30. The only way to get 75ohms from RCAs is to use matched pairs of plugs and sockets specifically designed for 75 ohms and specifically designed to be used with each other, like the WBT nexgen RCAs. You can use thoes but you'll need plugs and sockets not just one or the other.

Other then that you are only left with BNC, or F connectors as common 75ohm terminations.

All consumer equipment's RCAs are not ideal for their transmission and are mostly used for legacy reasons. If you look at pro gear you'll see 1/4", XLRs, Spekons, BNCs, etc. Even some better computer monitors have 5 75ohm BNCs on the back instead of a D15 connector.

 

[bryman79]

People talk about BNCs like they are the devil, trying to avoid them at all costs. I personally like BNC connectors... am I the only one? My Trompeters slide precisely together, and you get that turn-and-lock feeling. Kind of like the CB mic plugs. Although the locking feature is certainly more necessary on the BNCs than the mic connectors...

And as for cost, the BNC bulkhead from Trompeter for the 1505a, for example, cost me ~$9. Pricey for a BNC, but a lot less than boutique RCAs. And the cable termination BNC connectors that mate to the BNC bulkheads were ~$4. This is a lot less than the Eichmanns I have to use to keep my RCAs from getting scratched (I've posted about those wonderful "locking" RCAs that scratch the hell out the RCAs no matter how careful you are).

Bryan

 

[motherone]

Being a router monkey, I've used BNCs quite a bit, including crimping my own cables. BNC's are great if they're done right, but the tools are a little bit more expensive. I agree that thye're far better/cheaper than the "boutique" RCA connectors. I also wish that they'd been the standard for S/PDIF from the getgo instead of RCAs. Unfortunately, people are used to the venerable RCA jack, and lord knows that they only way that's going to disappear is when everything is digital through-and-through.

I'm going to eventually swap all the digital RCA connectors out of my equipment for BNC.. I have some Canare BNC cables, but they're only 1 or 2 foot cables.. Looks like I'll need to order some longer ones.

Tangent -- I really like your explanation.. It's the most eloquent way I've seen someone answer that question (very easy to visualize, especially since I'm a computer nerd as well..)

On an interesting side note, I keep reading about how terrible toslink is, yet time and time again, I see toslink putting out superior jitter perfromance compared to coax. I suppose when you get into megabuck equipment, there's probably a better chance of the coax being lower jitter, but on inexpensive gear, toslink seems like the way to go.

 

[bryman79]

Quote:

Originally Posted by motherone I keep reading about how terrible toslink is, yet time and time again, I see toslink putting out superior jitter perfromance compared to coax.

Can you elaborate a bit? Is the superior jitter performance in SPDIF transmission?

Bryan

 

[motherone]

Yes. Time and again I've read measurements of various components, and it regularly shows that the toslink seems to have it's jitter fall in the 200-300ps range, where Coax can be anywhere from 150-1000ps.

There's a quite a few published results from stereophile, as well as a great comparison on a review from What Hi-Fi or one of the UK rags that Monarchy included with my DIP Classic. In almost every case, the Coax's performance was far worse than the Toslink's.

I'll poke around and see if I can find any examples through google.

I should stipulate that all the Coax measurements I've seen are either RCA or 13W3 based -- none with BNCs.

 

[Voodoochile]

Canare does make true 75 ohm RCA fittings, you crimp the center pin, then insert it into the center of the plug and attach the shield. It works very well, especially with the nice digital video coax they have, but ideally requires the expensive crimping tool to be true.

It is just about as easy to retrofit your equipment with BNC connectors. Bear in mind that not all BNC is 75 ohm, either, so don't just grab some connectors and say "it's 75 ohm".

 

[Garbz]

Yes canare does make true 75ohm cables. But thoes cables cease to be 75ohms when plugged into a normal RCA jack. There was a very heated discussion on this on diyaudio resulting in jacks being tested using TDRs.

Yeah most coaxes are 50 ohms. People have choice, just make sure you get the right one.

I'm sure coax is outperformed in most cases because of the said problem. RCAs! It seems every idea that the audio world has get's bashed about and standardised, and compromised and comes out a smidgen of it's former self. It's kindof like a parlamentry session like that

I think i'll just leave all of this behind, buy a few NAIM components complete with their 5pin DIN interconnects as standard, and just let the connector and cable comapnies eat themselves

 

[Voodoochile]

Quote:

Originally Posted by Garbz just let the connector and cable comapnies eat themselves

I like that idea the best of any I've heard in a long time.