Hi All:
I have a kit DAC in a metal box and I want to provide it with working S/PDIF input.
You all might say that this topic has been beaten to death. But still I could not find a guide/textbook/method for beginners like me
.
I have read the posts here and then on diyhifi and the theoretical talk of Gurus’ just goes over my head. Please guide me in steps what to do.
This is what I currently plan to do. Take 6 inches of 75ohm coaxial cable, solder or crimp one end to 75ohm female chassis mount BNC. BNC is chassis isolated, i.e. BNC outer frame does not electrically connects to chassis. Solder the other end of this cable directly to PCB where it says S/PDIF or use existing terminal block. I read on posts about need to use 75R resistor, where should I place it if this is required step?
I just have a DMM and no oscilloscope or other equipment. I know some of you will write that they do not know what is on the PCB, “characteristic impedance” of PCB trace,…neither I know the answers to these nor I think DMM can help. Thus please give me your best practical guess of installing a BNC for S/PDIF in.
Regards
I have a kit DAC in a metal box and I want to provide it with working S/PDIF input.
You all might say that this topic has been beaten to death. But still I could not find a guide/textbook/method for beginners like me
.I have read the posts here and then on diyhifi and the theoretical talk of Gurus’ just goes over my head. Please guide me in steps what to do.
This is what I currently plan to do. Take 6 inches of 75ohm coaxial cable, solder or crimp one end to 75ohm female chassis mount BNC. BNC is chassis isolated, i.e. BNC outer frame does not electrically connects to chassis. Solder the other end of this cable directly to PCB where it says S/PDIF or use existing terminal block. I read on posts about need to use 75R resistor, where should I place it if this is required step?
I just have a DMM and no oscilloscope or other equipment. I know some of you will write that they do not know what is on the PCB, “characteristic impedance” of PCB trace,…neither I know the answers to these nor I think DMM can help. Thus please give me your best practical guess of installing a BNC for S/PDIF in.
Regards
You want the termination of 75R to be at the board end of the coax. You also want the traces to the receiver to be relatively short. Make sure the board doesn't already have an input resistor on it!
If you want to be anal, you can also use a properly terminated input transformer- there could be some benefit in certain situations. Don't worry too much about the gurus- it's not hard to make the termination Good Enough if what you care about is the audio output rather than some sort of theoretical perfection of the digital signal.
If you want to be anal, you can also use a properly terminated input transformer- there could be some benefit in certain situations. Don't worry too much about the gurus- it's not hard to make the termination Good Enough if what you care about is the audio output rather than some sort of theoretical perfection of the digital signal.
Most PCB's are not controlled impedance builds, and very few designs on this site warrant a controlled impedance build. I regularly have controlled impedance PCB's made, the controlled impedance traces though are the really high speed parts of the layout, DDR2/3 ram, gigabit Ethernet etc. For SPDIF, I2S, I2C and the rest I wouldn't worry to much about.
The lengths of the PCB traces are minimal compared to the length of the cable (and depending on the PCB build, will be in the 110-65 ohm range) and slapping a 75ohm resistor to ground is common practice and works. This is how the CATV (cable TV network ) is terminated, and those cables are higher bandwidth and longer than any SPDIF cable and they work fine.
More important is keeping the SPDIF signal and return path together, ie don't terminate the ground/shield of the cable at some star point, as SPDIF is digital they need to be closely coupled. If you are worried about ground noise then use a transformer and isolate the SPDIF signal as it enters the receiver.
Actually controlling the impedance of PCB traces is a pain, and the cost of the boards goes up quite dramatically. When you do have a controlled board for digital designs you are looking at usually 100ohm for diff pairs and 50 ohm for single ended, and as said we only look at them if we have to, and then only when we are getting clock speeds in the high MHz (ie 100MHz plus) so I wouldn't worry about the SPDIF interface. I have found that to corrupt digital signals to the point where they cannot be received correctly takes a lot of effort, at the end of the day the whole point of digital transmission is to be tolerant of a whole host of nasties such as noise, etc.
The lengths of the PCB traces are minimal compared to the length of the cable (and depending on the PCB build, will be in the 110-65 ohm range) and slapping a 75ohm resistor to ground is common practice and works. This is how the CATV (cable TV network ) is terminated, and those cables are higher bandwidth and longer than any SPDIF cable and they work fine.
More important is keeping the SPDIF signal and return path together, ie don't terminate the ground/shield of the cable at some star point, as SPDIF is digital they need to be closely coupled. If you are worried about ground noise then use a transformer and isolate the SPDIF signal as it enters the receiver.
Actually controlling the impedance of PCB traces is a pain, and the cost of the boards goes up quite dramatically. When you do have a controlled board for digital designs you are looking at usually 100ohm for diff pairs and 50 ohm for single ended, and as said we only look at them if we have to, and then only when we are getting clock speeds in the high MHz (ie 100MHz plus) so I wouldn't worry about the SPDIF interface. I have found that to corrupt digital signals to the point where they cannot be received correctly takes a lot of effort, at the end of the day the whole point of digital transmission is to be tolerant of a whole host of nasties such as noise, etc.
Correct?
SY and marce, Thanks!
The board has just two small capacitors before + and – input soldering pads, there is no resister or transformer of any kind.
SY, this is what I understand from your suggestion. I should solder a 75ohm resister across the + and – soldering pads and also solder the coaxial cable on the same pads.
marce, I appreciate your help and understand your point about digital. However, I was talking about the posts that refer to real phenomena of increased clock jitter even when data remains intact if there are many reflections due to Impedance mismatches or termination problems.
SY and marce, Thanks!
The board has just two small capacitors before + and – input soldering pads, there is no resister or transformer of any kind.
SY, this is what I understand from your suggestion. I should solder a 75ohm resister across the + and – soldering pads and also solder the coaxial cable on the same pads.
marce, I appreciate your help and understand your point about digital. However, I was talking about the posts that refer to real phenomena of increased clock jitter even when data remains intact if there are many reflections due to Impedance mismatches or termination problems.
what like taking a probably perfectly good crystal off a board with a short trace, and replacing it with an add on board, with a super dooper crystal, then add a twisted pair wire link a yard long and a different impedance, that will reduce jitter.
Only joking (slightly), I use SIV (signal Integrity Verification) and High Speed routing software (allows us to put constraints on signals such as allowable skew etc) software as part of my CAD system (full blown Cadstar), all clocks on a board are checked using the SIV software and a termination scheme chosen that works. This software takes into account the topography of the PCB as well as the drive and receiver characteristics (IBIS format). I can also model connectors and cables so we can simulate a whole system set up.
I actually worry more about a signal being monotonic than most of the other artifacts you see on digital waveforms, such as ringing etc, but even then we are talking about digital signals in what most would call the RF range. Keep things simple, make all clock lines as SHORT as possible, have a contiguous closely coupled ground plane (or co-ax/twisted pair cables), avoid excessive rise times in relation to base frequency and for most digital design you will see in the DIY audio world it will work; In fact quite often it takes real audiophile beliefs and determination on digital signalling and layout
Where it says SPDIF input on your PCB, put the DMM across the terminals on a resistance setting (no power to the board), you will probably find that it reads 75ohms.
Have Fun
Last edited:
Thanks infinia. It is no-name kit that I bought from eBay and trust me there is no resistor before receiver.
Thanks marce!
The things your wrote just went over my head
, I am a beginner. However last lines did make sense to me.This app note from Ti should explain the basics of digital signal propagation, without getting to deep!
Even though it states high speed design, the rules are applicable for ALL digital, if you follow best practice you will get the best results, and with a lot of today's devices having fast rise times it is becoming a necessity.
If you want any more info or a simpler explanation of any data let us know, it was one of the simpler texts I could find, and gives a good overview.
http://www.ti.com/lit/an/scaa082/scaa082.pdf
Monotonicity, is a mathematical function, but when used in conjunction with digital signals it refers to a change in direction on the rising or falling edge of a signal. (Figure 2 in the link below shows a picture of this). Having a monotonic rising or falling edge is critical to digital signals especially on clocks; as it is on these edges that the logic switches and the non-monotonic edge can cause false switching. A big part of signal integrity and digital PCB design is ensuring that the rising and falling edges are clean, this is more critical than other artefacts' such as ringing.
Design for Signal Integrity: Unit 02 Integrity of Digital Signals
Even though it states high speed design, the rules are applicable for ALL digital, if you follow best practice you will get the best results, and with a lot of today's devices having fast rise times it is becoming a necessity.
If you want any more info or a simpler explanation of any data let us know, it was one of the simpler texts I could find, and gives a good overview.
http://www.ti.com/lit/an/scaa082/scaa082.pdf
Monotonicity, is a mathematical function, but when used in conjunction with digital signals it refers to a change in direction on the rising or falling edge of a signal. (Figure 2 in the link below shows a picture of this). Having a monotonic rising or falling edge is critical to digital signals especially on clocks; as it is on these edges that the logic switches and the non-monotonic edge can cause false switching. A big part of signal integrity and digital PCB design is ensuring that the rising and falling edges are clean, this is more critical than other artefacts' such as ringing.
Design for Signal Integrity: Unit 02 Integrity of Digital Signals
hi new
Look up the part number for the clock recovery device. In the data sheet or other app notes, you will see typical methods on input data circuits, choose what is close to your PCB or mod it to adapt to better schemes like pulse transformers and such. use approved parts from their source list.
PS Is there a schematic for the PCB or what>you want help but you not have given any information on the kit you have. If it's not a turnkey kit and there is no instruction provided then what did you expect to happen.
Look up the part number for the clock recovery device. In the data sheet or other app notes, you will see typical methods on input data circuits, choose what is close to your PCB or mod it to adapt to better schemes like pulse transformers and such. use approved parts from their source list.
PS Is there a schematic for the PCB or what>you want help but you not have given any information on the kit you have. If it's not a turnkey kit and there is no instruction provided then what did you expect to happen.
Last edited:
Thanks marce!
infinia, the receiver is CS8414. Ya! It was turnkey type kit, my first one. However, I'll be starting one from scratch once I finalize which one to use.
The purpose of the post was to establish practical methodology for S/PDIF termination independent of what the underlying kits are.
infinia, the receiver is CS8414. Ya! It was turnkey type kit, my first one. However, I'll be starting one from scratch once I finalize which one to use.
The purpose of the post was to establish practical methodology for S/PDIF termination independent of what the underlying kits are.
marce thanks for the informative posts, while i have you here and i believe its within topic, i have read many times that a longer length (like 6-7') of properly terminated coax for spdif is better than a really short cable wrt reflections, is there any truth to this, simply a bandaid solution for improperly terminated lines, or just mumbo jumbo?
I have taken to using w.fl cables and smd bnc wherever possible and use usb->i2s in preference to spdif these days, but i still have an AES on lemo, 75r bnc with belden coax and a glass optical to round out the inputs on my dac, so i'm interested in your comment all the same, as i like your thorough but no nonsense approach backed with deep understanding.
I had to laugh at your external clock 'upgrade' post hehe, i find this a little humorous also
I have taken to using w.fl cables and smd bnc wherever possible and use usb->i2s in preference to spdif these days, but i still have an AES on lemo, 75r bnc with belden coax and a glass optical to round out the inputs on my dac, so i'm interested in your comment all the same, as i like your thorough but no nonsense approach backed with deep understanding.
I had to laugh at your external clock 'upgrade' post hehe, i find this a little humorous also
Last edited:
Qusp
Before I answer I am going to prepare some simulations to illustrate my answer, as I also saw this thread. I believe there are some interesting Audiophile beliefs building up regarding digital signal transmission, that I have found rather interesting.
Again I am collating some links and references as well to go with my answer, as I want it to be as compherehensive as possible, as I quite often get shot down in flames for some of my views, also the simulations will show the effects of cable length and terminations.
I believe this may be from the thread regarding SPDIF attenuators (!).
What I will say initialy is that we strive to minimise the length of any digital interconnect and select termination schemes by using our simulation software,
empiral data and engineering practices, and measurement
Before I answer I am going to prepare some simulations to illustrate my answer, as I also saw this thread. I believe there are some interesting Audiophile beliefs building up regarding digital signal transmission, that I have found rather interesting.
Again I am collating some links and references as well to go with my answer, as I want it to be as compherehensive as possible, as I quite often get shot down in flames for some of my views, also the simulations will show the effects of cable length and terminations.
I believe this may be from the thread regarding SPDIF attenuators (!).
What I will say initialy is that we strive to minimise the length of any digital interconnect and select termination schemes by using our simulation software,
empiral data and engineering practices, and measurement
hehe, no problem marce, yeah it was mentioned there in that thread, but i have seen it elsewhere here and on other sites too. it never made a lot of sense to me, but like all good audiophile myths there is usually just enough truth in there to make it interesting and gain momentum (though the context maybe often incorrect)
i look forward to your post.
i look forward to your post.
I had a look round and found some basic simulations I had set up around the time of some interesting (read humorous) threads regarding USB to SPDIF etc. I set up two basic co-ax lines with a driver and receiver, both 75r, one 50mm long another 200mm long, both un-terminated and terminated with a simple far end termination of 75r parallel down to 0V reference (shows how effective this simple termination is). The topography for the simulations are shown in co-ax#1.pdf and co-ax#2-terminated.pdf respectively. The stimulus was a 12MHz 50% duty cycle square wave, the line drivers have a 20-80% rise time of approx 3ns, should be about average for an SPDIF interface. I didn't model the connectors but can do with more time.
The simulation results are shown in the following PDF files:
IC1-IC3 is the 200mm co-ax cable
IC2-IC4 is the 50mm co-ax cable
I think the results speak for themselves, shorter is always better, and choosing the correct method of termination is critical and becomes more so as clock frequency and signal rise times increase. As to the accuracy of the simulation software, I use it for all clocks and high speed interfaces such as DDR/Gigabit interfaces and in conjunction with the electrical engineers have checked the results and waveforms using a 13GHz scope. I use the software to both check out my layouts and where we have a problem to simulate different termination options to find the best option.
I do believe that the cable length thread was getting into myth territory.
The best source of information for termination is Howard Johnson, High speed digital design, chapter 6.
When I do get more time, and training I want to do more audio based work with the software as I can also do analogue (similar to pspice, but it will take into account both the physical PCB layout and cables) and look closer at some of the digital ideas floating about. Hopefully we will be able to have some fun with the software in future. I also did simulations of the Hi-face vs cheap Chinese adaptor, that was around at the same time, and found that the very fast rise time of the hi-face (1ns) was causing the reflection problems, whereas the more benign rise time of the cheaper module (30ns) made it more tolerant of badly terminated interfaces.
The simulation results are shown in the following PDF files:
IC1-IC3 is the 200mm co-ax cable
IC2-IC4 is the 50mm co-ax cable
I think the results speak for themselves, shorter is always better, and choosing the correct method of termination is critical and becomes more so as clock frequency and signal rise times increase. As to the accuracy of the simulation software, I use it for all clocks and high speed interfaces such as DDR/Gigabit interfaces and in conjunction with the electrical engineers have checked the results and waveforms using a 13GHz scope. I use the software to both check out my layouts and where we have a problem to simulate different termination options to find the best option.
I do believe that the cable length thread was getting into myth territory.
The best source of information for termination is Howard Johnson, High speed digital design, chapter 6.
When I do get more time, and training I want to do more audio based work with the software as I can also do analogue (similar to pspice, but it will take into account both the physical PCB layout and cables) and look closer at some of the digital ideas floating about. Hopefully we will be able to have some fun with the software in future. I also did simulations of the Hi-face vs cheap Chinese adaptor, that was around at the same time, and found that the very fast rise time of the hi-face (1ns) was causing the reflection problems, whereas the more benign rise time of the cheaper module (30ns) made it more tolerant of badly terminated interfaces.
Cable length increased to 1500mm one has simple termiantion 75r to 0V, and one has no termination.
The termination can be further tweaked by adding a capictor to ac terminate the line, this would reduce the attenuation of the signal level. that does happen with dc terminated parralel end termination.
Forgot to add, the signal and reflections will cover a 1m cable in approx 5ns, so in 10ns your first reflection is back at the transmitter, but will already cause peaks and dips after 5ns. The 50mm cable the signal will do in approx 0.25ns, the 200m in 1ns, based on the rough velocity factor of co-ax cable being around 0.66C.
The termination can be further tweaked by adding a capictor to ac terminate the line, this would reduce the attenuation of the signal level. that does happen with dc terminated parralel end termination.
Forgot to add, the signal and reflections will cover a 1m cable in approx 5ns, so in 10ns your first reflection is back at the transmitter, but will already cause peaks and dips after 5ns. The 50mm cable the signal will do in approx 0.25ns, the 200m in 1ns, based on the rough velocity factor of co-ax cable being around 0.66C.
Last edited:
some good signal integrity sites:
beTheSignal.com
Welcome
Publication Index by Title - Dr. Howard Johnson
Speeding Edge consultants specialize in high-speed PCB and system design disciplines
"Right The First Time" Lee W Ritchey a free download, worth the effort.
Right the First Time
And the SI Bible:
"high Speed Digital Design" Howard Johnson
and a couple more links.
http://www.sabritec.com/technotes/PDF/High_Speed_Digital_Tutorial.pdf
Design for Signal Integrity: Unit 02 Integrity of Digital Signals
Data Communications Basics
beTheSignal.com
Welcome
Publication Index by Title - Dr. Howard Johnson
Speeding Edge consultants specialize in high-speed PCB and system design disciplines
"Right The First Time" Lee W Ritchey a free download, worth the effort.
Right the First Time
And the SI Bible:
"high Speed Digital Design" Howard Johnson
and a couple more links.
http://www.sabritec.com/technotes/PDF/High_Speed_Digital_Tutorial.pdf
Design for Signal Integrity: Unit 02 Integrity of Digital Signals
Data Communications Basics
wow, thanks mate, thats a great resource that i'm going to have to pick over thoroughly when i have a more functional brain, its 5.45am here after a long day. yeah the hiface spdif version had a fair few issues, the oem usb-i2s module isnt quite as bad, but i'm not using either anymore. thanks a bunch! 
in my own experiments proper line termination (and yes a basic filter too) and the w.fl, or u.fl interconnects has given best results, so yours dont surprise me. perhaps this could even be made a sticky with a few more contributions
in my own experiments proper line termination (and yes a basic filter too) and the w.fl, or u.fl interconnects has given best results, so yours dont surprise me. perhaps this could even be made a sticky with a few more contributions- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.