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In Reply to: RE: A very technical curiosity - how to assess the quality of a spdif stream ? posted by beppe61 on May 18, 2015 at 06:15:26
1. Loop the output back or send it to another sound card. Run for a long time and make sure that all the bits received are the ones sent. This can be done by recording the SPDIF output and comparing the source and destination files.
2. Use a scope and look at the eye-pattern. This will show the presence of noise and reflections on the signal as well as gross jitter. This will also show the signal levels.
3. Measure the clock rate with a frequency counter.
If the signal passes these tests then it's good. Whether the resulting sound is good or not will depend on the DAC that's connected to the SPDIF source.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Follow Ups:
"Use a scope and look at the eye-pattern. This will show the presence of noise and reflections on the signal as well as gross jitter. This will also show the signal levels."
Tony, I can see how to put the SPDIF bit stream on one scope axis, but how do you connect data rate to the other one? Can you refer me to a "how to" paper or similar source on the web? BTW, I have a pretty basic 30 Meg two channel single beam analog scope, and I would like to play around with the eye-pattern measurement. I looked at the HP article that you referenced, and their method seems to involve a lot of ancillary instrumentation. Thanks in advance.
You can trigger the scope on the data or various other ways, depending on your equipment.
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Hi and thanks a lot for the very valuable explanation.
So it can be done and with reliable results.
It was mostly a curiosity because i read many reviews about transport-dac matching.
However i have also to say that when a transport unit is good it is good with many dacs.
So to me lab testing of different transport solutions could be very interesting.
Then of course just a very good spdif signal is not enough.
Let's say that it would be a good start ?
Because i also remember matching of exceptional transports with just decent dac with a resulting sound that was quite something.
Thanks a lot again.
Kind regards,
bg
Yes, if a transport is good it will be good with many DACs. By the same token, if a DAC is good it will be good with many transports. However, it's best to evaluate the entire playback chain as a system. That's what needs to be good. :-)
Tony Lauck
"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar
Hi and thanks a lot and i agree completely of course.
But what makes me really nervous is how digital signal are transmitted from a unit (transport for instance) to another (dac).
I am a complete ignorant but the fact that reliable and effective standards for this duty are available and still not used makes me very angry.
For instance. Why do not use the AES/EBU that from what i understand is a decent standard everywhere ?
A transport usually is a mid to high price unit.
It is not a cheap product.
It is asked to send out a good quality digital signal.
it should deserve a AES/EBU or at leas a bnc connection.
This would make the transfer better in terms of quality.
And the same for the receiving dac.
I would like to see this AES in dac from 4-500 USD up.
Only one very good standard.
Instead i read of RCA not being impedance matched (?), toslink with jitter issue ... this is pure perversion.
In absence of good alternative i could understand but the solution is already here just to be caught.
Better S/N ratio, noise rejection ... i am inventing things here.
Then i also discovered that a trivial ethernet cable is much better for video signals than an expensive HDMI and i understand Shakspeare ... All the world's a stage ...
Thanks again.P.S. just to say ... same video card same display ... the hdmi out compared to DVI is pure shit ... just to say
Then i see video processors of tenth of thousand of dollars with hdmi, clearly to me a bad standard from various point of view.
It is clear that there is something really bad going on
It is the worst who wins
Kind regards,
bg
Edits: 05/20/15 05/20/15 05/20/15 05/20/15 05/20/15
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