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Old 12th September 2009, 08:23 PM   #1
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Default S/PDIF vs. AES/EBU

I'm building my first TDA1543 dac and I wonder if it should be better to use AES/EBU transmission between transport and dac. If I understand I could use 110 ohm CAT5 twisted pair instead of 75 ohm coaxial cable just replacing 75 ohm resistor on receiver side with a 110 ohm resistor.


From Wikipedia:
"The digital audio standard frequently called AES/EBU, officially known as AES3, is used for carrying digital audio signals between various devices. (...) A related system, S/PDIF, was developed essentially as a consumer version of AES/EBU, using connectors more commonly found in the consumer market. (...) The AES3 standard parallels part 4 of the international standard IEC 60958. Of the physical interconnection types defined by IEC 60958, three are in common use:

* IEC 60958 Type I Balanced 3-conductor, 110-ohm twisted pair cabling with an XLR connector, used in professional installations (AES3 standard)
* IEC 60958 Type II Unbalanced 2-conductor, 75-ohm coaxial cable with an RCA connector, used in consumer audio
* IEC 60958 Type II Optical optical fiber, usually plastic but occasionally glass, with an F05 connector, also used in consumer audio
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Old 12th September 2009, 10:41 PM   #2
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AES/EBU is only really going to show its advantages if you're sending it over long distances, via cables that aren't optimised for digital audio, and are susceptible to noise pickup.
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Old 14th September 2009, 08:32 AM   #3
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Okay, thank you. So I'll have:
cdplayer digital out (RCA connector) --> digital cable 75ohm --> DAC input connector (BNC 75ohm)
But how to connect BNC to CS8414 (inside the DAC)? Do I need a piece of 75 ohm coaxial or could I use a simple copper wire?
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Old 14th September 2009, 02:06 PM   #4
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Quote:
Originally Posted by Calamaro View Post
Okay, thank you. So I'll have:
cdplayer digital out (RCA connector) --> digital cable 75ohm --> DAC input connector (BNC 75ohm)
But how to connect BNC to CS8414 (inside the DAC)? Do I need a piece of 75 ohm coaxial or could I use a simple copper wire?
If your cable is short - say a few tens of cm, ordinary co-axial cable - audio line level, or microphone screened cable, will work fine. The frequencies used for SPDIF aren't high enough that characteristing impedance of cable is important. You may need terminating resistors though, because the line driver of the source wants to 'see' 75R.
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Old 14th September 2009, 02:29 PM   #5
AndrewT is offline AndrewT  Scotland
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does the line driver see 75ohms or 150ohms? The receive end of the cable is terminated with 75ohms to ground and the transmit end is fed from a 75ohm source impedance. The driver sees this as a total of 150ohms.

If the output of the DAC is balanced, then I would preserve the balanced signal as long as possible.
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Old 14th September 2009, 03:40 PM   #6
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does the line driver see 75ohms or 150ohms? The receive end of the cable is terminated with 75ohms to ground and the transmit end is fed from a 75ohm source impedance. The driver sees this as a total of 150ohms.
Your description sounds right, although the 'driver' has an inherent source impedance of 75R from the sound of it, in which case it is correct to terminate it with 75R at the 'far end'.
So, yes, ultimately if you consider the source as an ideal Thevenin source with a series resistor of 75R, then the ideal source 'sees' 150R in total.

Quote:
If the output of the DAC is balanced, then I would preserve the balanced signal as long as possible.
The OUTPUT of the DAC is a completely different signal and system to the SPDIF part - I'm not sure if your two queries are linked in thought.
The balanced analogue link is most useful if your preamp has a balanced line in. Otherwise you will need a balun somewhere, and it is best put as close to the pre-amp input as possible (an audio grade transformer here would work well). Balanced lines are useful if you have electrically noisy environments or long cables; otherwise the added conversion of bal-to-unbal may degrade the sound more than just using the unbal out on the DAC.
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Old 14th September 2009, 03:49 PM   #7
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Quote:
Originally Posted by Steerpike View Post
You may need terminating resistors though, because the line driver of the source wants to 'see' 75R.
I have a parallel 75 ohm resistor + series 10nF capacitors just before CS8414. Between input BNC connector and CS8414 I need just 10 cm wire so any coaxial will works (also a 50 ohms one?).

Please let me ask another question:
do digital interconnect quality affects sound quality in the same way this happens with analogue interconnects?
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Old 14th September 2009, 04:01 PM   #8
Pano is offline Pano  United States
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Not in the same way, no. But they sure can effect it! Since the receiver chip has to derive the clock from the spdif or AES signal, bad cables can make a mess of it. I keep a few around here just for demo purposes. Both coax and optical.

The most expensive cables are not always the best, either. Radio Shack actually sells some decent ones.
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Old 14th September 2009, 04:07 PM   #9
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Originally Posted by panomaniac View Post
The most expensive cables are not always the best, either.
So how select the right one? Electrical properties? Conductor and/or shield material? ...?
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Old 15th September 2009, 10:57 PM   #10
dangus is offline dangus  Canada
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Originally Posted by Calamaro View Post
Please let me ask another question:
do digital interconnect quality affects sound quality in the same way this happens with analogue interconnects?
If you believe the crazy people, digital interconnects affect sound quality just as much as power cords and magic rocks and crystals do. What I wonder is, how come these marvelously subtle influences are almost always positive. How long before some crazed audiophile begins hanging human ears around the room to improve the listening experience or electrocutes himself putting Cramolin on the pole transformer on the street. I also wonder how I can cash in on these wonderfully gullible people.

Anyway...

If you're DIYing the whole chain, consider using something like I2S which includes a separate line for clock as well as data. But if the DAC uses an ASRC or something else that removes jitter, that's probably overkill.
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