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Old 25th September 2002, 03:00 AM   #11
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This is an interesting post. Would you mind sharing the details of your PCBs (circuit layout & schematics)?
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Old 25th September 2002, 03:02 AM   #12
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Sorry, my post was for jwb. I'm interested in the LVDS stuff.
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Old 25th September 2002, 07:36 AM   #13
hifiZen is offline hifiZen  Canada
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Default "standard" for DIY digital interconnect?

Lookin good... I was going to use LVDS with a different connector for exactly this same purpose, but jwb, I think you've chosen a better one... Should we create our own little "standard" for high grade DIY digital interconnect?

Then we could exchange digital module designs, PCB layouts etc based on this scheme...

If there's enough interest, we should add this to the wiki. I'd be happy to collect the info and do the first draft write-up.
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Old 25th September 2002, 12:08 PM   #14
artnyos is offline artnyos  United States
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Default Should we create our own little "standard" for high grade DIY digital interconnect?

There is already a standard, SPDIF. This interface can be made to sound excellent if you know what the hell you are doing. The issues of impedence matching, pulse transformers, cable length, connector reflections, common mode noise rejection, optimum rise times, and hysterisis in receivers will not dissappear by jumping to LVDS. You think impedance matching for coax is hard, wait till you match twisted pair which has a differential AND common mode characteristic impedance.I think you guys had better understand the limitations and strengths of SPDIF before you throw even more complicated "solutions" at the problem.

Like Mark Twain said:

"Everyone complains about the weather but nobody does anything about it."

Sorry to rain on your parade.

Art
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Old 25th September 2002, 12:15 PM   #15
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Default There is one small thing you neglected

While all that may be true, what do you do about jitter in the reciever?

Yeah, yeah....I can hear it now......multiple PLLs with VCXOs.

Jocko
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Old 25th September 2002, 12:24 PM   #16
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Default Then we could exchange digital module designs, PCB layouts etc based on this scheme..

Hey... we could put on a show right here in the barn!
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Old 25th September 2002, 12:54 PM   #17
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Lightbulb Re: There is one small thing you neglected

Quote:
Originally posted by Jocko Homo
While all that may be true, what do you do about jitter in the reciever?

Yeah, yeah....I can hear it now......multiple PLLs with VCXOs.

Jocko
Hi Jocko,
No, no, just a 100MHz Asynchronous Reclocker before the DAC.
Does wonders to the sound; "metastability" problem solved!
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Old 26th September 2002, 03:42 AM   #18
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artnyos:
I agree with you that S/PDIF can be made to work well, especially if it is reclocked on the receiving end.

Yes, proper design guidelines must be followed for LVDS, but I don't consider these insurmountable.
(For those interested,
LVDS Design Guide
is an excellent source of information.)

My main motivation for an alternate interface is for a multichannel audio: direct digital from a DVD-A, DTS, or SACD source. S/PDIF doesn't support this without resorting to some form of compression.

1394 or USB could be used as an interface, but the overhead for these is much more painful than a proper interface that transfers discrete signals in real-time, rather than in packets.

As far as options for getting audio clock and data signals from one box to another, I've considered 422, PECL, LVDS, and even plain single-ended TTL (for a very short distance).

My conclusion was the same as jwb, that LVDS offered the best combination of performance and ease of implementation.

I like hifiZen's idea of trying to come up with a standard for this.
There could be some degree of flexibility for variations in the clocking and data formats.

I'll have to think a bit more about MDR vs RJ-45 modular connectors. MDRs are a more expensive solution, but they do offer more pairs. In order to limit the number of pairs needed for multichannel, I was considering an EPLD to switch between different data sources in the disk player (2 channel IIS, 6 channel IIS, 2 channel DSD, 6 channel DSD) and then pack the selected source together in a TDM type stream. The use of an MDR connector would possibly allow the data lines to remain separated, but this would also require at least one additional quad LVDS transmitter/receiver pair (almost the same cost as the EPLDs).

Anyway, I'd appreciate feedback.

Brian.
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Old 26th September 2002, 04:50 AM   #19
jwb is offline jwb  United States
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I like your ideas Brian. I'll try to post my schematics and layouts here when I get some time. This weekend, I suspect.

I don't think RJ-45 termination is going to work for a standard. As you correctly point out, that only gives you 4 pairs. You'd need five pairs for 6-channel audio and two clocks. Actually, if you know the word length, you can derive the word clock from the bit clock, so I guess four pairs would work. I'll have to think about it.

For that matter, you could multiplex more than two channels on one serial stream, through obvious methods. You would need more logic, though.

Another disadvantage of twisted pair over twinax is distance. You could easily run twinax across or between rooms. I don't know why that would be desirable, but there you go.
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Old 26th September 2002, 05:25 AM   #20
hifiZen is offline hifiZen  Canada
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Quote:
Hey... we could put on a show right here in the barn!
Say, is that you on the left there, Ren? You must be wearing your best duds for the hoe-down! I hope you call a good square dance. See you there...
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