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Old 26th September 2002, 06:05 AM   #21
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Brian,
If you choose to serialize and deserialize multichannel audio I am sure at least some of the benefits of doing away with the AES rx will be lost. Given that even in the extreme you are unlikely to have more than 8 individual channels, 16 data pairs does not seem unmanagable and as you intend to use a FPGA, even the low-end devices have LVDS I/O.

ray.
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Old 26th September 2002, 06:22 AM   #22
hifiZen is offline hifiZen  Canada
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Quote:
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.
Mine as well. People constantly post about wanting DTS decoders etc, when you can buy an inexpensive DVD player that has DTS decode built right in. MLP decode will become more common soon too, so why not just mod your player so you can tap the I2S busses and pipe that straight out to your external digital DIY stuff?

In my view, MDR is the way to go over RJ-45. When spending a few hundred bucks on a decent digital project, I'm happy to spend an extra few dollars for the superior interconnect. Don't forget that you may need to send other info such as preemphasis status or other flags, user data etc, in addition to the I2S data and clocks. So, there is even more reason to move away from RJ-45. Now, as for all this auxiliary data, perhaps we should specify something like I2C as the standard method of carrying "other" data. The benefit of going with I2C is that it is scalable without affecting the physical specifications of the interface. It can go unused for those who just want the pure audio data. At first I hesitated to add an interface like this in with the audio data lines. But, the impact on jitter performance actually should be basically nil anyway, provided the master clock originates at the DAC end. If the master clock is the source, what then should we do with the auxiliary data? Relegate it to an entirely separate cable?

Anyway, for a bit more inspiration, check out the I2S-e specs put forth by Ultra Analog, and as seen in Sonic Frontiers equipment.
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Old 26th September 2002, 07:18 AM   #23
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Default Connectors

RJ45 connectors are not the best regarding impedance. We've tested some of them at work, both for EMC and signal integrity, and some TDR (yes Jocko) measurements have shown that the pin arrangement leads to nice impedance discontinuities. IMHO, best (quality/price) connectors for our purpose are the type SonicFrontier uses : Sub-D shells with 50R coaxial inserts. They come in several flavours, from 2 to 5 places for coax, plus some room for "normal" pins. The drawback is that you have to make your own cable, but I use some lengthes of miniature 50R coaxs tied together with a heat shrinkable tube, and it works fine.

Just my $0.02
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Old 26th September 2002, 08:48 AM   #24
hifiZen is offline hifiZen  Canada
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Hm. Interesting. I would certainly expect the 50ohm coax connectors to perform best... actually you might do even better with separate SMA or SMB etc. connections for the main clock if you want to be really anal about impedance. The I2S-e connector is indeed very nice, but maybe a little limited for our multi-channel applications?

Did you do any measurements on these MDR connectors by any chance?

Thanks,
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Old 27th September 2002, 08:21 AM   #25
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Sorry, no measurements yet for MDR connectors.

Concening the Sub-D shells, I've attached a picture of various ones along with coax inserts. These ones are made by FCI (Framatome Connectors International), but other suppliers like 3M, Amphenol, Amp, etc are manufacturing them too. Up to 8 insert places are available on the same connector. They're cheaper than the same number of SMA/SMC/SMC/MCX connectors, and far easier to plug . Ok, their bandwidth is smaller (1GHz), but large enough in this application.

Edit : I know that Harting makes some miniature multipoint coax connectors, but they're apparently very hard to source. I'll try to find the web link back.
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Old 27th September 2002, 11:56 AM   #26
hifiZen is offline hifiZen  Canada
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Ooooh, nice! Cheaper than separate coax, but still pretty expen$ive, no?...
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Old 27th September 2002, 12:50 PM   #27
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Hmmmm, looks like someone has been taking an old H-P spectrum analyzer apart. Guess that means that they probably are expensive. And that they do work.

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Old 27th September 2002, 04:43 PM   #28
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Default Re: Connectors

Quote:
Originally posted by ftorres
RJ45 connectors are not the best regarding impedance. We've tested some of them at work, both for EMC and signal integrity, and some TDR (yes Jocko) measurements have shown that the pin arrangement leads to nice impedance discontinuities. IMHO, best (quality/price) connectors for our purpose are the type SonicFrontier uses : Sub-D shells with 50R coaxial inserts. They come in several flavours, from 2 to 5 places for coax, plus some room for "normal" pins. The drawback is that you have to make your own cable, but I use some lengthes of miniature 50R coaxs tied together with a heat shrinkable tube, and it works fine.

Just my $0.02
Did you try CAT5 rated RJ-45 connectors? I believe these have better symmetry in the pin arrangements than regular RJ-45, and are intended to be impedance matched with CAT5 cable.

I realize that most generic RJ-45 jacks split the signals into two rows (one row with longer pins), which screws up the quality of common mode rejection.

I've considered the Sub-D's with the 50R inserts, but the cost has made me leary of these. They're also a bit of a pain to wire. Also, if unbalanced coax is to be the medium, then LVDS isn't the best type of interface. I'd probably go with a 422 driver, possibly with transformers.

I don't want to come across as being too hung up on cost. I'll happily spend money to try out new parts and turn a board to play with them. (NPC wanted $190 for samples of their SM5816AF that I intend on using as part of this interface. This is no problem for me because it was clearly the best option for me to be able to process DSD signals at this time.) I just don't want to commit to a higher cost solution than is necessary, especially if it's to be a 'standard' that I'll have to pay for again and again. Of course I don't want to limit the performance potential either. It's a matter of figuring out the cheapest and easiest way of acheiving the desired goals.

The key word there is 'goals'. It's usually harder to figure out what precisely you want to do, than it is to do it.

Having said that, I think that if we want to try and come with some sort of DIY Audio Digital Interface Standard (DIYADIS?), it would be a good idea to temporarily turn the discussion away from the specifics of implementation, to try and come up with a set of performance and feature requirements, along with an outline of the general architecture.

I'll try to come up with an outline of my thoughts for requirements and topology in a later post.

Brian.
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Old 27th September 2002, 05:50 PM   #29
jwb is offline jwb  United States
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This is the reason I chose the MDR cable. It is a good cable tech, designed for the signalling technology, that you can buy right off the shelf. Standard unshielded twisted pair is right out, because it is unshielded. You will have a much harder time finding shielded network cable, because nobody uses it, and the jacks and plugs are more rare as well.

The only problem I can see with the MDR is it is no longer used for computer monitors, and therefore it might become difficult to find.
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Old 27th September 2002, 09:53 PM   #30
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Default Well, there is two little things you rejected

Art, do you ever have a positive post? I hate to ask that, but everywhere I see your posts, they are always negative.

SPDIF is a low speed interconnect. It was designed for low speed, it is low speed, period. The whole design methodology for SPDIF was a cheap and somewhat robust. Low jitter, if it was a consideration, was certainly not evident.

LVDS was designed for low noise ultra-high speed short range (5-10 meters), links. LVDS parts are speced up to 1 gigabit per second. It is not uncommon to run multiple links in parallel.

Will there be reflections in LVDS? Of course there will. However, LVDS edge speeds are so fast, that the "susceptibility period", i.e. the period during the transition where a reflection could cause a bit flip is so small, that you would need to really screw up the implementation to run into a problem. And unless you really screwed up the termination, the chances of the reflection causing a bit flip once the level is solid, is just about nill. Also keep in mind, that a reasonable level of hysteresis is built into the chips to avoid just such problems.

100 ps (typ) of pk-pk jitter is a spec for some current parts with no PLL to clean it up...
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