Jitter : Digital Patchbays, good or bad?

Robert Katz Leave a Comment

From: Kim

Ok I have been told that i can use a regular patchbay for my digtal equipment. I question the valitity of this request.(I’m talking about AES/EBU, Spdif). I would like to know if you have any thoughts on this.

I guess theoreticly it should work but what I’m seeing out in the world are a lot of active digital patchbays. So I guess what I like to know is this, have any of you built and used just regular patch bays for digital audio interconnects. Now I’m looking at just using a adc patch bay with a punchblock on the backside.

Will I have any signal disscrepencies ie ohm problems, the possiblity of blowing dacs ect…

Thanks kim

Dear Kim:

You won’t blow any DACs, but every impedance bump in the chain adds a degradation which will make your signal connections less reliable, and certainly add jitter, which adds distortion to the monitoring.

The punch block, even the XLR connectors (which are not truly 110 ohms), every point where one wire is connected to another…is a serious problem…these are all “impedance bumps”. The jacks are also not true 110 ohm. You may find your wire length only good to a few feet before you get crackles, pops, or no signal at all.

I advise against using standard “analog” patchbays for digital, unless you convert to 75 ohm technology and use professional (video level) patch bays, or a certified impedance matched active patchbay. The same is true of XLR “patchbays” for AES/EBU. Every XLR plug in the middle degrades the integrity of the connection. XLRs were not designed to be digital connectors. This was perhaps the biggest mistake made by the AES. It helped us to get into digital audio in the beginning, but it’s making lots of trouble now. I support the AES-3ID standard, which puts digital audio on 75 ohm coax with BNC connectors. Another valid technology is properly twisted pair CAT-5 cable, with RJ-45 connectors, running 10 base-T or 100-base T Ethernet. These can also be made with patchbays and cable runs that pass digital audio with high integrity.

I hope this helps!

Bob Katz

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