Tuesday, May 23, 2017

Project Drift

     This was at work -- I expect any project at home to create three or four more.  Work, that's not supposed to happen--

     It seemed simple enough: deliver audio and video signals from a specific source to a nice computer-based recorder, which holds thousands of hours in the space occupied by a large desktop PC.  High-definition video and, as it turns out, stereo sound.

     The video is simple enough: find the multi-output distribution amplifier (DA) on the one-line diagram -- aha! -- and then go find it in the real world.  H'mm, more outputs are hooked up than the drawing shows, but one of them feeds another DA and...yes!  There is exactly one output left!

     Now the audio.  Easy, right?  No.  It's only available in 5.1 channel surround, your choice of discrete digital (three coaxial cables) or encoded (one cable but it takes dedicated hardware or software to decode).  Email the department that needs this, which would they like?  More back and forth to define terms.  (You can't just grab the Left Front and Right Front channels: true 5.1 has most of the dialog in the center channel.)  Nope.  How about summed mono?  We've got that!  After multiple e-mails, that gets the big nope.  Surely there's a digital stereo version around?  (One coax, easy as pi!)  Ummm...no.  But there's analog, and here's an analog-to-digital converter with available channels.  ...I/O not on the cross-connect one-line?  ...Kind of not.  Fine, now where are the audio DA outputs?  Dig to find the three different names three different techs have given it, then find it is not on the one-line diagram under any name but there is a Road Map To The Punchblocks and there, after looking and looking, right there is the name.

     "Punchblocks" are an old form of telephone interconnect, and what we use are "66M" blocks, with fifty pairs of odd little forked terminals in two columns of 25; you use a special tool to force a wire down into them (and trim it short in one step -- or cut it clean off, if you turned the tool the wrong way, darn it) and it makes a good connection.  Later versions -- Krone and 110 blocks, ADC "split-cylinder" terminals and suchlike -- don't much care what size or kind of wire they eat, but 66 blocks came first and they like small solid wire, 24 to 26 gauge.  Our spool of the right stuff is, of course, missing.  It took 45 minutes to locate.  

     Punched down analog audio wiring.  Went to find the audio A-to-D/D-to-A testbox (very nifty, has indicator lights and a little speaker).  Not in its spot on the shelf.  Kept looking for digital audio testbox, found it on the bench after looking, fixing something unrelated, and looking more. Fished through a rat's nest of rack wiring to plug it into the digital output of the A-to-D, and...nothing.  Not a sound.  No little light to tell me the data rate, either, and that's funny, because if the A-to-D is even working, it would be on...  Went around to front of racks.  One A-to-D is all lit up, bargraph level meters fluttering, state indicators indicating data rate and meter mode and all of that good stuff.  The other one, the one on which I have been working?  No lights.  Not happy.  Back to the other side, grope to power input on back of device, moving fat bundle of audio wiring to get to it--  IEC plug falls out of connector: some clever neatnik has tie-wrapped the power cord to the audio wiring, with less-than-sufficient cable to keep the power cord connected!  Cut tie wrap, plug back in, and, hey la, the audio testbox begins shouting in my ear!

     I wired up digital audio to its own distribution amplifier (because I'm not going to get caught like this again), and prepared to run cables over to the recorder-thing itself, about twenty feet down the row of racks and on the far side of a control console.  ...There's no small coax in the usual spot.  Hunter around and found orange (digital audio) coax and grabbed a spool of violet (digital video) as well, plus the portable rack the holds them for easy unspooling.  Opened up the computer floor and went to get the nice, screw-together glow-in-the-dark fiberglass wire-pulling sticks...which were missing.  Found them (after half an hour searching), opened floor, poked the sticks through until I hit a Known Obstacle, opened the floor at the square AC-wiring duct and helped the stick over, went back to snaking the fish-stick along, added sections and....clunk.  Clunk?  But....there's nothing else in the way.  But there must be, so I got the electric screwdriver, opened the floor at a new and odd place, and found electricians had visited, leaving a big knot of flexible conduit the stick needs help over.  Got it past that and on to the destination, taped on the two coaxes, pulled them through, disassembled and put away sticks, and went to cut the cables at the start end.

     Glanced at my watch, realized it's a half-hour past quitting time and I still had to put my toys away before I could go home.  Which took another half-hour.

     You know the first or second question I'll be asked at work today?  "So...you get that recorder connected?"

     No.  No, I didn't.  Hand me that box of BNC connectors for 1855A, and I'll finish up that little job -- oh, and I'll need a 75-to-110 Ohm AES/EBU audio balun, and a short XLR to 1/4" TRS 110-Ohm cable, too.  What, we don't have any?

     This may take a little longer.

8 comments:

D.W. Drang said...

When I clicked on the post title I was debating with myself whether it was going to be another story from The Hidden Frontier, or a Tale of a project expanding to fill the available time and money budget, and maybe some that wasn't available...

Roberta X, remotely said...

Bit of both.

Alien said...

"....true 5.1 has most of the dialog in the center channel."

Huh. I did not know that. I guess I should try adding a center channel speaker.

Anonymous said...

Accumulated dirt can become conductive over the course of years can cause crosstalk 'issues' on adjacent terminals on punchblocks. An elderly ship like the starship may see that issue pop up unexpectedly.

I discovered this the hard way when my computer modem on dial-up started dumping my connection mysteriously. Moving the connection a few rows over cured that problem. Having the tools on hand from doing #5 crossbar switchgear work years back helped mightily.

I've heard that wire-wrap punchblock electrical connections are about the best there are in terms of *solid* electric connections, something about the sharp edges of the square pins digging into the soft copper Bell wire that's wrapped under tension. Can you confirm or deny that rumor?

(Bell labs were damn good at making bullet-proof telcom electrics...)

The Neon Madman said...

It's not just a job, it's an adventure!

Been there.

rickn8or said...

"...some clever neatnik has tie-wrapped the power cord to the audio wiring..."

Which causes you to invent colorful new words when you discover this is the source of 400 Hz hum on audio lines going to recorders.

But "Project Drift" I understand. Now add a two-way 200-yard walk across the ramp at each divergence from The Plan.

Old NFO said...

Ah yes... BTDT, at least you're in the SAME building... Had to cross connect two Avaya cabinets in separate buildings with fiber, only problem was there wasn't any spares... Two hour job ended up taking five days... sigh... At least you've got the cables in, MUCH better than we did! LOL

Roberta X said...

:)