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  #1  
Old 02-11-2004, 01:20 PM
Pete Gould Pete Gould is offline
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Default Info about 888 interfaces

We have an old 888 interface (Rev-A) connected to an Avid Symphony edit system. The problem with Rev-A 888's is that they were built before the standard of -20db digital = 0db VU analog was arrived at, and they lack the headroom to be calibrated that way (if you try it you'll end up with analog distortion at peaks). They are, instead, calibrated to -14 = 0VU, which is out of place with the rest of our infrastructure.

At the moment there are a lot of 888's out there on the secondary market -- one we have access to through another company is a Rev-H 888. We're wondering how to determine (other than by buying and installing it) if the Rev-H box was redesigned so it can be configured to -20 -- certainly it has undergone some other evident improvements.

The other thing we're wondering is if the 888|24 is plug compatible with the 888 Rev-A, in which case we'd get one of those. Thoughts?
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Old 02-11-2004, 03:32 PM
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TVPostSound TVPostSound is offline
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Default Re: Info about 888 interfaces

Might the interface be set for -10 instead of +4 internally???
I have not seen an 888 in the past 10 years that didnt do -20 ?????
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Old 02-11-2004, 05:29 PM
Pete Gould Pete Gould is offline
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Default Re: Info about 888 interfaces

Quote:
Might the interface be set for -10 instead of +4 internally???
I have not seen an 888 in the past 10 years that didnt do -20 ?????
Pretty much what I had thought originally -- but after opening it up, no, it's jumpered for +4 (in and out). The information that it won't do -20 came from Digidesign tech support during a support call.
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Old 02-11-2004, 06:01 PM
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Default Re: Info about 888 interfaces

I didnt know they had -14 888s

What I can tell you is that I have a REV C 888/16 stiing directly below an 888/24 attached to a MIX Cubed system on a Y cable, both are calibrated to -20dBFS=0Dbu @ +4 operating level.
I could almost swear that the 888/16's analogs sound better than the 888/24's!!!!!
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Old 02-11-2004, 06:14 PM
Rail Jon Rogut Rail Jon Rogut is offline
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Default Re: Info about 888 interfaces

I would also question that.. I've never seen an 888 that wouldn't work aligned at -20dBfs.

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Old 02-11-2004, 06:29 PM
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Default Re: Info about 888 interfaces

Me thinks the Digi tech might have been refering to the 96 IO which is -14, as is the DIGI001.

I've been using Protools since the "SoundTools" days in the early 90s (What a mess!!!)
I still remember the post house I was working for getting PT3 systems, and learning about the -20 dBFS rule in 1994. (read the date, below)





Specifications for 888 Audio Interface.

The 888 I/O is an audio interface used with Pro Tools Project (3.2 or later), Pro Tools III (3.0 or later), or Session 8 PC (2.5 or later). It connects to any of the following cards: PT Project, Disk I/O, Bridge I/O, or Session 8 PC I/O.

Analog Input/Output: Balanced XLR, pin 2 hot, internally switchable +4dBu/-10dBu per channel.
Output Impedance: 50ohms typical (45 min, 55 max)
Input/output Level Trim: 6-turn cermet trims.
Digital I/O: 4 AES/EBU pairs (XLR); 1 S/PDIF pair (RCA); 16-bit I/O on NuBus/PC, up to 24-bit I/O on PCI (software dependant).
External Sync: 256x sample rate (Super Clock) in/out, about 3 Vpp, BNC connectors.
NuBus Card Connector: 50-pin, SCSI2-type female.
Sample Rates: 44.1k or 48k.
Analog-to-Digital Converters: 1-bit Delta-Sigma;128x Oversampling; 16-bit (18-bit with PCI).
Digital-to-Analog Converters: 18-bit; 64x Oversampling.
Operating Level: +4 dBu nominal (factory calibrated to 18dB headroom).
Maximum Input Level: +26dBu.
Frequency Response: 20Hz to 20kHz, +/-1dB.
Signal-to-Noise Ratio: -104dB (A-Weighted).
THD + N: .004%, 0 dbFS @ 1kHz, 22 Hz - 22 kHz bandwidth.
Power Requirements: 90-260 VAC, 47-63 Hz, 30 watts, auto-switching.
Dimensions: 2U External rack mount device; 19" x 3.5" x 9.75".
Weight: 7 lbs.



ID:73 Created:02/06/1995


When calibrating my 888/24 and 888 I/O interfaces, I am getting the 888/24 to output a 1.23V signal but the 888 I/O will only output a 1.16V signal. (-20dB reference level). Both interfaces are functioning at +4dBu

This observation represents normal operational difference between the two interfaces. Both the 888/24 and original 888 are factory calibrated for 18dB of headroom. However, the 888/24 can be adjusted for more than 20dB of headroom but the original 888 can't.


We use the same Trim Pots on both interfaces ( 2K ohms ).
ID:7595 Created:02/21/2002



Anyone seen my roll of 456????
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Old 02-11-2004, 08:02 PM
Pete Gould Pete Gould is offline
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Default Re: Info about 888 interfaces

Okay -- let's go back to the beginning and I'll provide a bit more detail. I'm not primarily an audio guy -- I'm a video guy. And this particular 888 is connected to an Avid editing system, because Avid uses it as the audio I/O for its Media Composer XL and Symphony products.

We took delivery of the system, new, in mid-2000. For its first few years it was used in an exclusively digital environment, so I never had a reason to deal with the analog I/O. But it was recently moved to an environment where it had to deal with analog decks. Avid shipped these units calibrated with 0VU as -14, but the emerging standard -- at least on the video side of the world -- is -20. So we recalibrated the 888 for -20 by outputting a 1KHz tone at -20 into a unity gain mixer with known-accurate VU meters and increasting the ouput potentiometers on the front panel such that at -20 digital it reached 0VU on the mixer. Later when we were troubleshooting we did the same thing with the 888 connected to various decks and they all showed the same level (0VU with tone at -20 on the 888).

We then noticed that when playing back program material, distortion would occur on the analog outputs (but not the digital outputs) when the level reached about -14. Initially we thought maybe the unit had been jumpered for -10, thus meaning we were overdriving the output stage in order to zero the mixer, but we opened it and checked it, and it's jumpered to +4 (which of course is what the mixer and all our decks expect). When we recalibrated again to -14, there was no distortion on the analog side all the way up to 0db digital. Avid's documentation regarding recalibration to -20 has a sidenote that says that "some older" 888's cannot be recalibrated to -20, but Avid's tech support (which is not really audio oriented) had no details. So we called Digidesign, and their $3 per minute tech support line informed us that, indeed, we "probably" had one of those units, because the symptoms I was describing were consistent with that problem.

You now know everything that I know. Unfortunately I don't have another unit here to swap with, which would be the easy way of reaching a definite conclusion.
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Old 02-11-2004, 11:28 PM
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Default Re: Info about 888 interfaces

The proper way to calibrate the 888 is by using the calibration template in Protools, setting the signal generator to -20 1K signal. Measure (adjust the trim pots) the output voltage at the XLRs to 1.16 volts, 1.23 v on the 888/24. Then jump an xlr cable from each of the 888s output to the 888 input and adjust the input using the fader automation indicators.

You didnt state anything about calibrating the input. SOunds like you are overdriving the output DACs by 12 dbs.

Quote:
some older" 888's cannot be recalibrated to -20
They mean it can at best -18.
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Old 02-12-2004, 05:51 AM
Pete Gould Pete Gould is offline
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Default Re: Info about 888 interfaces

Quote:
The proper way to calibrate the 888 is by using the calibration template in Protools
No doubt that's true -- but we don't HAVE Protools. We have an Avid video editing system that happens to use an 888 for its audio I/O. And it doesn't have a calibration template, though it does have a tone generator. Worse, although I realize this thread began on a Mac forum (because my initial searches about 888's led me here), this Avid is PC based, and while there's a free 888 calibration utility in the downloads area, it is Mac only. We don't have a Mac in this shop, I'm afraid (we used to, but we were forced to change over when Avid and Apple got into conflict several years ago and Avid briefly abandoned the Mac; that was when we did our last system upgrade, and PC was the only thing available).
Quote:
setting the signal generator to -20 1K signal. Measure (adjust the trim pots) the output voltage at the XLRs to 1.16 volts, 1.23 v on the 888/24
Well, this could be the problem, since 0VU analog is 1.23v and not 1.16v. So if we're using a calibrated piece of audio gear's VU meters to measure with instead of directly measuring voltage at the outputs, adjusting the trimmers to produce 0db analog with a -20db digital tone applied would be producing 1.23v, not 1.16. Presumably the result would then be that it was overdriven at peaks.
Quote:
jump an xlr cable from each of the 888s output to the 888 input and adjust the input using the fader automation indicators.
For that, you presumably need the calibration utility or the calibration template in Protools, neither of which do we have. At the moment, though, the problem isn't on analog input; it's on analog output of digitally-originated material that is brought IN via AES/EBU but has to be recorded to an analog output medium (Betacam-SP videotape). Which is, of course, the whole problem, since most digitally-originated video has a soundrack set up for -20.

Shall I take it from your post that ALL 888's prior to the 888|24 need to be set up with an output of 1.16 for -20db 1K, regardless of revision? If so, I wish I knew if we could swap in an 888|24 or if the software would know how to drive it. Sounds like that is our problem -- the analog gear in our shop would be expecting to see 1.23 as 0VU, not 1.16. The alternative is to leave the outputs calibrated to 1.16 at -20, but then we're pushing up the analog inputs of any deck we record on in order to compensate for the lower output level.


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  #10  
Old 02-12-2004, 09:28 AM
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Default Re: Info about 888 interfaces

You still dont get it!!! The INPUTS must be calibrated too!!! You are putting a signal throught the input, and calibrating the output. If the input was also calibrated to -14, you are miscalibrating the outputs.
What city are you in?? If youre close Ill calibrate it for you.
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