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mig 15
02-11-2002, 11:07 AM
I'm mixing to my Sony Dat machine via the optical out,with no overs on the PT master fader or the Dats' meters, but when I tried to bring the tracks back into PT optically (to compile a quick ref cd) the inputs in PT were peaking all over the place.Can anyone explain this-I thought digital level was constant between machines/platforms etc. Presumably meters are calibrated differently? I'm now concerned that the mixes I send off to be mastered next week will be full of overs.
Thanks in advance.mig

popps
02-11-2002, 11:53 AM
Hmmm.... that's odd. Try checking your input levels in your hardware panel. That shouldn't matter, but it's worth a try. Also, check the back of the DAT and make sure there's no boost/cut option. Also, you may try the SPDIF option and see if that gets you a better result.

Robert U
02-11-2002, 01:48 PM
I have experienced the same thing many times. I got a little suspicious in the beginning but after a while I got used to it since everything sounded right, i.e no digital distortion.


-Robert

mmiikkee
02-11-2002, 02:16 PM
I know what MIG15 is talking about. I have always tried to figure out the same thing. Say a DAT is outputting 0db at it's peaks ( on the DAT LED meters ). Go SPDIF/RCA or Optical out from the DAT machine to the 001s' SPDIF or Optical. Set up a track in PT, set the tracks' input to digital...engage Rec., and the PT input meter is clipping into Red. But the signal on the DAT is clean, no digital distortion...and the DAT meters are not clipping. There's some type of mismatch here?

Bastiaan
02-11-2002, 02:37 PM
there is a difference between digital clipping and 0db. clipping means the signal is trying to be louder then possible. 0db means you've hit the maximum volume. often clipping and 0db are considered the same, but there is a subtile difference...

Mr T
02-11-2002, 02:51 PM
DATs and ▀Digital, for example, don't use the same level reference as many other devices do. For instance a 0dB level on an analogue mixer or many DAWs will be equal to -20dB on a DAT or a ▀Digital.
Use 1KHz signal if you need to check your levels. A 1KHz signal in PT set to 0dB should result in a -20dB level on the DAT.
0dB on a DAT is a LOUD signal (close to clipping and distortion).

valvebrother
02-11-2002, 03:10 PM
After spending more time than I'd like to admit doing dubs of professional radio spots to DAT, DGS, 7.5, 15 and 30ips analog, here is the industry practice as I was taught it for digital transfer:

0db analog 1KHz tone -> -18db DAT/DGS

Your peaks should register at about -6db on the DAT, NO MORE!

D

Park Seward
02-11-2002, 05:21 PM
Some terms are getting confused:

0 dBu Preferred informal abbreviation for the official dB (0.775V); a voltage reference point equal to 0.775 Vrms.

+4 dBu Standard pro audio voltage reference level equal to 1.23 Vrms.

0 dBV Preferred informal abbreviation for the official dB (1.0V); a voltage reference point equal to 1.0 Vrms.

-10 dBV Standard voltage reference level for consumer and some pro audio use (e.g. TASCAM), equal to 0.316 Vrms.

0 dBm Preferred informal abbreviation of the official dB (mW); a power reference point equal to 1 milliwatt. To convert into an equivalent voltage level, the impedance must be specified. For example, 0 dBm into 600 ohms gives an equivalent voltage level of 0.775 V, or 0 dBu (see above); however, 0 dBm into 50 ohms, for instance, yields an equivalent voltage of 0.224 V -- something quite different. Since modern audio engineering is concerned with voltage levels, as opposed to power levels of yore, the convention of using a reference level of 0 dBm is obsolete. The reference levels of +4 dBu, or -10 dBV are the preferred units.

0 dBr An arbitrary reference level (r = re; or reference) that must be specified. For example, a signal-to-noise graph may be calibrated in dBr, where 0 dBr is specified to be equal to 1.23 Vrms (+4 dBu); commonly stated as "dB re +4," that is, "0 dBr is defined to be equal to +4 dBu."

0 dBFS A digital audio reference level equal to "Full Scale." Used in specifying A/D and D/A audio data converters. Full scale refers to the maximum peak voltage level possible before "digital clipping," or digital overload (see overs) of the data converter. The Full Scale value is fixed by the internal data converter design, and varies from model to model.

If a level exceeds 0 dBFS, it will make an "over".

There is also some confusion about the difference between peak and VU ballistic meters. Many audio people set a VU meter 6db lower than a peak meter when dubbing.

Identical digital recorders will let you clone the material at the same levels as long as the input and output levels are set at unity. If the DAT recorder does not have a detent setting, you may not be at unity. However, you would think that a -10 reading on a DAT would be the same as a -10 on a PT system as long as the levels were set at unity.. One way to find out is to use another digital recorder and see where both the levels read there.

dBHEAD
02-12-2002, 12:24 AM
Are you actually hearing the overs? Remember, a PT track will indicate a clip at full code.

Still, if it wasn't happening when it was sent, it shouldn't happen on the way back.

Also, I'm assuming you do mean DAT and not ADAT. If you haven't created a master fader for your session in PT, you actually could be clipping when sending to the DAT but you'd never see that on any individual track.

mig 15
02-12-2002, 04:29 PM
Sorry it's taken a while to get back here-been pretty hectic, but thanks for your many informative replies.I spoke to an engineer at the mastering place today-he was happy that the dat machine wasn't showing overs but suggested backing the pt master fader back 1 or 2db if I was still concerned.I'm going to run the original dat into another dat machine to see if I get the same result.Plus I'll do the 1khz signal check that Mr T and the valvebrothers suggested.
Regards.mig.

Chris Coleman
02-12-2002, 11:10 PM
There are some confused people here. When you're doing digital to digital transfers (via optical or S/PDIF output from 001), whatever's coming out of PT should be exactly what's showing up at the DAT - EVEN IF THAT'S -.5 dBFS (full scale, digital reference). If you are not clipping pro tools, then you won't be clipping on your DAT machine so long as you are passing signal digitally. Mr T and valvebrother would be correct in their level-setting statements (to an extent, keep reading) if you were coming out of PT and into your DAT via analog connections, but your application dictates that you are using the optical or S/PDIF outs, which do NOT have input trims on DAT machines anyhow. So - the main thing you're dealing with here seems to be meter calibration, sample duration for overs, etc., but this does NOT seem like a level problem. Also, make sure you're clocking everything properly, if the DAT machine or PT doesn't know what to make of all the 1's and 0's and WHEN, you will have errors and possibly clipping.

No matter what, pay no attention to anyone telling you to use 0dBFS tones out of PT, THAT makes no sense at all - if you've got 0dB out of Pro Tools equalling -20dB on a DAT machine, that's a full 20dB of headroom you're not using and you're basically throwing away more than one fifth of your useable dynamic range (which is roughly 96dB for a 16 bit system) - that's like chopping off more than 3 whole bits from your digital word (which is 16 bits) for NO REASON, resulting in LESS than 13 bit sound quality. Look, if this is a music master going to a CD dupe house, GO TO THE WALL, just make sure you've got a great limiter holding you JUST under all-bits-on (0dBFS), like around -.3dB (Waves L1 comes to mind)... if it's a work tape or a dub, then yeah, keep it under -6 to -4dBFS. Whatever you do, keep making music and have fun with all these bits n' samples.

Oh, and the mastering engineer in me asks to you kindly leave other mastering engineers a few dB of headroom to tweak with (for EQ and dynamics work). In other words, don't send a mastering guy something that's already been mastered - mix your songs to where they sound good and natural with some breath, and the mastering house will take it from there.