cutting DC Offset

[tape]

Does anybody here knows what problems are related when cutting material that contains a DC offset that is drifting?

I expect that this might give tracking problems during playback and possibly distortion...

I know that the DC offset should ofcourse be fixed prior to cutting, I am just wondering if it's possible to cut material with extremely drifting offset....I suspect that there might also would be a difference depending on if the material is stereo or mono....

[TotalSonic]

A digital track with a heavy amount of DC offset will manifest itself as having some amount of sub-low end rumble in the audio. A high pass filter (aka "low cut" or "HPF") will get rid of it.

Best regards,
Steve Berson

[JayDC]

from what I understand, a DC offset can damage cuttingheads..

[TotalSonic]

from what I understand, a DC offset can damage cuttingheads..

I am not so sure about that. Damage to cutterheads is much more likely to occur from high amplitudes of high frequencies rather than low amplitudes of ultra-low ones. And there's often some amount of very low level DC offset present in analog gear - so just because you can measure some in an analyzer does not necessarily mean that it is problematic. Again - the application of an HPF will get rid of DC offset - and in general even a low crossover point for the HPF (i.e. 10Hz) can still eliminate it.

One thing I should note that is the one true piece of evidence of problematic DC offset is a "click" whenever you start or end the playback of a digital audio file even when you start or end this at what should be an area of silence.

Best regards,
Steve Berson

[gold]

DC can damage cutting heads. It will also reduce headroom. Having DC current around a permanent magnet isn't a good idea.

[TotalSonic]

DC can damage cutting heads. It will also reduce headroom. Having DC current around a permanent magnet isn't a good idea.

Paul -
thanks for that clarification.

Best regards,
Steve Berson

[tubefan]

Depends on the magnitude of offset and type of head magnet material. Ceramic magnets are harder to demagnetize at normal temperatures, rare earth magnets nearly impossible. Alnico magnets are likely the only type at risk in heads. So everyone with Westrex, Grampian, Presto, early Neumann, and other older heads may have this problem.

Neumann and Ortofon amplifiers maintain a small DC voltage through the heads to measure the coil temperature. Say 800mV to 1.2V, can't remember which number. Some variable depth systems (Capps) for the Westrex system added DC voltage to the cutterhead amplifiers to adjust depth.

All that said, it is a good to avoid having significant DC offsets in your cutterhead coils due to program content...

[JayDC]

DC can damage cutting heads. It will also reduce headroom. Having DC current around a permanent magnet isn't a good idea.

That's what I thought, coz when my RCA MI-4887 Blew, I could not figure it out, because the levels where not too high. On closer examination I found a rather strange DC offset spike around the time that the head blew..

[opcode66]

I know you can correct the program material in a DAW like ProTools. To bring everything down to zero line. I don't think that would alter the integrity of the audio.

[JayDC]

I know you can correct the program material in a DAW like ProTools. To bring everything down to zero line. I don't think that would alter the integrity of the audio.

yes, most daws can remove the dc offset, i didn't even check before i blew my rca cutterhead.. now i always do

[opcode66]

Sorry to hear about your cutterhead

Thanks for posting this however! You might save someone elses head!

[JayDC]

Sorry to hear about your cutterhead

Thanks for posting this however! You might save someone elses head!

yeah, it happened a while back, a shame too coz its have more high end then the 1-c i'm using now.. I'd send it to gib to rewind, but the last time he worked on it, he damaged it.. So I am reluctant..

I think I might just buy a coil winder off ebay.. there are some that will count backwards, so u can use it to unwind the coil, so you know how many turns u need to rewind it..

another trick i was told about was: unwind the coil, straighten the wire, cut a new wire the same length, and rewind.. I am told even though the amount of turns might not be the same, it should be just about there, and should not effect the performance..

[gold]

Sure, it's all a matter of degree and it depends what the source of the DC offset is. It's a good idea to keep the DC offset on the SAL74 power amps tightly calibrated...