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Is it possible to normalize the highs on a tape?

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Is it possible to normalize the highs on a tape?

Postby mikehende » Wed Jul 08, 2020 5:26 pm

Hey guys, I have a cassette tape [TDK] from 1987 which was copied from an original on a Ghetto Blaster Dual Cassette box. However while recording seems the tape head was loose so the final recording ended up having some spots with no audible highs.

Some years ago I had tried using the eq and normalizing features of either Audition or Audacity software [can't remember which] to try to get the highs constant but it did not work. My thinking has always been that you cannot add highs to parts of a track which does not have the highs in the first place.

I was just told by a Sound technician that guys who "Produce" tracks might be able to do this which is why I am checking in here now. So guys, is this really possible please?
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Re: Is it possible to normalize the highs on a tape?

Postby blinddrew » Wed Jul 08, 2020 5:34 pm

You're right in that you can't boost what isn't there.
What your Sound Tech chap might be talking about is using an exciter to generate higher order harmonics, but how that might work in this situation... :problem:
There's nothing stopping you trying it though, digitise the recording, find one of the quiet spots loop it round and play with an exciter plug-in until (and if!) it gets you closer to what you want, then automate it in and out as required.
But I don't hold out much hope for getting anything sensible out of it really.
James P or Tim G will have a lot more experience in this area though, I'm sure they'll be along at some point.
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Re: Is it possible to normalize the highs on a tape?

Postby Hugh Robjohns » Wed Jul 08, 2020 5:36 pm

It won't have been a loose head -- that would cause all sorts of far more serious problems.

Most likely is that your tape has suffered from drop outs -- areas of the tape where the oxide has fallen off -- which typically affects the higher frequencies more than the lower ones. Or possibly you had intermittent head clogs in the recording machine or poor head/tape contact... but my money would be on dropouts from tape damage or general wear and tear.

As to whether it can be restored, that depends on the specific nature of the problem and the material.

If it is dropout, and on a music tape, I'd probably approach fixing it with a spectral editing tool. This can be used to manipulate the level of specific frequencies at specific points in time, and often specific sound elements can be copied and pasted in from elsewhere to replace missing content.. So a missing hi-hat or snare beat could be restored, for example.

But this is very laborious and time-consuming manual work and thus expensive if contracted out!

And while it may be possible to improve the material to some extent there are no guarantees and it is unlikely to be perfect whatever the case!
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Re: Is it possible to normalize the highs on a tape?

Postby mikehende » Wed Jul 08, 2020 5:43 pm

Hugh Robjohns wrote:It won't have been a loose head -- that would cause all sorts of far more serious problems.

Most likely is that your tape has suffered from drop outs -- areas of the tape where the oxide has fallen off -- which typically affects the higher frequencies more than the lower ones. Or possibly you had intermittent head clogs in the recording machine or poor head/tape contact... but my money would be on dropouts from tape damage or general wear and tear.

Thanks guys. I can't see that it would dropouts from wear and tear as the tape as that way the very first time I had used it which was like a couple weeks after my friend had made the tape for me.
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Re: Is it possible to normalize the highs on a tape?

Postby Hugh Robjohns » Wed Jul 08, 2020 5:46 pm

Well, it could be related to the quality or age of the tape itself, or its storage (heat is not good for tapes!) . The spring-loaded pressure pad in the cassette housing could also be exerting poor contact pressure, or the tape tension in your own playback machine could be inconsistent...

The possible reasons are myriad!

Given the extremely narrow track width and the extremely slow tape speed, it's amazing that cassettes work as well as they can... and not even slightly surprising that they suffer problems like this quite frequently.

I spent most of my teenage car driving with a cassette made by a friend of Pink Floyd's DSOTM and that had a dropout in it from new. I got so used to that sound that I struggled to cope with the proper thing when I upgraded to CDs! :lol:
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Re: Is it possible to normalize the highs on a tape?

Postby mikehende » Wed Jul 08, 2020 6:15 pm

I have many tapes which were made in the 80s and most if not all of them still play with the same or around the same quality as when they were made, it's only this tape in question which had that issue from day one.

Friend of mine told me yesterday he as a Mac software which might add the highs where needed. I am waiting on him to send me the name of it so I can try to see what happens.
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Re: Is it possible to normalize the highs on a tape?

Postby MOF » Wed Jul 08, 2020 6:22 pm

I presume the original tape is no longer available?
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Re: Is it possible to normalize the highs on a tape?

Postby Hugh Robjohns » Wed Jul 08, 2020 6:23 pm

Me too... although I rarely play them these days, I still have them and they are mostly still good.

As I said, from your description this is most likely a drop out, caused either by physical damage, or age, or a tape formulation problem. It happens.

Rather than try to 'fix it' which is inherently difficult and unlikely to be perfect, would it be possible to make another copy of the (good?) original onto a new blank tape? That would be a much easier, quicker and better solution.

Edit: MOF beat me to it!
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Re: Is it possible to normalize the highs on a tape?

Postby mikehende » Wed Jul 08, 2020 6:44 pm

I had tracked down the guy who had the original tape some months ago and he thought he had it stored in his Garage but after checking he could not find it.
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Re: Is it possible to normalize the highs on a tape?

Postby Tim Gillett » Wed Jul 08, 2020 8:54 pm

If the fault was in the original playback and merely copied onto your copy, which is all there is now, that doesn't sound too promising. We call that a 'printed in' fault. But hard to be certain. An expert works with what there is and tries to extract everything from the tape. Probably the most common mistake is less than ideal playback of the tape. Sometimes a mildly creased section of tape can be improved by literally ironing it. Foreign matter can cause dropouts. Most cassette tape can be expertly wet cleaned. I know of one company which has developed a wet cleaning system specially for cassette tape. The usual reason is contamination due to flooding. On most cassettes there is an unused section down the centre of the tape where nothing is normally recorded but sometimes it is. That area can be accessed by deliberately misadjusting the play head's height, but that's only possible on certain playback decks and you need to know what you're doing. Azimuth angle is critical with cassettes. It may be possible to dynamically track a moving azimuth error. The result may not be a perfect fix but at least an improvement. It just depends on how thoroughly you want to explore all the options.
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Re: Is it possible to normalize the highs on a tape?

Postby innerchord » Wed Jul 08, 2020 10:06 pm

Hugh Robjohns wrote:But this is very laborious and time-consuming manual work and thus expensive if contracted out!

As Hugh said.
Some of my work involves such tasks. It's forensic analysis and plastic surgery for audio, and can be very hard work!

Rebuilding a few seconds of audio is one thing, but a whole album quite another. :crazy:
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