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Restoring a cassette recording - NR question

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Restoring a cassette recording - NR question

Postby Steve A » Thu Dec 01, 2011 1:06 pm

I've been given a very old (most likely early 80s judging by the look of it) cassette tape of home recordings to try and recover. It's on a cheap unbranded ferric tape and very hissy. No way of knowing if it was recorded with Dolby or not. As you might expect, Dolby (B) certainly tames some of the hiss but dulls the signal considerably as well. I wondered if anyone has any thoughts as to whether it is best to assume Dolby was used and try and improve the recording after capturing it with Dolby on, or whether I am best leaving it off and simply relying on software noise reduction and EQ, or indeed - as I suspect - on something this far gone, it isn't really going to make any difference at all!

The tape has been 'liberated' from the person in question by colleagues with the idea being that they get it recovered as a surprise/ambush so I can't enquire further on how it was made but it's almost certainly recorded on a music centre with a cheap mic, certainly not in a studio.
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Re: Restoring a cassette recording - NR question

Postby Hugh Robjohns » Thu Dec 01, 2011 1:24 pm

I would assume Dolby B was used, but replay the tape without Dolby decoding because the self-erasure of HF over the passing eons will have made the tape rather dull and lifeless if you try to decode with Dolby B. The very modest HF compression that Dolby B introduces is fairly benign and the audio is usually quite acceptable without being decoded.

If Dolby C or S had been used you're in a whole different world of difficulty!

So, extract without any decoding, EQ the audio to taste, and then use a finger-print-style noise reduction process to help lower the noise floor to something more acceptable. Don't go mad trying to clean it completely -- you won't succeed and it'll sound horrible. Aim to reduce the noise by 3-6dB and listen out for warbling artefacts. Several passes with small amounts of reduction on each will always sound better than one pass with lots of noise reduction dialled in!

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Re: Restoring a cassette recording - NR question

Postby Steve A » Thu Dec 01, 2011 1:42 pm

Great advice Hugh, thank you. The compression aspect of 'B' was the thing that was making me think twice about going without but now I understand perfectly why I needn't have worried!
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Re: Restoring a cassette recording - NR question

Postby James Perrett » Thu Dec 01, 2011 4:54 pm

One thing to try if you can open up your cassette deck is to adjust the level going to the Dolby decoder. Giving the decoder a higher level will reduce the effects of the high frequency loss. However, don't do this until you have an alignment tape so that you can set it back again.

Otherwise follow Hugh's advice - especially with regard to going gently on the noise reduction.

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Re: Restoring a cassette recording - NR question

Postby Guest » Thu Dec 01, 2011 7:03 pm

Well, I suppose there's not much to stop you transfering it in the various ways to see which is preferable before any processing/editing. With regard to cleaning up, I think it often depends on what you're trying to achieve. Some people can accept the 'that's the document' idea. Others simply won't (for example, listen to Robert Johnson) because of the period quality -despite it being acceptable to (very musical) people of the time. For me, these things can have their own charm.
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Re: Restoring a cassette recording - NR question

Postby ef37a » Sat Dec 03, 2011 9:28 am

It is also very likely that the azimuth is out of whack giving a loss of HF but as with the other "tweaks" suggested, do not attempt to alter this unless you know what you are at and if you still want to have a go first make a test tape with white noise recorded on it at about neg 20 ref Dolby.

BTW: In a properly setup system Dolby B (or A or C!) does not cause any HF loss. This criticism was leveled at the system in early tests but it turned out to be a foible of human hearing, reduce the NOISE content in a signal and the subjective impression is that of HF loss. When the noise was artificialy put back into Dolby processed signals peeps were quite happy!

If you are on a budget you might like to download the 30day trial of Sony Soundforge. The noise reduction apps seem very good to me, certainly cleaned up my daughter's punk 45s a treat!

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Re: Restoring a cassette recording - NR question

Postby James Perrett » Sat Dec 03, 2011 10:44 am

ef37a wrote:It is also very likely that the azimuth is out of whack giving a loss of HF but as with the other "tweaks" suggested, do not attempt to alter this unless you know what you are at and if you still want to have a go first make a test tape with white noise recorded on it at about neg 20 ref Dolby.

Good point Dave - I adjust the azimuth for every cassette transfer that I do as there is always a little variation. The way I do it is to just listen for maximum treble - but don't go too far either side as things get confusing if the azimuth is very wrong. One tip is to listen in mono while adjusting azimuth as the correct azimuth is much easier to hear when both channels are combined in mono. If I want to adjust it back to optimum I just use a known good tape with plenty of treble content.

I would add that I have a cassette deck dedicated to transfers so I'm more worried about matching it to the tape I'm transferring rather than keeping it aligned properly. If you still record to cassettes then you may want to think twice before adjusting things like azimuth as I find that once I've adjusted it on a machine I'm always tweaking it.

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Re: Restoring a cassette recording - NR question

Postby Hugh Robjohns » Sat Dec 03, 2011 1:44 pm

ef37a wrote:In a properly setup system Dolby B (or A or C!) does not cause any HF loss.

Er... yes and no. In a properly set up system with new tape recorded and played back over a modest time period, absolutely. What goes in comes out (more or less), and there shouldn't be any programme loss at HF. And yes, in the early days people sometimes did perceive the reduction in broadband hiss as a loss of HF programme. Strange thing, the human hearing system, sometimes!

However, all tape recordings suffer self-erasure over time, which afects mostly the high frequency content, and it's worse on some tape sizes and formulations that others. Cassettes tend to suffer more than open reel.

The result is that when you come to play back your old cassette (even after hust a few years, let alone forty!), the HF content on the tape itself will have diminished considerably. The Dolby B system is expecting to find elevated HF content and tries to restore it to the correct level... only because it comes off the tape at the wrong level it ends up being reduced way below where it should be. Hence a dull and lifeless sounding recording.

Thankfully, because Dolby B is relatively gentle and unsophisticated (unlike Dolby C and S), the HF boost that the system aplied during recording is usually pretty close to the HF loss through self-erasure over time. So replaying an old tape without Dolby B is usually reasonably close to the intended tonal balance, and certainly has enough HF to do something with it. Of course, there will also be elevated HF noise, but that can be dealt with using the more sophisticated tools we now have available.

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Re: Restoring a cassette recording - NR question

Postby ef37a » Sat Dec 03, 2011 2:43 pm

Yes Hugh,
all understood but "at the time" this was a very common myth, that "Dolby cuts treble" it didn't. Of course, if the reverse expasion was switched off a treble lift was experienced along with the original noise but if people WANTED to listen to hacked up, noisy music that was their affair!

But then the duplicated musicassette was never going to be audiophile grade, even if the azimmuth matched the replay deck and the EQ was accurate AND the Dolby levels were spot on from the factory, it could not compare to vinyl, except of course for the absence of clicks! How many home recorders were setup for the tape types used? Often more than one type to boot..Effectively none and HF was the first casualty.

It is ironic that the digital technology that made CD cheaply available was also used for auto-bias/setup for tape, but sadly just too late. I had a Dolby HX Denon auto machine and with TDK AD it was superb.

And yes of course Hugh, all thing tend to droop with age.

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Re: Restoring a cassette recording - NR question

Postby Exalted Wombat » Sat Dec 03, 2011 4:27 pm

ef37a wrote:Yes Hugh,
all understood but "at the time" this was a very common myth, that "Dolby cuts treble" it didn't. Of course, if the reverse expasion was switched off a treble lift was experienced along with the original noise but if people WANTED to listen to hacked up, noisy music that was their affair!

Sometimes, simply switching Dolby B off when playing a muddy old cassette gives as much subjective improvement as any other attempt at "restoration".
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Re: Restoring a cassette recording - NR question

Postby Hugh Robjohns » Sat Dec 03, 2011 7:52 pm

Sorry to hear About the droopage, Dave ;)

It might be worth mentioning for anyone wondering, that Dolby HX isn't a noise reduction system at all. It is actually an automatic bias modulation system that adjusts the bias level to counter the over bias effect that can otherwise happen with strong hf content in the audio. It was mostly employed on the final generation of cassette recorders, but it was also used in the last studer reel machine, the a807.

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Re: Restoring a cassette recording - NR question

Postby Steve A » Mon Dec 05, 2011 2:19 pm

In the end I got listenable enough results from applying Hugh's original workflow. The tape was pretty far gone and I'm pretty convinced there wasn't really much in the way of treble to recover whatever way I attacked it - I suspect in equal part due to the self erasure aspect that Hugh mentioned, the distinctly low-fi nature of the original recording and the horrid light shade of brown of the tape which probably merits the use of Toilet Duck rather than Isopropyl Alcohol to clean my heads with! But I appreciate all the tips and I found it especially beneficial to understand why it's not essential to re-engage Dolby B which was the main point of theory that I wasn't sure about. Cheers guys.
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Re: Restoring a cassette recording - NR question

Postby Phil Reynolds » Mon Dec 05, 2011 2:53 pm

Hugh Robjohns wrote:Several passes with small amounts of reduction on each will always sound better than one pass with lots of noise reduction dialled in!

hugh

I'm about to start on a fairly massive cassette archiving program of my own, and I'd never have thought of this. Cheers, Mr R!
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