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ARSCLIST  October 2013

ARSCLIST October 2013

Subject:

Re: Advice needed on removing / minimizing tape bleed-through

From:

Tom Fine <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Fri, 25 Oct 2013 08:18:52 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (135 lines)

Hi Ted:

Much better!

The wonders of modern DSP in careful craftspeople's hands!

The next frontier will be figuring out how to grab the just musical content out of a noisy grooved 
disk and then un-do the problems of groove distortion and disk wear. I hope Carl Haber's work leads 
there -- scan the groove and then come up with some kind of Photoshop-like algorhythm to "heal" 
groove wear and the material on the groove surface that produces playback noise (I'm assuming that 
comes down to rough-surface shellac, which would need to be differentiated from minute lateral 
changes in the groove, ie soft-dynamic music content). I am optimistic that such a system will 
emerge in my lifetime. Imagine few-dozen-dollar software that enables you to scan your 78RPM disks 
on a high-resolution flatbed, then "heals" the ravages of time and the problems with the original 
shellac compound and saves a clean,crisp audio file to your hard drive.

-- Tom Fine

----- Original Message ----- 
From: "Ted Kendall" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, October 25, 2013 7:56 AM
Subject: Re: [ARSCLIST] Advice needed on removing / minimizing tape bleed-through


> Tom - is this better?
>
> I couldn't agree more - some tapes we only get one good shot at, and the aim has to be to preserve 
> as much information as possible.
>
> Cedar Retouch (now much imitated) is indeed a sort of audio photoshop with a spectral display - 
> time is the x-axis, frequency the y-axis and amplitude the z-axis, done with colour. The things 
> you can fix with it (and the mess you can make if you are careless) are nobody's business - 
> dropouts, bumps, clicks, pre and post echo, some distortions, etc, etc. I like it, use it all the 
> time, and I paid for it!
>
> The nice thing, as I've said, is that you can pinpoint the offending noise in time and frequency, 
> so the amount of original material you modify is minimised - and often the print is greatest at 
> two or three spot frequencies. If you take those bits out, the ear can't hear the rest.
>
> On 25/10/2013 11:53, Tom Fine wrote:
>> Ted, any chance you could modify your comment style to put them up top or in-line with the 
>> previous text? Sometimes your comments are many page-downs away, at the bottom of a long thread.
>>
>> I have to agree with Ted on this point:
>> "To use agressive and irreversible techniques on an analogue original is surely to be discouraged 
>> when other techniques are available."
>>
>> A tape with bad print-through is more likely than not to be old and maybe there are limited 
>> opportunities left to transfer it without major playback issues. So, a clean, high-resolution 
>> transfer should be made and then you can use copies of the digital file to try DSP remedies. I do 
>> doubt that analog remedies would be any more or less effective than DSP circa 2013.
>>
>> I did suggest in my first reply that one could attempt analog solutions to the problem, but I've 
>> changed my mind on that. I can't think of an analog tool that would work markedly better from 
>> today's sophisticated DSP "remedy" software. And the many playback passes required for 
>> experimentation might be very bad for an old source tape.
>>
>> Another thought that occured to me was try working in one of the spectral programs, like Sony 
>> Spectral Layers. I haven't had time to figure this kind of method out yet, but plan to 
>> investigate. I'm suggesting this because I noticed in an AES presentation that spectral-display 
>> information was used to show out artifacts of ancient splices were rendered inaudible without 
>> effecting the music content. I assumed that a spectral editor was used to identify and "heal" the 
>> splice artifact. Perhaps the Cedar program Ted recommended is just such software?
>>
>> -- Tom Fine
>>
>>
>> ----- Original Message ----- From: "Ted Kendall" <[log in to unmask]>
>> To: <[log in to unmask]>
>> Sent: Friday, October 25, 2013 5:32 AM
>> Subject: Re: [ARSCLIST] Advice needed on removing / minimizing tape bleed-through
>>
>>
>>> On 25/10/2013 05:40, John Chester wrote:
>>>> At 10:51 PM 10/24/2013, Malcolm Rockwell wrote:
>>>>> There may not be a major problem here. What has printed through is the audio from the next 
>>>>> layer of tape, correct? With digital manipulation being what it is today it should be simple 
>>>>> enough to grab the full volume layer of audio, attenuate it, flip the waveform and apply it 
>>>>> "over" the printed through signal. There will probably be artifacts but if you fiddle with 
>>>>> various parameters for a while, such as EQ, you will probably be able to find an acceptable 
>>>>> solution to your problem. I'd apply this to softer passages and leave louder material well 
>>>>> enough alone, though.
>>>>> It's worth a try.
>>>>> Comments?
>>>>
>>>> There are a number of problems to consider.
>>>>
>>>> First, the printed recording is not the same length as the original.  The delay between 
>>>> original and print changes as tape pack diameter changes.  Seems to me that for a tape which 
>>>> has been stored tails out, the print is longer than the original.  (Delay from original to 
>>>> print increases as tape pack diameter on the take up reel increases.)  The original recording 
>>>> can be speed-shifted, but you need to figure out how much to shift it.
>>>>
>>>> Second, the frequency response of the printing process is not flat.  According to
>>>> http://www.aes.org/aeshc/docs/3mtape/printthrough.pdf
>>>> "The worst print-through occurs at a wavelength equal to 27 * C. C is the total tape caliper in 
>>>> mils. For a typical 2
>>>> mil mastering tape, the worst wavelength for print-through would be about 12.6 mils. When 
>>>> running at 15 ips, this
>>>> would be a frequency of about 1200 Hz."
>>>> Note:  there's an error in their formula, as printed in the on-line document.  It should read 2 
>>>> * Pi * C.  But they're correct that it's worst at about 1200 Hz.  This is confirmed by the 
>>>> October 1980 JAES article "The Print-Through Phenomenon" by Bertram, Stafford and Mills.  It 
>>>> includes a graph of print-through vs. frequency.
>>>>
>>>> BTW, this article also states that "print-through ... can be reduced if [the tape] is 
>>>> repeatedly rewound.  The amount of print reduction ...can reach as much as 7 dB."  In their 
>>>> tests, this required 6 rewindings.  "The rewindings should be consecutive with an optimum 
>>>> storage time between rewindings to achieve maximum reduction.  The optimum storage time may 
>>>> depend upon the individual tape."
>>>>
>>>> Third, is the printing process linear or non-linear?  The 3M document cited above says it's 
>>>> linear.  Camras, in the 1988 edition of "Magnetic Recording Handbook", says it's not, and that 
>>>> the ratio of the original to printed signal varies with the level of the original signal.  I 
>>>> think Bertram et al.  are saying it is linear, but I must admit that I have not yet entirely 
>>>> digested this long, complex article.  Hopefully it is linear, because modelling a non-linear 
>>>> transfer function will not be much fun.
>>>>
>>>> So....  If the printing process is linear, the other problems seem manageable.  But it will no 
>>>> doubt require a fair bit of fiddling to get the cancellation signal lined up in time and 
>>>> amplitude with each objectionable echo.
>>>>
>>>> -- John Chester
>>>>
>>> I routinely use Cedar Retouch for print-through problems, whether pre- or post-echo, and usually 
>>> find that removing the two or three most prominent components is enough to push it back into the 
>>> noise. To use agressive and irreversible techniques on an analogue original is surely to be 
>>> discouraged when other techniques are available.
>>>
>>>
>>
>
> 

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