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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 13:32:32 -0400

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (131 lines)

Hi Malcolm:

I think click and pop removal software looks for level spikes of short duration, not for repetitive 
patterns. I don't think time between ticks and pops matters, it's more spike duration and percent 
above average level. This is why if you over-use that kind of DSP, it squashes percussives like drum 
hits. I have to admit that there are some de-click tools that now really just remove ticks. Pops are 
still too long duration, and all software I've tried or gotten test examples of leaves artifacts on 
the underlying musical content. Pops can be dealt with more easily in spoken-word and other 
less-complex sound environments. Ticks are of such short duration that you can spank them down and 
the mind will fill in the tiny void, as long as the void is tiny enough and the ticks aren't spanked 
down lower than the surrounding music level so a "white void" is created (the "white voids" are very 
noticeable). The guy who figured out how to scrape off a little bit of oxide to reduce tick levels 
on tape dubs (was it John R. T. Davies?) figured this out decades before DSP. The guys who used to 
make tiny deletions of the tape where the tick peak was were messing with the time-domain, which is 
noticeable to people with a good sense of rhythm and most other careful listeners. The 
oxide-scraping method leaves the time domain intact but messes with the amplitude of a microsecond 
of time, which is less noticeable to the listener because the brain can fill in the tiny amount of 
missing content.

Of course, the very best method I have heard for fixing a tick and even shorter-duration pops is to 
use the waveform editing tool and simply draw out the spike, freehanding in the missing waveform. 
It's a skill, but it can be learned by people not good at drawing with pencils like myself. You 
can't tune tick removal software to do this right all the time, because you're tuning it to reduce 
ticks to a certain level, not to re-draw the wave where the tick was, following the contours before 
and after. I betcha spectral editing could come pretty close, though. You'd "heal" all the elements 
of the tick except those exactly in the content frequencies.

-- Tom Fine

----- Original Message ----- 
From: "Malcolm Rockwell" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, October 25, 2013 12:59 PM
Subject: Re: [ARSCLIST] Advice needed on removing / minimizing tape bleed-through


> Seems a variable delay time algorithm has already been written, else how would one de-click a 78 
> that has a repeating click across the grooves at, say, 90 degrees (like a scratch or a repaired 
> broken record)? Same algorithm, different application.
> Ted Kendall's idea of "removing the two or three most prominent components (of the echo signal 
> being) enough to push it back into the noise" is a good one, as well.
> And, of course, we never fiddle with the analog master. There should be no need to if it is intact 
> and can play through.
> Malcolm
>
> *******
>
> On 10/25/2013 12:41 AM, Tom Fine wrote:
>> Ok, this is what I meant when I was questioning whether a de-echo plug-in would work:
>> "The delay between original and print changes as tape pack diameter changes.  "
>>
>> John said it better than I.
>>
>> De-echo software that can "chase" echo of varying delay times would have to be quite 
>> sophisticated, unless it's just an automated noise gate (ie it looks for spikes below a certain 
>> level and kills them). Again, I haven't tried this software and I'm not a code-writer, so I have 
>> no idea if it would do the job on print-through.
>>
>> Richard Hess asked for a sample of bad print-through. I don't have any on my hard drive and don't 
>> have time to hunt for and transfer a bad-example tape. Anyone who has old 2-track duped tapes on 
>> 1-mil stock probably has a candidate for experimentation. Richard, what about your old RCA 
>> 2-track tape, or that Mercury 2-track I gave you a few years back? I'm sorry but I don't have 
>> studio time for experimentation right now, maybe a little bit of time next month. Another good 
>> candidate would be any 1/4-track early 60's acetate 1-mil duped tape in your shelves. The smaller 
>> tracks and usual lower level may or may not make the print-through a worse problem (maybe less 
>> dynamic range between original signal and echo, but also maybe lower level echo of lower-level 
>> signal, I'm no expert).
>>
>> -- Tom Fine
>>
>> ----- Original Message ----- From: "John Chester" <[log in to unmask]>
>> To: <[log in to unmask]>
>> Sent: Friday, October 25, 2013 12:40 AM
>> Subject: Re: [ARSCLIST] Advice needed on removing / minimizing tape bleed-through
>>
>>
>>> 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
>>>
>>>
>>
>
> 

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