Hi Tom -
Seems there is no simple answer. Reminds me of the way the Motors auto
repair manuals would begin describing the process of rebuilding a motor:
"With engine on bench..."
Thanks for your comment.
Malcolm

*******

On 10/25/2013 7:32 AM, Tom Fine wrote:
> 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
>>>>
>>>>
>>>
>>
>>
>