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.
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
>> 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.
>> 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
>> "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