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Thread: Calibration tones on pre-recorded tapes?

  1. #11
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    Quote Originally Posted by Fred Thal View Post
    Charlie, you're on the right track. The industry has been using analog to digital conversion at the signal inputs (for signal capture and storage) in almost all of the high-performance audio test equipment designed since the early 1980s.

    And the computing power that's available for audio signal analysis (DSP) with today's laptops fitted with a good sound card embarrasses much of the older surviving stand-alone instrumentation (like spectrum analyzers) that was once dedicated to audio measurement.

    I believe the Scarlett you mention is an outboard USB interface with two mic preamps, and not a sound card. (Maybe it ships bundled with one?) In any case, before connecting a pro tape machine like yours, be certain that the balanced analog inputs of your interface comfortably accepts studio line-level audio without any danger of input overloading.

    Then, you'll be busy researching what software to install and run. Another steep learning curve lies ahead, but there are some great places and forums on the web to find help with this.

    As for what signals I use, well, I'm not in the business of making tape copies. So perhaps the question ought to be what signals should be used by those who are in that business. It would be great if it could be standardized (across all the tape makers). I've long thought about it and had hoped the more knowledgeable among them would form a producers consortium with a qualified technical committee. Maybe I'm only a hopeless dreamer.
    Fred,

    Many thanks. I'm beginning to get this now. I am certainly starting too understand the use of computers to analyse the audio signal. My late friend, Stewart, had a large computer desk in his home studio with spectrum analysis going on all the time. sadly, he passed away before he could pass on this to me. I can see how using DSP, you can gauge repro head azimuth.

    You mentioned special tones, but the video clip shows how you can do this just using the music on the tape. Is that a sensible and reliable way to do this - looks simple enough to do - trying to maximise the signal across the freq range (i.e. get more yellow across screen instead of blue).?

    As I'm a newbie to this, could you suggest either some resources, or perhaps a budget priced A2D and suitable software for a Mac?

    Best wishes,

    Charlie

  2. #12
    [Industry Expert] Senior Member Fred Thal's Avatar
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    Quote Originally Posted by topoxforddoc View Post
    . . . Is that a sensible and reliable way to do this . . . ?
    Sure, if you have no tone provided on a tape for setting azimuth, then hunting around for the optimum azimuth alignment by using spectral analysis of the recorded program can be an excellent solution.

    But the process is basically akin to searching around without any clues. So it's hardly convenient.

    My point in starting this thread (and some earlier ones, as I've been complaining about this for over ten years) is that selling expensive tape copies that do not include even eight or ten seconds of useful azimuth alignment signal is inexcusable. Especially so, if the product purports to be a serious high-end offering.
    Adolph Friederich (Fred) Thal
    Technical Director and Founder, Audio Transfer Laboratory
    Managing Director, ATAE ataudioeng.com

  3. #13
    [Industry Expert] Senior Member Fred Thal's Avatar
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    Quote Originally Posted by topoxforddoc View Post
    . . . could you suggest either some resources, or perhaps a budget priced A2D and suitable software for a Mac?
    Sorry, but USB sound cards and their associated audio signal analysis software are clearly outside my area of expertise. I'd be about the last person you'd want advising you in this area. For a good start, I suggest you ask on Amir Majidimehron's Audio Science Review forum.

    As for me, in my daily work building and testing new analog tape reproducers, well I'm still (very happily) using older, dedicated hardware like the Audio Precision System Two and the Sound Technology 1510A Tape Recorder Test System.

    And a collection of other legacy gear from GenRad and HP. Importantly, unreliable test equipment in the lab is a huge liability for any professional. Maintaining this old legacy hardware is a very big and very time consuming deal, unfortunately.
    Adolph Friederich (Fred) Thal
    Technical Director and Founder, Audio Transfer Laboratory
    Managing Director, ATAE ataudioeng.com

  4. #14
    VIP/Donor [VIP/Donor] microstrip's Avatar
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    Quote Originally Posted by Fred Thal View Post
    Sure, if you have no tone provided on a tape for setting azimuth, then hunting around for the optimum azimuth alignment by using spectral analysis of the recorded program can be an excellent solution.

    But the process is basically akin to searching around without any clues. So it's hardly convenient.

    My point in starting this thread (and some earlier ones, as I've been complaining about this for over ten years) is that selling expensive tape copies that do not include even eight or ten seconds of useful azimuth alignment signal is inexcusable. Especially so, if the product purports to be a serious high-end offering.
    We have an easy answer to this problem - using a tape with the calibration signals any one can compare the setting obtained using both methods and see if they are really equivalent. Do you know of any one reporting on such tests?

    Some audiophiles take these subjects very seriously - we have members that adjust tonearm height for each LP.
    Under construction around a pair of Wilson XLF's , the Forsell Air Force One, the ARC Phono 2SE and the DCS Vivaldi 2.0 stack : ARC REF40 + ARC REF250, TA OPUS MM2 +TA XL digital, TA XL gen V power cables and CenterStage feet ...

  5. #15
    VIP/Donor [VIP/Donor] microstrip's Avatar
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    Quote Originally Posted by topoxforddoc View Post
    (...) As I'm a newbie to this, could you suggest either some resources, or perhaps a budget priced A2D and suitable software for a Mac? (...)
    Unfortunately I have no experience with Mac's, but if you have access to an Windows PC the full function demo version of SpectraPlus works for 30 days. www.spectraplus.com/.

    I have used it with the EMU tracker USB Pre USB2.0 with great success.
    Under construction around a pair of Wilson XLF's , the Forsell Air Force One, the ARC Phono 2SE and the DCS Vivaldi 2.0 stack : ARC REF40 + ARC REF250, TA OPUS MM2 +TA XL digital, TA XL gen V power cables and CenterStage feet ...

  6. #16
    [Industry Expert] Senior Member Fred Thal's Avatar
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    Quote Originally Posted by microstrip View Post
    We have an easy answer to this problem - using a tape with the calibration signals any one can compare the setting obtained using both methods and see if they are really equivalent. Do you know of any one reporting on such tests?

    Some audiophiles take these subjects very seriously - we have members that adjust tonearm height for each LP.
    No, I don't know of anyone reporting on that, probably because there's no need to.

    Again, if you accept the definition of optimum repro head azimuth as being that setting that provides the peak short recorded wavelength response from a tape in question, then obviously that result will also be confirmed with spectral analysis of the playback of the audio program. As I'm sure you know.

    It's not surprising that someone would make a tonearm adjustment if they understand optimum VTA and what happens to that setting when you place records of differing thicknesses on a turntable. It's a pretty darn good analogy to setting optimum azimuth with magnetic tape. Thanks for making it.
    Adolph Friederich (Fred) Thal
    Technical Director and Founder, Audio Transfer Laboratory
    Managing Director, ATAE ataudioeng.com

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    Fred and microstrip,

    Thank you for the helpful replies. I suspect that few "high end' tape suppliers will change practice and put tonnes on the tapes. Much of that stems possibly form the fact that the end consumer probably doesn't understand and just assumes that you thread the tape and press play.

    Charlie

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    The importance of reference tones highlighted here in Studio Sound 1976 on page 38 paragraph 3 onwards

    http://www.americanradiohistory.com/...nd-1976-11.pdf

  9. #19
    [Industry Expert] Senior Member Fred Thal's Avatar
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    Quote Originally Posted by topoxforddoc View Post
    The importance of reference tones highlighted here in Studio Sound 1976 on page 38 paragraph 3 onwards
    Note that in 1976, the term "standard reference tone", as mentioned by Chakraverty below, often meant a series of at minimum four, and much better of up to ten or more discrete tones, for level and across band verification of frequency response in accordance with the employed recording equalization.

    In the 1980's, some of the more technically astute and quality-focused recording engineers began recording sweeps of up to 120 discrete tones, so that playback equalization could be checked across ten octaves with one-twelfth octave resolution. Using purpose-built tone generators, such sweeps could be run in as little as thirty seconds.

    While Chakraverty was discussing tape playback in the lacquer mastering stage (excerpt below), his comments apply also to making tape copies, in those days called "tape dubbing".

    Arun Chakraverty, CBS Studios, from the November 1976 article in Studio Sound :

    "It is a source of perpetual headache for the cutting engineer to find that the master tape does not incorporate the standard reference tone, ie absolutely level, Dolby tone, as well as azimuth alignment
    tone. Naturally any discrepancy caused by the absence of such essential aids will be transferred along the accompanying programme material to the disc.

    Generally, the most common faults with copy masters include an indifferent frequency response, high frequency intermodulation and aberrations in the bass registers. There are two principal reasons
    for the large number of poor copies presented: 1) The importance of tape machine alignment for both master and slave is often neglected and, occasionally, barely understood. 2) The importance
    of the dubbing engineers' job is very much overlooked and definitely underestimated within the recording industry; dubbing is sometimes considered to be on a par with tea making."

    Not mentioned in the above piece is the very large susceptibility for accumulating (or compounding and convolving) time-base error that is inherent to most commonly employed processes of tape dubbing or tape replication. This time-base corruption problem is mostly avoided in the process of vinyl record mastering and replication (stamping).
    Adolph Friederich (Fred) Thal
    Technical Director and Founder, Audio Transfer Laboratory
    Managing Director, ATAE ataudioeng.com

  10. #20
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    Quote Originally Posted by topoxforddoc View Post
    Fred and microstrip,

    Thank you for the helpful replies. I suspect that few "high end' tape suppliers will change practice and put tonnes on the tapes.
    Charlie
    Good luck with that one!!
    Bruce A. Brown
    Puget Sound Studios
    Stereomojo reviewer
    Seattle, WA


    Even a blind squirrel gets a nut once in a while!

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