Calibration tones on pre-recorded tapes?

Fred Thal

[Industry Expert]
Jul 15, 2016
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I'm told that there are now about thirty different makers of pre-recorded tapes.

Consider what could happen if any one of them were to start placing two minutes of calibration tones at the head of each reel.

Their customers might object. Perhaps quite strongly! After all, they're paying for recorded music, not for calibration tones.

But on the other hand, where is there any quantifiable quality assurance of the final product? (For that matter, how can we even check azimuth?)

The solution is to use smarter tones. This means that the information is put there for those who wish to extract and use it. Yet, it's also not objectionable (in length) for those who choose to ignore it.

Smarter obviously means NOT using a series of widely spaced discrete frequencies (and certainly not any tones of thirty seconds duration per frequency, as was commonly done by old school recording engineers in the 1960's and 70's).

No, we'd use much, much faster sweeps instead. Those sweeps could then be ingested (and stored digitally) for subsequent analysis. We'd then have meaningful quality assurance (accessible by independent measurement performed by the end user) and the important ability to accurately align the user's tape playback equipment to a given tape. That would be a true, high-end approach to analog tape playback.

We've had the technology for doing this for more than 35 years. Are any of the pre-recorded tape makers using it?
 

topoxforddoc

Well-Known Member
Feb 20, 2015
67
6
138
Cheltenham, UK
Hi Fred,

As someone who has been taught to do it the old fashioned way (30 secs each of 1k, 10k, 100 & 15k), when the tone durations are long enough to adjust on line up, the sweeps are a new thing to me.

So how do the smart tone sweeps work, when you're checking repro levels and azimuth before playing the tape? Am I right in saying that if the whole sweep is recorded at 0dBU, then checking repro level is straight forward-ish - you can adjust HF repro level when the sweep gets to the HF sweep and normal repro level when you hear the mid tones around 1k. Do you just adjust azimuth when it gets to the higher freq part of the sweep?

As I don't have an A2D or a large studio, I donut think I can ingest the sweeps digitally. Everything I do is old fashioned analogue with a pair of PPM meters.

Charlie
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
Hi Fred, . . . Do you just adjust azimuth when it gets to the higher freq part of the sweep?

No. You cannot perform any meaningful examination or adjustment of a reproducer's playback response without first establishing that the repro head azimuth is optimally adjusted to the recorded tape in question. (For any readers questioning this, I'm not stating an opinion. I'm stating incontrovertible scientific fact.)

So it follows that the azimuth check must come first, before measuring or adjusting playback response.

There are several different methods known for rapidly adjusting repro head azimuth. Ten seconds of a special signal tone would be more than enough time. (Some of the methods are indeed clever. Check the literature. I'm not going into them here.)

The bottom line for the educated WBF readership is that one has to wonder how anyone could be promoting tape playback as a serious audiophile medium if there are no azimuth calibration tones on the pre-recorded tapes.

Does someone believe that their tape will play back correctly or optimally on any known reproducer without an azimuth check and adjustment?

Or, is it that someone believes that achieving flat response across the audio spectrum is actually unimportant, or that retrieval of the information contained in the upper most octaves is optional or even unnecessary?

It appears that only three of us are following these recent WBF, elementary analog tape technology related threads. Are any of today's thirty pre-recorded tape makers also interested in these topics? Somehow I suspect maybe not.

So there's probably room for yet one more. A new, thirty-first pre-recorded tape producer. Now, if it were someone who cares deeply enough about audio quality, someone both knowledgeable and passionate about uncompromised analog audio, someone who cares enough to actually set-out to do tape right, there could possibly be an opportunity for them. Until that day most people may never know how good analog tape can actually sound.
 

topoxforddoc

Well-Known Member
Feb 20, 2015
67
6
138
Cheltenham, UK
Fred,

Just been looking at spectrum analysis for repro head azimuth adjustment, like here.

https://vimeo.com/22343864

Can I use my Macbook Air and a standard USD 2 channel sound card like a Focusrite Scarlett 2i2, or do I need something more sophisticated?

What special tones do you use? I'm afraid that I'm not a member of AES - I'll have to read that paper on azimuth you posted on the previous thread. I don't know how many surgeons the AES has on it membership roster!

Thanks

Charlie
 

Bruce B

WBF Founding Member, Pro Audio Production Member
Apr 25, 2010
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www.pugetsoundstudios.com
On the tape we produced, I put 1k, 100Hz and 10k tones at the tail. I think every tape should have these.
 

microstrip

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May 30, 2010
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On the tape we produced, I put 1k, 100Hz and 10k tones at the tail. I think every tape should have these.

At what level?
 

microstrip

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May 30, 2010
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0dBvu at 250nWb/m

I had an anonymous CD with such signals after a few demo tracks. One day I left the room while it was playing, 100 Hz is one of the resonances of a 6.40m long room I had at that time, everyone in the house was frightened!
 

topoxforddoc

Well-Known Member
Feb 20, 2015
67
6
138
Cheltenham, UK

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
Fred . . . Can I use my Macbook Air and a standard USD 2 channel sound card like a Focusrite Scarlett 2i2 ? . . . What special tones do you use?

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.
 

topoxforddoc

Well-Known Member
Feb 20, 2015
67
6
138
Cheltenham, UK
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
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
. . . 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.
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
. . . 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.
 

microstrip

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May 30, 2010
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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.
 

microstrip

VIP/Donor
May 30, 2010
20,806
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(...) 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.
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
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.
 

topoxforddoc

Well-Known Member
Feb 20, 2015
67
6
138
Cheltenham, UK
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
 

topoxforddoc

Well-Known Member
Feb 20, 2015
67
6
138
Cheltenham, UK

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
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).
 

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