The importance of playback equalizations

Fred Thal

[Industry Expert]
Jul 15, 2016
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Probably any knowledgeable analog tape professional is aware that the long-outdated 15 in/s IEC2 recording and playback equalization standard (formerly known as NAB) has serious issues in the regions approaching both frequency extremes. This was established more than four decades ago and lab measurements and the findings of expert listeners remain in solid agreement. I don't recall this as ever being controversial. You could easily repeat the tests today.

So, if someone is reporting that they prefer the sound of the IEC2 15 in/s equalization standard over that of the IEC1 15 in/s standard (formerly known as CCIR), my own first instinct and suspicion would be to look closely at their equalization chain, both in recording and playback.

Just as a carefully and correctly performed measurement of a phono stage's accuracy in tracking the RIAA curve is an essential first step to revealing, quantifying and distinguishing why different phono playback electronics can have audibly different sonic characters, so too it is with analog tape. Arguably, even much more so.

Unlike the case with vinyl pressings (the consumer medium), a series of calibration tones were placed on each master tape reel by recording professionals who wanted to be certain that playback could always be performed with the exact needed complementary equalization.

It's long been accepted that even mild response anomalies are disqualifying in high-end audio. (Or that they ought to be.) No knowledgeable or serious listener would ever accept a randomly adjusted graphic equalizer being inserted into their playback system. But that's what you might be doing if you connect after-market tape playback electronics to a repro head without verifying that the system plays back a calibration tape with acceptably flat response.

In my work, using a measurement protocol with one-twelfth octave resolution (typically measuring over nine or ten octaves with one hundred or more discrete frequencies) I observe certain accepted professional tape playback electronics designs that can routinely align to a given equalization standard to within about one-half a db, from 40 Hz to 22kHz.

Distressingly, I have also measured evaluation non-professional units that cannot be brought closer than eleven dB at some points. Caveat emptor. Any readers of this forum who are thinking of purchasing specialized tape playback electronics would do well to first verify the performance of the unit. At the very least, be certain that what you are buying closely tracks your specified equalization curve with the repro head supplied.
 
Last edited:

Folsom

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Oct 25, 2015
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Funny, I never thought about that. I assume many tapes are already equalized if they're sold as just music tapes, but for any that are literally masters from direct copy the situation is different.

In the recording industry this has caused a lot of gruff over the years. For example I believe it was all the old Capitol machines had identical tones (EQing) among all their tape machines used with the master tapes that would be mastered for being cut to vinyl. For them it might be easy to dig out old masters and do a digital mastering. But for everyone else that had tones (EQing) written down on paper along with the tape, they're leaving unknown mixing to the now digital masterer if they don't have the same machine and notes... or for the person who got a copy of a master. Heh, what does one do? :eek:

I presume the method you mention helps you get an approximation of tones to use?
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
Steve Williams writes: I have no idea about NAB vs IEC equalization


It's a technical decision to be made in recording production. It's mainly about how best to utilize the later generation improved tape formulations at 15 in/s. One immediately clear benefit of IEC1 is that there ought to be less tape hiss audible than with IEC2.

Yet, as I think Tony Ma was suggesting, whether IEC1 or 2 was employed shouldn't really matter all that much to the end user. Just hope that whichever was chosen was also correctly implemented. And be sure to always set your tape playback electronics to correctly match the recording equalization that was used for making your copy of any particular tape. Otherwise, you'll be completely lost with unknown response aberrations being introduced in your playback.

This is why you should verify your tape playback system's performance in the amplitude domain. Does it in fact closely follow the equalization curves? A reproducer alignment calibration tape is used to determine this.

It follows that making any reference to the purported sound of different playback electronics, without also detailing their measured responses against a specific equalization curve, will be misleading. The listener would likely experience widely varying perceived tonal qualities, yet would have absolutely no basis for making any valid conclusions. You've inserted a graphic equalizer with unknown settings.

To my knowledge, none of the producers of tape copies today are putting a comprehensive series of ladder tones or sweeps on the copy reels. This is unfortunate, in my opinion.
 

Ron Resnick

Site Co-Owner, Administrator
Jan 24, 2015
16,017
13,346
2,665
Beverly Hills, CA
Hi Fred,

Thank you very much for starting this interesting thread!

If a tape was recorded originally with one equalisation, and if a copy of that tape is recorded, is there any merit to the notion that the copy should be made with the native equalisation of the original master?

For example, if an original master tape was recorded using NAB, and a copy is made of that tape, is there any reason the copy should also be equalised in NAB –– the native equalisation of the original tape? Or does it not matter if a tape originally recorded in NAB is copied in IEC?​
 

Steve Williams

Site Founder, Site Owner, Administrator
Probably any knowledgeable analog tape professional is aware that the long-outdated 15 in/s IEC2 recording and playback equalization standard (formerly known as NAB) has serious issues in the regions approaching both frequency extremes. This was established more than four decades ago and lab measurements and the findings of expert listeners remain in solid agreement. I don't recall this as ever being controversial. You could easily repeat the tests today.

So, if someone is reporting that they prefer the sound of the IEC2 15 in/s equalization standard over that of the IEC1 15 in/s standard (formerly known as CCIR), my own first instinct and suspicion would be to look closely at their equalization chain, both in recording and playback.

Just as a carefully and correctly performed measurement of a phono stage's accuracy in tracking the RIAA curve is an essential first step to revealing, quantifying and distinguishing why different phono playback electronics can have audibly different sonic characters, so too it is with analog tape. Arguably, even much more so.

Unlike the case with vinyl pressings (the consumer medium), a series of calibration tones were placed on each master tape reel by recording professionals who wanted to be certain that playback could always be performed with the exact needed complementary equalization.

I suspect that no knowledgeable or serious listener here would ever accept a randomly adjusted graphic equalizer being inserted into their playback system. It's long been accepted that even mild response anomalies are disqualifying in high-end audio. (Or that they ought to be.)

In my work, using a measurement protocol with one-twelfth octave resolution (typically measuring over nine or ten octaves with one hundred or more discrete frequencies) I observe certain accepted professional tape playback electronics designs that can routinely align to a given equalization standard to within about one-half a db, from 40 Hz to 22kHz.

Distressingly, I have also measured evaluation non-professional units that cannot be brought closer than eleven dB at some points. Caveat emptor. Any readers of this forum who are thinking of purchasing specialized tape playback electronics would do well to first verify the performance of the unit. At the very least, be certain that what you are buying closely tracks your specified equalization curve with the repro head supplied.

FWIW these two tapes are the best recorded tapes I have ever heard and yes it was NAB
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
Hi Fred,

Thank you very much for starting this interesting thread!

If a tape was recorded originally with one equalisation, and if a copy of that tape is recorded, is there any merit to the notion that the copy should be made with the native equalisation of the original master?

For example, if an original master tape was recorded using NAB, and a copy is made of that tape, is there any reason the copy should also be equalised in NAB –– the native equalisation of the original tape? Or does it not matter if a tape originally recorded in NAB is copied in IEC?​

Hello Ron,

The short answer, arguably, is no.

A critical step when making any copy of an analog master tape, is to make certain that the reproducer's (that's the name given to the special machine used for master tape playback) repro electronics are exactly aligned (in other words, are complementary) to the equalization on the master. Flat response on playback (reproducing) is the goal. The calibration tones on the master reel are there to help confirm this.

Say you have a 15 in/s master recorded in the 1960s on Scotch 111 tape, with the old NAB (IEC2) equalization. So that equalization is what you need to dial in on the reproducer's electronics.

But over on your copy production line, you'll undoubtedly be using modern tape and you are completely free to now use any equalization (and tape formulation, track width and speed) that you might wish. Production considerations for the copy generation should obviously be based on a solid understanding of the technical details of what you're doing and the tape stock you are using.

If done right and using modern tape, a 15 in/s IEC1 copy should come out better than if you had made that copy in NAB (IEC2). Here's a short overview of the reasons why:

http://mrltapes.com/equaliz.html
 

Fred Thal

[Industry Expert]
Jul 15, 2016
161
11
123
Funny, I never thought about that. I assume many tapes are already equalized if they're sold as just music tapes, but for any that are literally masters from direct copy the situation is different.

In the recording industry this has caused a lot of gruff over the years. For example I believe it was all the old Capitol machines had identical tones (EQing) among all their tape machines used with the master tapes that would be mastered for being cut to vinyl. For them it might be easy to dig out old masters and do a digital mastering. But for everyone else that had tones (EQing) written down on paper along with the tape, they're leaving unknown mixing to the now digital masterer if they don't have the same machine and notes... or for the person who got a copy of a master. Heh, what does one do? :eek:

I presume the method you mention helps you get an approximation of tones to use?

Actually, practically all analog audio tapes employ record equalization. There are many different equalization standards that can be sorted according to tape speed.

Consider vinyl mastering (the process of cutting a lacquer) where equalization is introduced in order to better cut the signal into the groove in a way that facilitates optimum playback quality. To play the resulting record, a complementary playback equalization (RIAA) is employed.

So it is with tape. The record equalization is there to optimize getting the signal onto the tape. But there are different equalization standards we need to be aware of. The issue is making certain that your complementary playback equalization is correct. (Not introducing serious response errors.)

Also, by definition there really can be no such thing as masters made from direct copy.

Employing more data points (such as from a one-twelfth octave tone ladder sequence) is done to better characterize the accuracy (or lack thereof) of the playback equalization response. Special calibration tapes are required for this.
 

c1ferrari

Member Sponsor & WBF Founding Member
May 15, 2010
2,162
51
1,770
Practical question, Ron -- thanks!
Thanks for your reply to Ron's question, Fred.
 

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