NAB, CCIR, IEAC emphasis curves

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NAB, CCIR, IEAC emphasis curves

Post by vbsh » Tue Apr 28, 2015 3:10 pm

Hi all,

My english is not perfect, so I will try to put it as simple as possible.

I was reading an article about reel to reel tape recording and there's one thing that really confuses me. It mentions that the NAB and the CCIR curves are decreasing the high frequencies at pre-emphasis stage,
but at the post-emphasis or de-emphasis stage (if I'm correctly using these terms as synonyms), there are complementary curves that boost the high frequencies, so that in the end we'll get a flat frequency range. So far I get this. However, in the other chapters, it mentions that the high frequencies should be boosted at recording. But wait a minute, they just said that the NAB and CCIR curves are meant to roll off high frequencies. Isn't this contradictional or I'm not understanding it right?

Source: http://www9.dw-world.de/rtc/infotheque/ ... ec_06.html

Also, many online articles and online books define emphasis as (I'm paraphrasing): "Boosting the high frequencies at recording and lowering them back at playback", but in the case of NAB or CCIR there's no boosting, but attenuating of those frequencies at the recording stage. Maybe they meant RIAA or maybe something about telecommunications?

Then, in the following article, we can see tests of various tape recorders' frequency responses, all of them NAB calibrated, and none of them is flat, of course. Then whats the point of all this de-emphasis thing if it doesnt give flat response in the end? Please check the pics there.

Source: http://www.endino.com/graphs/

Thank you very much.

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Re: NAB, CCIR, IEAC emphasis curves

Post by Mo-Tech » Wed Apr 29, 2015 5:23 am

vbsh wrote:. Then whats the point of all this de-emphasis thing if it doesnt give flat response in the end?
Number or spec-wise, analog is never as "perfect" as digital. The point of de-emphasis probably being to give the flattest curve possible considering the complexities involved in analog electronic circuitry, tape emulsion and tape path topology? I.e. averaging them to most circumstances, but as we know tape decks vary to extremes in their design add the vast difference in magnetic media properties, hence each one having it's own sonic character. Analog is alive so to speak, it varies a little even within the the same tape batch on the same machine, it's never a sterile flat static entity many perfectionist number- and graph seekers think it should be. If it would be digital-like perfectly clean, sterile and perfectly flat, the analog tape would been dead much longer time ago IMO (remember the "success" of digital R2R machines?)

Or is it another IEC vs NAB debate?

In this case, to put more oil into flames - IEC sounds better than NAB! :twisted:

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Post by vbsh » Wed Apr 29, 2015 6:27 am

Thank you for your reply Mo-Tech. This solves one of the questions, I guess. And no, this is not a IEC vs NAB debate. I'm just trying to understand both of them.

What confuses me most is that, they say:

High frequencies should be lifted when recording, but when you look at the graphs, you see that the high freq are rolled off, both in NAB and in CCIR. Is this contradiction or am I missing something?


Image[/img]

source: http://www9.dw-world.de/rtc/infotheque/ ... ec_06.html

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Post by Snarl 12/8 » Wed Apr 29, 2015 1:22 pm

My interpretation of that is that you need to lift them manually (use brighter mics, sources, eq, etc.) because they get rolled off by the recording medium. But I could be way off my rocker.
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Post by jhharvest » Wed Apr 29, 2015 5:46 pm

Well, I think there is a misunderstanding. In the section 6.2 there is this sentence:
The standards give clearly defined and relatively easy producible curves for the frequency response of the flux on a recorded tape, the so-called standard flux curve. Such flux curves can only be produced or measured under laboratory conditions.
The graph in the paragraph shows what CCIR and NAB have defined the flux curve (i.e. the "frequency response of the flux on a recorded tape") as being. So to counteract this, the inverse of this needs to be applied at the recording stage.

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Post by vbsh » Thu Apr 30, 2015 3:09 am

Snarl 12/8 wrote:My interpretation of that is that you need to lift them manually (use brighter mics, sources, eq, etc.) because they get rolled off by the recording medium. But I could be way off my rocker.
No, we are talking about circuits (eq filters) built-in in the recording preamplifiers that lift or attenuate certain frequencies automatically. These eq curves are applied on the recording side and a complementary curve is used on the playback side.

Yes, there is a loss of highs when recording and so they were increasing them (pre-emphasis) at the recording side, but then, the graph shows something completely different. It's so confusing.
jhharvest wrote:Well, I think there is a misunderstanding. In the section 6.2 there is this sentence:
The standards give clearly defined and relatively easy producible curves for the frequency response of the flux on a recorded tape, the so-called standard flux curve. Such flux curves can only be produced or measured under laboratory conditions.
The graph in the paragraph shows what CCIR and NAB have defined the flux curve (i.e. the "frequency response of the flux on a recorded tape") as being. So to counteract this, the inverse of this needs to be applied at the recording stage.
jhharvest, your interpretation makes sense, but are you sure?

Meanwhile, I found other articles such as this:
http://laocaudiosociety.net/tech/NABtoIEC.pdf
and this:
http://home.comcast.net/~mrltapes/equaliz.html

So, I'm afraid that things are much more complicated.

QUOTE: In the 1950?s the National Association of Broadcas ters (NAB) adopted a standard in which the high frequencies on the recording side were rolled off above 3150Hz. They also decided that the low frequencies needed to be lifted below 50Hz.

So, they intentionally rolled-off the HF and boosted the LF at the recording stage by using certain circiuits. The graph that I posted before shows what their circuits were doing.

then one more confusing sentence,

QUOTE: "By the mid-to-late 1960s, the wavelength response of the tape had improved so much that a high-frequency cut is necessary to get the standard flux on the tape, and thereby flat overall response."

Then why in many places, pre-emphasis is defined as "LIFTING the HF".
I mean "TO EMPHASIZE" something means to lift it up, to make it stand out.

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Post by jhharvest » Thu Apr 30, 2015 7:55 am

Very interesting. Yes, I was wrong. That Ariel Jansen article indeed very clearly says that HF frequencies need to be cut at recording stage to avoid oversaturating the tape.

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Post by rhythm ranch » Thu Apr 30, 2015 9:23 am

Perhaps the the confusion is related to two different topics being discussed as one: record/playback equalization and noise reduction.

Pre-emhpasis for noise reduction is explained here: http://www.soundonsound.com/sos/1996_ar ... noise.html
Sound on Sound wrote:The forerunner to modern noise reduction was a system known as pre-emphasis/de-emphasis, which works by applying a high frequency (HF) boost to signals during recording, then applying an equal degree of HF cut during playback. In theory, this restores the programme material to its original state while applying top cut to any tape hiss that may be present. Due to the response of the human ear, and because successive octaves of sound each double in frequency range (and hence the energy level of white noise also doubles every octave), random noise is most noticeable at high frequencies, which means that pre-emphasis can bring about a worthwhile reduction in noise. The other reason that pre-emphasis works as well as it does is that in most naturally occurring sounds, the high frequency signal components are much lower in level than the low frequency components, so a significant amount of high frequency boost can be applied to a 'typical' signal before the high frequency level comes up to that of the low frequencies. If this were not the case, pre-emphasis would offer no advantages as any significant degree of HF boost would simply cause the signal to overload.

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Post by vbsh » Thu Apr 30, 2015 11:47 am

rhythm ranch wrote:Perhaps the the confusion is related to two different topics being discussed as one: record/playback equalization and noise reduction.

Pre-emhpasis for noise reduction is explained here: http://www.soundonsound.com/sos/1996_ar ... noise.html
Sound on Sound wrote:The forerunner to modern noise reduction was a system known as pre-emphasis/de-emphasis, which works by applying a high frequency (HF) boost to signals during recording, then applying an equal degree of HF cut during playback. In theory, this restores the programme material to its original state while applying top cut to any tape hiss that may be present. Due to the response of the human ear, and because successive octaves of sound each double in frequency range (and hence the energy level of white noise also doubles every octave), random noise is most noticeable at high frequencies, which means that pre-emphasis can bring about a worthwhile reduction in noise. The other reason that pre-emphasis works as well as it does is that in most naturally occurring sounds, the high frequency signal components are much lower in level than the low frequency components, so a significant amount of high frequency boost can be applied to a 'typical' signal before the high frequency level comes up to that of the low frequencies. If this were not the case, pre-emphasis would offer no advantages as any significant degree of HF boost would simply cause the signal to overload.
rhythm ranch, thanks! maybe you have a good point, but many websites and online books mention NAB, CCIR, Tape EQ and emphasis as if they are all synonyms. Or at least it seems so. Check here for example:
http://www.dilettantesdictionary.org/in ... tion_curve
or here: http://www.soundfan.it/en/recorders.html

Meanwhile, I found the book "The Art of Sound Reproduction" by John Watkinson, where in the Chapter 9.7 titled "Pre-emphasis AND equalization", he explains about these things as if they are something different.

QUOTE: "Both of these are processes in which frequency-dependent filtering is applied. They differ in principle and achieve different purposes. Pre-emphasis is part of the interchange standard for the tape format and is designed to reduce hum by boosting low frequencies in the record process and applying an equal and opposite attenuation on playback. Equalization is necessary to compensate for losses and deficiencies in the record and playback processes whilst maintaining interchange".

But then, why boosting the LF, and no mention of the HF?

and there is this scheme which shows emphasis and EQ circuits as separate things.

Image[/img]

Then in the book "Audio Electronics" by John Linsley Hood it says:

QUOTE:"in tape recording it is assumed that the total inadequacies
of the recording process will lead to a r?manent magnetic flux in the tape
following a recording which has been made at a constant amplitude which
has the frequency response characteristics shown in Fig. 1.8, for various
tape speeds. This r?manent flux will be as specified by the time-constants
quoted for the various international standards shown in Table 1.3.
These mainly refer to the expected HF roll-off, but in some cases also
require a small amount of bass pre-emphasis so that the replay de-emphasis
- either electronically introduced, or inherent in the head response -
may lessen 'hum' pick-up, and improve LF signal-to-noise (S/N) ratio.
The design of the replay amplifier must then be chosen so that the
required flat frequency response output would be obtained on replaying a
tape having the flux characteristics shown in Fig. 1.8."


Image

So here they ASSUME how the frequency response of the magnetic flux will look like on the tape, and they take the necesary measures to counteract it? This is what jhharvest said in the previous reply.

But then, other sources say "Oh no, that's not the assumed freq response of the flux, it's what we intentionally apply on the recording side by using a certain circuit. We intentionally boost or cut certain frequencies, it's not a consequence of the nature of the tape or the machine"

I'm totally lost now. The person who can explain this in a concise way should get a Nobel prize.

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Post by themagicmanmdt » Fri May 08, 2015 3:02 pm

Hey everyone -

The topic of tape's recording equalization, and what is happening, is the single biggest misunderstood topic when it comes to tape machines.




Ok, here's the summary:

- Due to the physical nature of the tape recording process, there is a short wavelength (ie, high frequency) loss 'built into' recording to tape.
- To overcome this, an EQ boost is used to overcome the tape mechanism's loss.
- If all of this boost is provided in repro, we have wayy too much noise.
- If all of this boost is provided only on record, we saturate the tape too easily and soon for high frequencies.

- Splitting the difference, and providing a HF boost on both RECORD AND REPRO is what happens.

- The misnomered 'cut on repro' is the physics of 'what happens'. This is not an effort of 'noise reduction'. This is 'what happens' when you use tape.


Here's the classic paper that illustrates this best:

http://home.comcast.net/~mrltapes/mckni ... ements.pdf


Start on page 7 (463 on the bottom.)
Figure 6 shows the *straight up* loss that physics deals us when using tape. (Think of it this way for now. There is a second part of the playback amplifier 'total' design, that I mention at the end, but is not a part of the EQ standard. Read on...)

You can start reading section 3... especially 3.1.... ' Division of the Equalization'....

Page 8 (464), fig 7 shows, wonderfully, the idea of how much 'boost EQ' is used for 15ips/sec, NAB eq.




Of course, yes, the goal here is to balance the two issues:
- noise cause by too much repro boost,
- HF saturation caused by too much record boost.


Different EQ standards have been proposed to balance between the two, depending on which one you care more about.

To make things even more interesting, the record EQ and the playback EQ, although boosting, are not identical in design, but rather designed to work 'in tandem'. (Page 8, 3.2: Equalizer Response Shapes.)



so, that's the 'NAB/IEC EQ' part of things.


*but wait! there's more than just this that happens!

...the 'differentiating playback' nature of inductive heads. the amplifier also has another design consideration to 'balance out', once again, a physics principle of tape machine reproduction. this is a 'given', and not actually part of what the isolated topic of NAB/IEC EQ does and why it was instated.

summary of compensating for the 'differentiating playback':


- Inductive head playback looks like a line with positive slope, starting from low freq's and going higher. This is another natural phenomenon. This is called 'differentiating frequency output of playback from an inductive head'.
- Playback amplifier plays this back as the opposite, a line with a negative slope.


This is said, in a short paragraph, and without charts, here:
http://home.comcast.net/~mrltapes/equaliz.html
- Third paragraph.


*******************
So, it is necessary to consider the playback amplifier as having two distinct 'roles':
- one, to provide the integrating 'cut' to the natural phenomenon of the record heads' 'differentiating playback'. (After you do this, then this is how the graph of Page 7, figure 6 previously mentions.)
- two, to further combat HF loss using the dual rec/repro boosts in relation to an agreed-upon EQ standard.

******************




The last thing is that the thought of 'noise reduction', by boosting before and reducing after, is the concept of, well, noise reduction. However, classical noise reduction isn't done by EQ, but rather by:

- compressing the high frequencies
- printing them 'hotter'
- expanding the high frequencies at the same ratio they were compressed,
- then bringing down the playback level of the HF,
- which has the effect of reducing hiss/HF noise.



If I've missed something, please correct me! Yet, I've been in too many situations where people get blue-faced at me. This all makes sense to me, and Jay laid it out wonderfully with those graphs I presented.


:D

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Post by vbsh » Sat May 09, 2015 8:32 am

themagicmanmdt, thank you very much for your detailed reply. I read the link that you posted. Thanks for that. I understand a lot of it, but I must admit that I don't understand everything. I'm trying to put everything into few sentences that everyone would understand. You already did that, but I wish it could be even simpler.

So far, I learned this (maybe I got it wrong):

1. THIS IS ONE THING. LET'S CALL IT JOHN.

There are HF losses during the recording process (there're some causes, long story). It's an inevitable evil. Those frequencies are not cut intentionally, it's just happens because of the characteristics (imperfections) of the tape recording technology. It's physics and you can't do anything about it. In such case, the frequency response chart will show something like this: \. The HFs go down and the LFs stay up (see fig. 6 on p. 7 in the PDF, in the original p. 463: http://home.comcast.net/~mrltapes/mckni ... ements.pdf).

Here we are not talking about noise reduction, but about losses. To overcome these losses they invented this EQ thing. Boost is achieved with certain electronic circuits and it takes place BOTH during the recording AND playback. Each of the electronic curves has certain values and in tandem they give the desired result.

It's NOT like: they only boost electronicaly during recording, and later, at playback, they cut that boost electronicaly with a complementary curve (see fig. 7, p. 8 in the PDF, p. 464 in the original).

No, the electronic playback curve is not inverse of the electronic recording curve, it's not it's "mirror image".

The playback curve is actually the inverse of the RECORDED FLUX FREQUENCY RESPONSE CURVE, i.e. "THE FREQUENCY CHARACTERISTICS OF THE THING THAT HAS BEEN RECORDED ON THE TAPE" (source: http://home.comcast.net/~mrltapes/equaliz.html).

2. THIS IS A SECOND THING. LET'S CALL IT PAUL.

During playback there will be another problem: the differentiating playback problem. The frequency response chart will show a line which rises 6 dB per octave. It will look like a diagonal line like this: /. See picture here: http://www9.dw-world.de/rtc/infotheque/ ... ng/332.gif .

Here we have a low frequency loss. To overcome this there will be AN EXACTLY INVERSE CURVE (complementary) in the playback amp. The playback amp will do like this: \.


Now as I understand:
- JOHN represents the boosting process during rec and repro.
- PAUL represents the fixing of the repro problem that looks like this: /.

What I don't understand is:

1. What the NAB and IEC standard curves represent?
NAB pic here: http://www.soundfan.it/FOTO/REGISTRATORI/nab_curve.jpg
IEC pic here: http://www.soundfan.it/FOTO/REGISTRATORI/iec_curve.jpg

When the people at NAB and IEC made them, they meant:

a) After we record something without any eq, we think that it will look like this on tape. It's just physics, we do not do anything here. We don't cut, we don't boost. Thats what you get on the tape by default. WE DON'T INTERVENE HERE.

OR they meant:

b) When recorded WITH a recording EQ, the frequency response of the tape flux will look like these curves. Thanks to them, now we know how much EQ we should apply at playback.

c) These curves represent what we will intentionally do to to the signal electronically at the recording stage (some cut here, some boost there)

d) Something else.

Thanks.

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Post by themagicmanmdt » Sun May 10, 2015 11:43 am

Hey! That's a good re-type! That's it!


I hunted down where the 'NAB and IEC' curves you talk about came from.

Those are the flux curves of what a calibration tape looks like without the repro EQ HF boost. Without that context, it looks a bit confusing.




Here's a little more thought (and, this is just my experience using tape and hacking into my Scully 280):


Different formulas of tape throughout the years, and especially talking about the formulas used 'before' the development of 456 - there were a lot of tradeoffs.

I believe that the NAB 'boost' at LF's upon record was a way to deal with the tapes (and machines!) of it's time to get less noise and deeper response.

Furthermore, NAB 'boosts' starting at a lower frequency (3150), which, also, working in tandem with the more 'modern' tape formulas, that we don't get a flat response, and there is too much boost:

This is mentioned here:
http://home.comcast.net/~mrltapes/equaliz.html

"By the mid-to-late 1960s, the wavelength response of the tape had improved so much that a high-frequency cut is necessary to get the standard flux on the tape, and thereby flat overall response. This makes the recording equalizer hard to design, inflexible (hard to get flat for different tapes), and terrible for utilization of the tape -- there's way too much HF "overhead" room, and way too much noise to go with it."

Also, read his intro paragraphs here:
http://home.comcast.net/~mrltapes/mckni ... ing-eq.pdf


So, Jay mentions here that since tape got 'better', ie, more resolution of high frequencies (short wavelengths), that there is less HF loss on tape, which turns out that, when you calibrate your machine to a NAB reel, you get your playback EQ looking great with an MRL (flat tape), then you switch to calibrate your record side, and the record NAB EQ doesn't work 'flat' without actually cutting that record-side EQ!


Yes - there ends up being a strange thought here: 'So, is the playback EQ technically 'tape dependent''?' In a way... yes!

So, what people are finding is that using an EQ that doesn't 'boost' as much - such as IEC, which starts at 4500hz - the result is easier to calibrate with modern tapes.


And, yes, this is why Jay has proposed an EQ that boosts even LESS - it was developed 'in tandem' with the more 'modern tapes he was using in 1974.



This is why NAB, nowadays, has the most hiss - there is barely (if any!) actual true boost happening on record. The tape doesn't need it.


My 280 has a lot of hiss at 15ips NAB, especially when I recently used some reels of ATR. It was because of what I mentioned above.

However, I've experimented and changed the 30ips record curves on my 280. I wasn't quite getting a flat response, but rather I had to juggle between getting 'flat' high frequencies with about a +2dB boost in the upper mids (around 4-7k), or, if I got the mids flat, I didn't have enough boost in the highs.

This is a sign that the EQ shelf is set at too low a frequency. 30ips = IEC.

I inadvertently found that Jay's proposal is the solution - shift the EQ shelves higher. Modern tape is better. It doesn't need as much heavy handed EQ.

I haven't calculated it, but I can see from sweeps and toying around that what I dialed it 'by ear' and 'by meter' shows that my repro shelf is probably centered around 9k.

My 'new dinosaur' now sweeps full range with no more than a half a dB variance up to 20K!

Of course, I don't plan on having the 8 track multitracks sent to another place for any reason.

It would be foolish to do a 2tk master tape machine on something custom. You couldn't send it anywhere to get copied, or cut to vinyl, or anything!

Unless you own your own in-house lathe...

:D



Hopefully this makes sense on why people like IEC a little better.

It's mainly because of the tape formula.

I'm sure if you were using ancient tape, that you might like NAB.


And, that's another reason why NAB @ 15ips sounds 'huge' - not only the head bump, but the big LF boost! Those LF's are probably saturating the tape on instruments like kick and bass guitar, and that tape wully-bully-ness can be just what the track needs. Sounds more like a 'warm blanket' than a 'hi-fi bass sound'.

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Post by vbsh » Mon May 11, 2015 1:56 pm

Thank you very much themagicmanmdt. Now I understand the process like 90%. Not bad at all.

There're just couple of more thing that interest me. I posted this picture previously and I'm trying to understand what this circuit really does.



Image


I checked the book where this image comes from ("The Art of Sound Reproduction" by John Watkinson) and the explanation is something like this:

- The box "a" (pre-emphasis) adds that typical NAB LF boost. It pre-emphasises the LF's. Makes sense.

- The box "b" (EQ) compensates in advance for the HF losses of the recording head. Makes sense.

- The box "c" (integrator) compensates for the repro head LF losses and it lowers down the HF, because the repro head boosts them like this: /. The box "c" has a complementary curve to the recorded flux which is now played back. Makes sense.

- The box "d" (de-emphasis) de-emphasises the pre-emphasis (i.e. that NAB LF boost). Makes sense.

Everything is clear to me here, except two things:

1. We said that the HF boost takes place BOTH during the recording AND playback. During the recording the HF's are boosted by the box "b", but then where's the HF boost during the playback?

There must be some explanation, cause I don't see any box that says something like "boosting the HF's during playback".

Maybe it will happen naturaly due to the characteristics of the repro head? Or maybe it will become boosted through all those mathematical additions and substractions of the eq curves?

2. What if we don't use NAB. What will the boxes "a" and "d" do? There will be no pre-emphasis of the LF's.

Or maybe the HF boost by the box "b" will be counted as "pre-emphasis"?

Thanks.

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Post by themagicmanmdt » Sun May 17, 2015 10:36 am

He says it in the next paragraph:

"In the absence of replay head losses, this gives a flat frequency response (especially at 30ips)... at lower speeds, gap and thickness losses are significant and further eq with a high frequency boost (f) is needed..."

This is really true! When I run 30ips, I find that my trimpot is at a place where it's only adding the slightest bit of HF playback boost.



Not using the NAB LF boost gets rid of the LF parts of the discussion.

It's actually part of the 'sound' to hear those sub frequencies...ahem...'blow up and get fuzzy', if you print a mix hot.


Hmm!

I think we're there!

Thanks for letting me type a lot!
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