Tape Tech 101 Questions

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fstrat76

Member
I am trying to get more of a technical understanding of tape and its properties. I thought I'd post some questions to our resident Tape Professors here as most of the information I am looking for is pretty much scattered around the net and not easily decipherable, at least not by me.

I read everywhere about different tape stock and bias compatibility and this or that tape switch setting for consumer grade machines and such.

However, I haven't really found anything on bias compatiblity and how to determine (technically) if two different tapes are bias compatible or eq compatible.

For example, I've read that Scotch 206 is bias compatible with RMGI SM911 (Ampex/Quantegy 456) except that 206 is +3 and SM911 is +6 (more questions on that later).

In addition to the 206 and SM911, I own some older BASF LGS 35 and LP 35. I think I've read that LPR35 is the same as SM911 but the only difference is the tape thickness. But what about LGS 35 and LP 35?

I have two charts, one for BASF tapes and one for Scotch that lists tape properties for each tape stock over the years. The charts show production dates, base material, thickness, backing, and the goodies such as Coercivity, Retentivity, and Remanent values.

Example:

1. BASF LGS 35 Coercivity is listed at 310, Rententivity Gauss 900, Remanent 900.

2. BASF LPR 35 Coercivity is 320, Rententivity Gauss 1650, Remanent 1800.

3. Scotch 206 Coercivity is 320, Rententivity Gauss 1050, Remanent (flux lines) 0.93.

Questions:

How do these values listed in the charts relate to each other and how can I tell if certain tapes are bias compatible or eq compatible with other tapes?

So what exactly does "bias compatible" mean? What about EQ Compatible?
 
Ignore the EQ stuff as far as it concerns bias...THAT has to do entirely with the EQ curve on your machine. If your machine is setup for NAB curve, you record something and take it to your buddy's machine that is the same format but IEC curve its not going to sound right.

Your question about +3, +6 etc...that is a measure of how high a flux level in nanowebers per meter the tape can handle before reaching 3% distortion right? There isn't a direct correlation with this figure and the tape's bias requirement, but there is some relationship because typically a higher output tape has a thicker oxide coating and will require more bias signal to properly excite the oxide so its ready to receive audio from the record head.

The bias requirement of a tape is ALSO driven by the oxide formula itself and that's why I say that the standard operating level of the tape is not a direct driver in the bias requirement. It CAN be but it depends on the oxide formulation FIRST.

Here's my short answer: I've never been concerned with bias compatability because even tapes that are "bias compatible" between different manufacturers are still going to have some variation...there is even likely some variation between batches of tape of the same model and manufacture. So I've never been concerned because I figure if I'm putting a different tape on the machine, if I'm going to do it "right" (take that with a grain of salt), I'm going to setup the bias.

I suppose the wrinkle here is if you are working with a "consumer" machine that doesn't have easily accessible bias trimmers but rather a two or three-position switch for bias level. Other's will have to chime in there as far as what setting to use for tape X.
 
sweetbeats said:
I suppose the wrinkle here is if you are working with a "consumer" machine that doesn't have easily accessible bias trimmers but rather a two or three-position switch for bias level. Other's will have to chime in there as far as what setting to use for tape X.
Click to expand...

My A-4300SX manual has a chart with all of that info. I would assume most TEAC decks with bias switches have as well.
 
I'm not so much looking for switch settings but more so what the tape specifications, such as what coercivity value, means in relation to bias settings.

While I agree the correct way is to bias a machine for one type of tape and stick with it, it's not necessarily practical all the time.


For example, for tape BASF LGS 35, is that a high output tape? +0 or +3?

Can I tell from just having those Coercivity, Rententivity, and Remanent values?
 
As Cory said that it's best to fine tune your machine with whichever tape you plan to use so +1 to that.

Generally speaking, I always look at Coercivity as defining the amount of bias the tape needs and Retentivity as relating to the operating level of tape. Note that operating level can change but not necessarily the bias, which would explain why a +6 tape can be "bias compatible" with a +3 tape. Thus it is telling from your examples, that the LGS is lowest operating level tape, probably around "+1", while the LPR is +6 and the 206 is at +3. Interestingly their "bias compatibility" is in the same ballpark.
 
Interesting, but if I look at the basf chart here:

http://www.aes.org/aeshc/docs/basftape/basftapes.html

Look at basf sm900, which is +9 correct?
It's coercivity is 360, rententivity 1340, and remanent 2410.

Then look at sm 911, which is +6, coercivity is 320, rententivity 1450, and remanent 2320.

So would it reason that the sm900 should have higher rententivity than sm911 if the output level were based on rententivity?
 
Yeah, I wish it all lined up perfectly every time but it doesn't. It also has to do with type / kind of oxide used and not necessarily thickness, as Cory mentioned, so that's why I suppose it can vary that way, as per your example. The SM900 / SM911 is nearly the same (thickness wise) and yet op level is significantly higher on the 900 and so is bias requirement. I think it's due to different oxide mix used.

Still, I feel my "formula" is good enough for rough estimates. For instance, I know by looking at Coercivity I can at least know where a certain tape stands as far as bias vs another tape. Same thing about estimating operating level. I feel that for the most part, this works well, ballpark.
 
Thanks cjacek. Some of it makes sense. However, I don't mean to punch holes in theories, but if I look at Scotch tape values, the rough formulas don't seem to apply.

If we can say that Scotch 226 is Ampex 456 and BASF SM911 bias compatible, the coercivity should be close to the BASF SM911 value of 320. But the spec shows Scotch 226 coercivity at 360, exactly as with SM900.

Unless 226 is not bias compatible with 456, which it may not be, and I'm just throwing it out there, then there is still something missing in the equation.

That of course could be the oxide formula, which we don't have values for.

So unless something is eluding me, the charts seem meaningless without other values or measurements and we are relying not on hard data but what others "say" the specs are.

(However, interestingly, as it should be, RMGI SM9xx is pretty close to BASF SMxx values)
 
Could indeed be a different oxide formula or a value we haven't accounted for... but, isn't it cool to know that something which is oft touted as "bias compatible", for example, like your 226 or even 468, turns out to need more bias just by looking at Coercivity, have the ability to look at and decipher specs? Still, none of this makes any difference if you set your recorder up from scratch. Good topic tho.
 
It is good to know also when you are in the "ballpark" as you say, when you just want to slap a tape on a machine to know that it will be at least be close to compatible (or not). Agree though that a machine should be biased appropriately.

Looking at those charts, it's also interesting to see the progress over the years.

And yes, I would like to see or hear about those other unaccounted values that might help better determine compatibility.
 
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