high frequency roll-off on RTR?

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Hi_Flyer

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so I was experimenting with my new (to me) deck, printing a few mixes from digital to tape, experimenting with how hard I could hit tape and puch it into the red, just sorta getting to know the deck, etc...

anyway, I noticed some lost high end. Not "considerable", but definitely noticeable... Obviously, this is to be expected, to a degree. But I guess my question, is how much loss of high end is to be EXPECTED? At what point so you suspect lost high end is due to bad tape or calibration issues? I understand this may be hard to explain on a BBS, but until I get some of the right test equipment and learn how to do this crap on my own, I'm sort of guessing.

Part of my curiosity stems from my doubts that the dude that sold me this deck had went over it and aligned it and everything, like he said he did.
 
You want to make sure you are not hearing what is a normal bump in low end with a machine that runs at 15ips that is maybe making you think that there is less high end. (Called head bump) I think.
If you have a computer progarm that will let you record some tone into your reel to reel and then send them back to the computer for comparison would give you a idea of what is going on.
 
The frequency response is actually higher (though not as flat) on most open-reel decks compared to digital. However, high frequency response diminishes as level increases. So, if you are pushing the levels well into the red it will sound fuzzier and warmer, which is a combination of increased distortion and decreased high frequencies.

Other factors like bias and head alignment will also affect high frequency performance. For example, if the machine was biased for GP9, but you are using 456, it will sound dull. And if the head is not making optimum contact with the tape you will lose high end.

Another thing that comes into play when going to tape is increased low end from “head bump” frequencies, which peak at about +2 dB around 70 Hz for the MX5050-8. Normally this makes analog tape @ 15 ips sound fuller and meatier compared to a digital source.

You could even have all of the above factors working together, which would result in a whole lota’ low end and squashed high end.

:)
 
OK makes sense. I was pushing signal pretty hard into the red, I'll have to test at some more moderate levels and see if the high frequency roll off is still there.

The guy that sold this machine said I could use 456 *or* GP9, at least I think it was GP9. It might have been 499, I'm asumming those are both +9 formulations? Whatever it was, I assumed it was a +9 tape. So I dunno if that means he has it set at some kind of middle ground between +6 and +9 because it didn't really make sense to me that it could be set up for both. Ideally I would like to have it set up for 456 cuz I already have a couple reels of that.

I still would like to check some of this stuff on my own, I guess I need a calibration tape and I don't really feel like shelling out $200 for one right now... what computer software can you use to generate tones and use a tool to calibrate the deck?

What would be wrong with using the built on Osc on the deck to record test tones to tape?
 
456 and GP9 are different classes and are not bias compatible. 499 and GP9 are in the same class. You might be able to use 456 OR GP9, but you have to calibrate to one and stick with it. So you can't use 456 AND GP9. Perhaps that’s what he meant.

Unfortunately, you can’t do a full calibration without a pre-recorded calibration tape. Calibration tapes are a standard reference just like a yardstick is for measuring distance. If you don’t know where the machine is set in relation to a known reference you cannot proceed.

You can find other calibration tapes on eBay for less. IMO, the TEAC YTT-1144 and YTT-1144-2 are better for the ½” 8-track format, which TASCAM invented. The YTT-1144-2 calibration tape is the newest, but harder to find. It is about the same as the original, but with the addition of a tone to set speed.

:)
 
no, he told me I could use both without changing the bias on the deck.

He gave me a couple reels of 3M 996, which I guess is +9? I'm pretty sure its sticky shed though. how do I tell for sure?

I'll have to keep an eye out for those tapes, I definitely would love to pick up a calibration tape on the cheap...
 
Hi_Flyer said:
no, he told me I could use both without changing the bias on the deck.

He gave me a couple reels of 3M 996, which I guess is +9? I'm pretty sure its sticky shed though. how do I tell for sure?
Click to expand...

Yeah, he was mistaken about using those tapes interchangeably. I would use 456 or SM911 if I were you.

3M 996 is in the same class as SM900 and GP9. If he gave those tapes to you he may have calibrated the deck for it. That would mean 456 would be over biased, and over driven even at 0 VU... that is if he set the machine to get the best signal-to-noise for +9 tape.

The only way to check for sticky-shed is to run the tape. Best to do that in Fast-Forward and Rewind so as not to contaminate the heads. I don’t think of 996 as having classic sticky-shed, but it may have heavier than normal shedding of oxide. I’ve heard some reports of shedding, but have never seen it myself.

Is your 456 branded AMPEX or Quantegy? Do you know when it was made? Sticky-shed is another thing that will kill high frequencies.
 
I was actually testing with SM911, not 456. I have one or two reels of 456, one of them is old and probably sticky.
 
That would mean 456 would be over biased, and over driven even at 0 VU...
Click to expand...

so if I can determine for sure that it is biased for hotter tape, then is it possible to still use a +6 tape like SM911/456 but just back off 3 dB on the levels? I guess I'm asking if the difference is simply 3dB on the VU meters? Or will the EQ curves be screwed up? I'm not as worried about S/N ratio as I want to go as hot as I can, for most of the stuff I record, grit and distortion is exactly what I want (if I want crystal clean and clear I'll just go to hard disk!).

I'm still trying to wrap my mind around bias, I need to read the manual when I get a chance and really try to digest this stuff.
 
The bias frequency is different for +6 and +9 tapes. If your bias is set for one, the other one will sound wrong. No way around this that I know of.
 
Just a small note.....................make sure you demag the heads and transport if in doubt.;)
 
Bias level

Whoopysnorp said:
The bias frequency is different for +6 and +9 tapes. If your bias is set for one, the other one will sound wrong. No way around this that I know of.
Click to expand...


I'm sure you meant to say bias level....

The frequency of the bias signal is set in the design of the deck (perhaps there is a deck that changes bias frequency but I have not used or worked on one).

The level of the bias signal is set at calibration for the specific tape formulation. Some tapes (406, 456) are bias compatable and can be interchanged. Other formulations are not bias compatable and need to have the bias set specifically for them. It is not exact but comes in "classes" if you will.

Record with the bias level too far off optimum and you will hear increased distortion.
 
overview

Hi_Flyer said:
so if I can determine for sure that it is biased for hotter tape, then is it possible to still use a +6 tape like SM911/456 but just back off 3 dB on the levels? I guess I'm asking if the difference is simply 3dB on the VU meters? Or will the EQ curves be screwed up? I'm not as worried about S/N ratio as I want to go as hot as I can, for most of the stuff I record, grit and distortion is exactly what I want (if I want crystal clean and clear I'll just go to hard disk!).

I'm still trying to wrap my mind around bias, I need to read the manual when I get a chance and really try to digest this stuff.
Click to expand...

Bias is like the carrier signal on AM radio. You need it to play (let's not carry this anology too far).

Bias is a HF signal (100 kHz and up depending on the deck) that is applied at record time. The bias level is set by increasing the bias signal level until distortion is at a minimum and linearity max. Typically, you increase the bias level while recording a test tone until the tone is played back at the maximum possable. This reprensents the most efficient transfer from the record head to tape.

THEN you continue to increase the bias level (which erases some of the tone you are recording) until the minimum distortion of the tone ie reached. We do not have distortion meters in general so the tape/deck manufacture recomments some "over-bias" level (increase bias level until the test tone is X dB lower than the peak tone level). On 456 type tape this is typically 4 dB or so. On 499 type tapes is might be 6 dB over bias.

The lowest distortion point depends on many things (including the shape of your heads....) but in general+- a few dB has minimum effect.

Some engineers set overbias by ear.

-------

The max recording level is in essence the ability of the tape to accept signal level without distorting (well 3% distortion).

A tape like GP9, 499, 996 has a lot of ability to accept the audio signal. thick oxide and perhaps a better "formulation"! 456 has a thinner and 406 less so.

You can calibrate your deck for a +9 tape and drive it "hot" which moves the noise floor down. You could calibrate the deck to +6 and record on a +9 tape and get the same noise floor but add 3 dB headroom.

If you want distortion calibrate the deck to +6 (456) or +9 (499) and record on 406 at a level that gets you the distortion and compression that you want.

Let's hope I added to the thread :D
 
Hi_Flyer said:
so if I can determine for sure that it is biased for hotter tape, then is it possible to still use a +6 tape like SM911/456 but just back off 3 dB on the levels?
Click to expand...

As Whoopysnorp said, you can't get around the bias issue. The different classes of tape have a different bias range, and each model within a class can be tweaked even tighter if desired. The level issue isn’t as much of a problem as bias. SM911 doesn’t sound bad when pushed, but how much you can push it beyond redline depends on the machine’s electronics, track width, speed, and nominal fluxivity @ 0 VU.

A full calibration is really the first step when buying a pre-owned tape deck. You never know who has done what to it, or if they knew what they were doing.

:)
 
Great information in this thread. Beck and EVM. I calibrated mine first by the MRL but then tweaked it slightly by ear with better results. The thing with that is remembering to write down to what degree you changed it. It takes a while to grasp how all this works. EVM you painted a good picture for me by describing the bias as an am signal.
 
evm1024 said:
I'm sure you meant to say bias level....

The frequency of the bias signal is set in the design of the deck (perhaps there is a deck that changes bias frequency but I have not used or worked on one).

The level of the bias signal is set at calibration for the specific tape formulation. Some tapes (406, 456) are bias compatable and can be interchanged. Other formulations are not bias compatable and need to have the bias set specifically for them. It is not exact but comes in "classes" if you will.

Record with the bias level too far off optimum and you will hear increased distortion.
Click to expand...

Ah, I didn't understand it was level rather than frequency. Same principle though.
 
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