外国人对NAB两轨磁带和DIN立体声（蝴蝶头）信噪分析

红灯记

Playback of NAB 2-track tapes on a DIN Stereo (Butterfly) head2018-03-022007-09-12

I received a query from Sweden today asking

I have a Studer machine with butterfly heads with which I’d like to reproduce
tapes recorded with normal two track heads. Theoretically, how much more noise,
in dB, would I get from playing the “empty” part of the tape?

Let’s look at the assumptions.

Studer talks about 2 mm and 0.75 mm guard bands.

If we take the width of the tape at 6 mm (6.35 mm to be exact, but we have to leave a little guard band at the edges) that gives us NAB tracks of 2 mm and DIN tracks of 2.6 mm

As a quick crosscheck, 2 mm is 79 mils which is between the Ampex standard of 75 mils and the the NAB standard of 82 mils. Close enough.

Let’s first review the calculation for the case of NAB vs DIN with matched heads.

Signal goes up 20 log (2.6/2.0) or 2.28 dB because it’s all in phase
Noise goes up 10 log (2.6/2.0) or 1.14 dB because it is random
for a net S/N improvement of 1.14 dB

For the NAB tape played back on the DIN heads,

Signal doesn’t go up
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Net S/N degradation of 1.14 dB

One other major consideration: IF there is JUNK in the NAB guard band, then all bets are off. This includes centre track time code (CTTC) as well as gapped erase heads on the recorder (common on most consumer recorders from the ReVox A77 on down). Pro machines that used Woelke erase heads only had gapped erase heads when there was CTTC (at least in the Studer A810 and Sony APR-5000 series). The plain stereo machines that I’ve seen have had the “checkerboard” erase heads that fully erase the entire width of the tape and if they miss anything it’s right down the centre and still won’t impact the DIN playback heads.

I am a purist about this and regularly swap DIN/NAB heads on my A80s when I have DIN recordings — I can also do it on my APR-5000s and my A810s but most DIN recordings get the A80s as they sound a hair better than the APRs which sound a hair better than the A810s, but all are really quite close.

With all the math, the reality is that many archives are using DIN playback machines here in North America as the largest group of surplus, well-cared-for machines to hit the market came from CBC which used DIN heads in many applications.

I would rather play an NAB tape on a Studer A807 with butterfly (DIN) heads than on a ReVox A77 with NAB heads if there  is no centre channel junk.  And since I think the A807 isn’t as good a playback deck as the  A810, the same holds true more so with the three above-mentioned machines. But, I’d rather play an NAB tape on a Sony APR with wideface Applied Magnetics heads than on a Studer A80 with butterfly (DIN) heads.

Content and care of original recording also matter. If these are special recordings and there are enough of them, I’d go to the trouble of putting an NAB head in a spare headstack for your machine.

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文超玩胆

英语没学好，等于文盲

红灯记

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宏声3

我太难了