Analog Tape Trivia

[LarryF]

Hi Jez,

You can record sound on tape without using bias, it will just be very distorted. The bias was primarily to reduce distortion. A secondary effect or unintended or un-needed effect, was that the bias level changed high frequency response due to self erasure by the bias signal. High frequency response is primarily a function of the equalization circuits, which were pretty extreme. Since it was more difficult to change equalization, a recording engineer could tweak the bias to change high frequency flatness. The question on the test is quite good because there was a lot of misunderstanding about the function of the bias and this question brings it to light.

 

Let's simplify the point. First a designer of a deck would pick a tape based on cost, quality and Christmas gifts from the vendor. With the tape running, the bias would be adjusted by the designer for minimum distortion. Then recording equalization would be tweaked for flat response when the recorded tape was played back. The playback response of that deck would have already been set by playing a master tape recorded to whatever standard was being met, NAB, CCIC, 3-3/4, 7-1/2, 15 ips. You also had to take into account how many times the master tape had been played, as high frequencies wore down faster. After all this, you shipped the machine to all your customers.

 

The customers would use the specified tape and all was well. Except they had different vendors, price points or just wanted to be different. With different tape, they got different high frequency response. The bias was relatively easily to adjust on most all machines to flatten the high end response. The distortion would not be optimum now, but that wasn't as obvious as frequency response. The saving grace was distortion and response on harder or softer tapes magnetically, went in the same direction. So more or less bias for distortion and/or response tended to be in the same direction as you changed tape types.

 

In sum:

1. Bias was to reduce distortion. The sound was really  horrible without it.

2. A secondary effect was to change high frequency response. The designers would have preferred that bias had nothing to do with response.

Best Regards,
Larry Fisher
Lectrosonics

[mikewest]

In the 80's I was asked by 3M to give a lecture on audio tape to their sales force.

 

Here is an extract:

 

The Germans perfected HF bias and used it to effect for recording Hitler's speeches.

Using recordings they made it appear that he was giving speeches all over Germany.

 

By the end of the war some 174,890 Km of tape had been produced in Germany

(or 480,000 x 7 inch rolls of tape) and the rest of the world had produced none!

 

 

When the allied forces moved into Germany at the end of the war, their policy of

dividing the industrial giants resulted in the factories of IG - Farben being separated.

Their factory at Wolfen became AGFA and their larger one at Ludwigshafen was

to become BASF.

 

mike

[soundtrane]

9/10... not bad for a 44 yo...