Quote:
Originally Posted by paulsherwin
...I seem to remember that the two different layers resulted in a non linear response, and that the tapes weren't physically stable, though it's a long time ago and I may be misremembering.
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I did a little reading on Ferrichrome cassettes. It seems there could be a dip in response around 5kHz. I'm guessing it was above the frequency where the Ferric tailed off but below that where the Chrome kicked in.
The Chrome was a thin layer on the outside, so intimate with the head, while the Ferric was behind, separated from the head due to the thickness of the Chrome layer, with the result (I'm guessing here) that the Ferric layer probably had what's called a "spacing loss", which inevitably results in a loss of highs. Similar to a dirty head, at least for the Ferric layer.
Interestingly this is also why record bias generally was always somewhat of a trade off. The particles on the outer layer closest to the record head's bias field got an overdose of bias while those at the back of the magnetic later got an underdose. In practice we overbiased the back layer a little where most of the lower frequencies were recorded, but not too much or we would overbias (partially erase) the layer intimate with the head, where the high frequencies were recorded. Record bias was a tradeoff.
In the earlier days, bias was set using a mid frequency like 400 Hz or 1000 Hz, aiming for a slight level drop below maximum due to overbias. Later on a high frequency like 10kHz was used as it was more sensitive to bias changes. For a reel to reel machine different level drops at 10Khz were specified for different tape stocks, such as -4db, -5db, -6db etc.
It seems the advent of the Japanese super ferrics from such as TDK and Maxell with their much better high frequency performance made the complexity and cost of the dual layer Ferrichromes redundant, and perhaps outperformed them overall.
I have a few old 70's Sony cassette machines with the Ferrichrome position. I dont recall using it perhaps because I rarely came across a Ferrichrome tape when recording.