Hi, Lou,

In order to understand the time constant perspective, one needs to 
realize that most equalization curves (Ampex Master Equalization and 
perhaps others excepted) utilize single pole shaping networks, so the 
amplitude response changes at the rate of 6 dB per octave or 20 dB per 
decade for all curves.

The time constant refers to the 3 dB down "corner frequency" of the 
curve. This can be provided as a time constant or as a frequency.

There is some looseness in this specification because the frequency 
values have been slightly changed to fit into the ISO preferred 
frequency series.

One of the wonderful Web references is
and specifically this for your current question

You will note that in the above-referenced page, 3180 Ás is used when 
referring to 50 Ás, but Jay, I think, uses 3150, the ISO preferred 
frequency. This amounts to about a 1 % ambiguity or (I think) about a 
0.09 dB ambiguity.

I hope that helps.



On 2012-10-28 4:50 PM, Lou Judson wrote:
> Excellent compendium of cassette lore, Richard!
> I'm nearly done with a project of transferring over 120 workshops originated on cassettes, from one to seven C-90s each, from 1979 through the mid 90s. As the speaker travelled around the country and amateurs did most of the recording, they are all over the map in terms of quality! Fortunately it is just spoken word, and the final delivery of MP3s is justified...
> I worked with cassettes during all those years myself producing radio interview programs, so I have extensive experience, and all your references are helpful.
> I have one question though that has mystified me for years: how does one translate the 3180/120  or /70 specification into frequency response? I am seriously math-challenged, and though I know extremely well how normal and high bias tapes act and sound, how does one translate that spec into a frequency curve? I don't think I have ever seen a direct chart comparison... If there is one on your site i have not seen it yet.
> I'm an audio engineer, not an EE, so I can operate them but not design or repair the gear...
> Thanks,
> l
> Sent from Lou Judson's iPad 2
> Mobile 415-271-8070
> On Oct 28, 2012, at 1:14 PM, "Richard L. Hess" <[log in to unmask]> wrote:
>> Ahh, the beloved cassette...where do I begin?

Richard L. Hess                   email: [log in to unmask]
Aurora, Ontario, Canada           (905) 713 6733     1-877-TAPE-FIX
Quality tape transfers -- even from hard-to-play tapes.