You certainly created an interesting thread on this list!
I agree with just about all of the information already posted.
Let's look at the numbers a bit.
The 20 kHz loss at 3.75 in/s of a head with a typical gap length of 100
µin or 2.54 µm is about 6 dB without any gap loss compensation. If we
use a narrow-gap head of about 50 µin or 1.27 µm, the 20 kHz gap loss at
3.75 in/s is about 1.3 dB.
The gap length in your Studer A807 is about 3 µm which, uncompensated,
would provide an 8.67 dB loss at 20 kHz. That's what the "Treble"
adjustment is for, more or less.
So, if we take a nice round number of 20 kHz as the highest frequency
that we can reliable reproduce with common heads at 3.75 in/s, it then
stands to reason that at 1.88 in/s, we have a 10 kHz ceiling and at
15/16 in/s, the ceiling falls to 5 kHz. That is still better than the 3
kHz limit on hardwired telephones.
If we look at the digitization equipment, 96 ks/s provides about 45 kHz
of bandwidth. When that is slowed down by a factor of four, the sampling
frequency becomes 24 kHz with an effective bandwidth of about 11 kHz.
So, the head gap length is more restrictive than the digitization system.
In the example with the 3 µm Studer head, there is a null at about 27.85
kHz so when slowed down four times, that null corresponds to about 7
kHz. The audio between 5 and 7 kHz will be rolled off. For example 6.25
kHz will be down about 18 dB.
Certainly digitizing this yourself will work to at least ascertain what
is on the tape. These could be quarter track or half track.
While the Uher was pleasant to listen to at 15/16 in/s, I don't think it
has any phenomenal frequency response.
The ReVox C270 logging series manual claims an upper frequency response
(at - 3 dB) of:
15/32 in/s - 3 kHz
15/16 in/s - 6 kHz
1.88 in/s - 12 kHz
This provides little opportunity for anything approaching high fidelity
until you get to 1.88 in/s. Typically (and for many reasons including
coating thickness and overall optimization of the format and heads for
7.5 and 15 in/s) 1.88 in/s never performed as well in open-reel machines
as it did in cassette machines, with Nakamichi, by and large, leading
the pack in cassettes.
The ReVox (by Studer) C270 was the last model manufactured that
addressed these speeds and is a cousin of the A807 in some respects. I
know some people on this list have C27x loggers (x=track format, they
were available in 4- and 8-track versions).
While IASA TC-04 section 5.4.10 lists 15/16 in/s as "undefined" the
Studer manual, at least, informs us that the EQ for 15/16 in/s (though
the manual has a typo saying "5/16" but that speed does not appear to be
otherwise offered) is
15/16 in/s EQ = 200 & 3180 µs
15/32 in/s EQ = 400 & 3180 µs
So, the math is in line and if you digitize at 96 ks/s you probably
will not degrade the high-frequency response of the recording as the
repro head gap length is the limiting factor.
On 2011-01-12 1:04 PM, George, Stephanie wrote:
> Hello, All -
> I've run up against an interesting scenario (for us, anyway) and I'm asking for some feedback from the collective wisdom of this group.
> We've been doing a lot of our own digitization, but we've now run across a reel to reel tape that was recorded at 15/16 speed. It doesn't appear as if any of our reel to reel recorder/players offer that as an option, so, obviously, we'll need to send it out.
> How common was this recording speed and are there any general assumptions I might be able to make about the circumstances of the recording (i.e., Were there consumer-grade recorders available with this speed option? Years it might have been a popular option?)
> Thanks in advance --
> Stephanie George
> Center for Oral and Public History
> California State University, Fullerton
> (657) 278-3693
> [log in to unmask]
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.