8 May
2006
8 May
'06
6:31 p.m.
7. Reel to Reel Tape, Speeds and Software
Equalisation
(Steve Fosdick)
Hi, Steve,
I can record the output of the tape machine running at
7 1/2 inch/sec and
then use software re-sampling (sox, or sndfile-resample) to get a file
that plays at the correct speed but I wonder if I should also be applying
some equalisation in software as multi-speed machines usually have
different equalisation constants for different speeds.
The idea thing would be to record it at 88.1kHz, then you'd only have to
decimate it (i.e., downsample by 2, with no filtering) to get the result
you're after. Reconstruction filters on playback would be giving you free
anti-aliasing. :)
This would also have some (really mild) noise reduction benefits.
So, does the equalisation compensate for the record
process or playback
process at a given speed or a combination of both. If it compensating
only for playback then I should be OK, otherwise I presumably should be
applying extra EQ in software - anyone know how I could calculate what,
assuming I have the usual LADSPA plugins at my disposal and that the
signal has already been through the playback EQ for 7 1/2 inch/sec?
You probably don't need to get very uptight. I think what you're
referring to are the NAB/IEC/DIN EQ curves, but I don't believe those were
ever specified for the speeds you're talking about. If you were worried
about the response for 15ips or 30ips, then sure, it would make sense.
But the odds are that the EQ circuits at your disposal are fairly funky to
begin with, so I'd really recommend doing it by ear. (It always chafed a
little, but even doing pro level mastering, that's what it comes down to
for a lot of tape decks.)
In fact, unless you were given a set of calibration tones for the
partcular machine that *made* the recordings, you really have no other
choice.
This is a really long way of saying, "Yes, EQ compensates for _both_
record and playback."
HTH.
Phil M
P.S.: If these are mono recordings, you will get about 6dB of noise
reduction by summing the two channels of a stereo playback machine to
mono!
--
Dept. of Mathematics, 342 Machray Hall
U. of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
Office: 446 Machray Hall, 204-474-6470
http://www.rephil.org/ phil at rephil dot org