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Re: [ARSCLIST] RIAA EQ software
Greetings all,
I do not know if this is important in this discussion, but, as I
understand it, the reasons for playback EQ in tape and in discs are
different. In playback of discs, you are trying to de-emphasise what
was emphasised during recording, and vice-versa. In tape, it is a
physical characteristic of magnetic induction that output will increase
6 dB per octave; and then there are the playback head losses above a
certain frequency, related to head gap and other factors. Therefore, a
correct and complete playback curve for a recording "straigh off the
head" must be tailored to the specific playback head (and transport)
used, and cannot be taken from a "standards" book.
There is an excellent explanation of this here:
http://tinyurl.com/5wl8cx
Cheers,
Marcos
On Aug 25, 2008, at 12:14, Richard L. Hess wrote:
Hi, Folks,
I think perhaps this discussion could become more valuable if we think
about some of the different classes where we might need to apply this.
Since I don't DO discs, I would like to start with an analogy from the
tape world.
No, Tom, I don't connect my A-D directly to the head and do
NAB/CCIR/IEC equalization in the software. For one, you need an
integrator as well as the equalizer as the voltage output rises 6 dB
per octave off the tape head.
BUT, I do record the machine outputs and the Dolby/dbx/Telcom outputs
as TWO stereo pairs when I'm doing music work. I may not deliver the
non-decoded file, but I may. In this way, if there is ever any
question about the processing, we can do the analog re-processing
without having to replay the analog tape.
I also use the DSP to correct for using instrumentation recorders for
playing back audio, so in that sense I do add the NAB curve (or some
approximation that better matches the actual sound on the tape since
one might suspect that the recorder wasn't very accurate to begin
with). As you know, instrumentation recorders are essentially constant
current record which means that there is some playback equalization
for the thickness loss and other losses in the tape path. The only
pre-equalization is typically to adjust for high-frequency losses in
the 300 kHz - 2 MHz range, depending on tape machine and that's only
adjusted at the highest speed (60-120 in/s).
Conversely, I attempt to dial out all audio EQ when I'm reproducing
instrumentation tapes on audio recorders--but that probably won't
happen again now that my instrumentation recorder population has
increased beyond all reasonableness <sigh/smile>.
There is another use I make of DSP and that's correcting odd off-speed
replay when I need to, but, for my common off-speed replay (like 3.75
-50% varispeed to obtain 1.88 in/s), I have a separate preset that has
been equalized to the 1.88 in/s MRL tape on the machines.
Now back to discs.
I would think that there is a time and a place to use both systems. I
would guess that there might be a good reason to use hardware RIAA EQ
(and by the way, the National Association of Broadcaster's publication
of that curve is now available as are the tape standards at
www.richardhess.com/tape/history/ ) for playback of "late model" discs
and use software for the wide variety of other EQs.
There is a movement afoot (partially pushed by TracerTek -- the people
who sell DC7 and flat preamps) that flat transfers with digital EQ is
superior. I see Chas. Lawson's similar post, but I'm surprised analog
components drift as much and as quickly as he said, but I don't doubt
it happened.
I think that dividing the discs into different groups and then
discussing each group might be more informative than lumping all discs
together.
The original question was for RIAA but there are few RIAA discs that
we should be transferring, even in Canada, as the true RIAA standard
came into effect what, 55 years ago? I don't think it was adopted
immediately, was it? I recall seeing London/Decca FFRR recordings (but
maybe they were done with the RIAA curve and they just used the
acronym for marketing) into the 1960s and perhaps the 1970s.
The freeware from Australia that I pointed Scott to last night does
"delta" equalization, so it appears that you can tell it that you used
an RIAA preamp to make the transfer and then it changes RIAA to FFRR
or whatever.
Finally, when I have attempted to do RIAA playback, I have noted a
WIDE variety of apparent responses from built-in preamps in receivers.
I use an RTS-405 preamp which has adjustable R and C cartridge
loading and I first adjusted it by ear based on several LPs and a CDs
that I believed to be mastered from the same source. After doing that
I found I was within about 2 dB of the CBS test record at 12 kHz. I
tweaked a little more to bring the CBS test record into alignment.
This setup is much brighter and has more detail (without sounding bad)
than most of the preamps in receivers. Of course, that exacerbates
surface noise, but, when properly adjusted there was surprisingly
little difference between the CDs and the LPs other than the noise of
the LP.
This has led me to speculate that at least one source of the critique
of CDs as "sounding harsh" comes from a combination of overly bright
speakers and rolled off RIAA preamps so that some of the disc noise is
reduced.
So, I think this whole area needs further discussion and narrower
categories.
Cheers,
Richard
At 09:09 AM 2008-08-25, Tom Fine wrote:
OK Charles, I'll take the bait ;) ...
What DSP RIAA "decoder" do you recommend? Do you also do this for
tapes (ie take a flat feed at either head levels or no-EQ amplified
level and decode in the computer)?
-- Tom Fine
Richard L. Hess email: richard@xxxxxxxxxxxxxxx
Aurora, Ontario, Canada (905) 713 6733 1-877-TAPE-FIX
Detailed contact information:
http://www.richardhess.com/tape/contact.htm
Quality tape transfers -- even from hard-to-play tapes.