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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: [log in to unmask]
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.