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: [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.
|
