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ARSCLIST  March 2010

ARSCLIST March 2010

Subject:

Re: Disc EQ in the digital domain

From:

Parker Dinkins <[log in to unmask]>

Reply-To:

Association for Recorded Sound Discussion List <[log in to unmask]>

Date:

Wed, 10 Mar 2010 21:25:06 -0600

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (68 lines)

on 3/10/10 6:38 PM US/Central, George Brock-Nannestad wrote in part:

> For this reason it would appear that going flat to your A-D, going into noise
> reduction (not here discussing the various characters of noises) and then
> perform the EQ digitally, while you are at it, would give the best effect.
> However, as more and more noise reduction is using physiological properties of
> the ear for noise masking, possibly this approach will be thwarted, as the ear
> does not like to listen to a non-boosted bass from a pre-vinyl record, and so
> the wrong masking procedure would be applied if you had not already corrected
> the response in the pre-amp. It is probably a compromise situation.

George, hi.

In the early days of digital audio, click removal from disks was thwarted by
well meaning people who used the Packburn, low pass filters, or even manual
oxide removal.

Automatic digital click removal consists of detection and then repair. Since
a click is usually just a burst of white noise, any muffling of that burst
made automatic detection and repair more difficult, and sometimes even
impossible.

Click detection was so important that, in some cases, you would get good
results by creating a duplicate file using a high pass filter, and using
that file to flag the areas to be declicked. The high pass file would be
removed and the original file put back in its place so actual repairs could
be made. 

As a logical progression, click removal before imposing the RIAA playback
curve significantly improved in the click detection process because the RIAA
playback curve reduces high frequencies.

With modern tools, transferring flat, removing clicks, and then imposing the
RIAA playback curve is not as essential to the click removal process as it
once was - but it's certainly the conservative and correct approach for
archival work because we don't know what further developments will occur
with digital technology. And it doesn't hurt in the click removal process.

Digressing: I'm not sure I would describe the click removal process in terms
of masking, because masking has a particularly strong and different
connotation with lossy audio compression. In click repair the actual work is
done by interpolation, in which the damaged audio is actually replaced with
new audio using algorithms based on what came before and after the click
damage. If the click duration is short enough, almost anything can be used
to replace it. The problem occurs when the damage is a longer duration, and
especially when the damage occurs with an instrument we are familiar with,
like a piano. In a longer problem area, you can sometimes build across by
interpolating from one side and then the other until they join each other.

Back to the main point: the conclusion of R.S. Robinson in my previously
cited 2007 AES Paper 7185 was that in most cases the bass resolution
truncation was 1 bit or less, which is negligible in light of the 24 bit
resolution capability of modern converters, and in all events is more than
compensated for by enhancement in the higher registers, where aural acuity
is greatest (like those Fletcher and Munson boys described).

Ironically, linear phase digital eq can cause problems because its
implementation requires combining bands after changing each band's
individual delays in an attempt to create a linear phase eq (which cannot be
perfectly achieved). Ironically, this can be first audible in the lower
registers. For that reason, some people prefer regular, precision digital
eq. 

There's probably lots of poorly implemented digital eq in the world, too.

-- 
Parker Dinkins

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