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Canyon 's Collection

Grateful Dead   02/18/1971

Capitol Theater, Port Chester, NY
Source SBD -> Dolby A Reel -> Dolby A decode -> DAT -> Sound Forge NR -> CD-R
Media audio   3 discs
Notes hd1
Tech Notes SBD -> Dolby A Reel -> Dolby A decode -> DAT -> Sound Forge NR -> CD-R

The Sound Forge noise reduction was used to attenuate a very narrow
frequency band (in the vicinity of 8000 Hz) which was present as
interference on the original reels. See my notes below for details.

The original reels of the Port Chester run of '71 were audibly
tainted due to a cable problem. The result is a tone which hovers
around 8000 Hz with an amplitude of about +20 dB above the noise
floor. This translate to a very annoying pitch, especially during quiet
sections, which is in the old-tv-tube or lousy-hard-disk or maybe even
the-sound-you-hear-after-you-slam-your-head-against-a-concrete-wall
class of 'whine'. Because this interference is 'monochromatic' it's
easy to get rid of if you have the processing power and a quick
FFT algorithm. I used the filter-shaping capabilities of the Sonic
Foundry Noise Reduction Plugin to tailor a very narrow notch filter
which attenuated the interference without affecting the rest of the
signal. And it's all done in the digital domain. Huzzah!

As a side note, I actually ended up crafting many filters to process
these shows. The frequency of the interference actually drifted anywhere
from 7700 to 8100 Hz during a given show. This was probalby due to a
slightly wandering reel speed. It made my work harder but when I tried
a much wider filter I started to be able to hear the attenuation in the
music. The filters generally go like this:

25 dB /- - - - - - - -\
/ \
/ \
0 dB | |
7830 7900 7970

The end result is no more annoying whine!

I also removed some of the nasty mic feedback in the 2/18/71 Bertha
using this type of filter.

Leigh Orf <orf@mailbag.com>
Trades Allowed Yes
Owned by Canyon · Last Updated May 23, 2023