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Re: echoplex - hi freq overload



At 05:08 PM 3/10/98 -0800, Bret wrote:
>Recently I started looping my Alesis HR-16 drums with the echoplex dp.
> Prior to that I had input mostly bass guitar, and electric guitar.  

hey, I loop my old HR-16B sometimes. I like that old guy. I'm a sucker for
live reverse snare. Not to mention micro length delays with lots of
feedback. You'd never imagine a simple drum machine could be so fun.....

>While looping high hats and cymbals, I noticed that I have to be very
>careful with the input level (keep input LED only green), or I get an
>unpleasant distortion.  Per the input LED I seem to be able to input a
>much larger signal for bass drums (or bass guitar and guitar) and
>snare  than the cymbals. 

The loop section is more sensitive to high frequency sounds. If you
experience that high freq clipping, you just need to turn down the input a
bit, and it'll clean up. We did use preemphasis/deemphasis around the
convertors, which is why high frequencies will tend to clip before lower
frequencies do. The point of it was to shape the noise out of the 
convertors
to be less noticeable. (another perceptual thing, high frequency noise is
more irritating and noticeable).

The level LED on older units is a bit misleading. The orange and red points
were set too high, so it didn't indicate clipping soon enough. You 
mentioned
changing R7 to 75k, that will set the LED in a better range. It will also
help it match the feedback LED better. People had a lot of trouble with
that, because you naturally want to set it where it's just turning red, 
like
you would with tape. And in the old case that was way too much. It's better
to put it where it's just turning orange. I think they changed this in the
production, so is shouldn't be so much a problem on newer units. Still, I
always put it where it's just hitting orange, so there's a little headroom.

Setting it with the drum machine would be fairly easy, just set up a
sequence with crash cymbals at max volume, and on the plex turn the the mix
to wet, feedback to min, and set a real short loop with overdub on. Then 
you
will be hearing the input entirely through the digital section and it 
should
be easy to set the level right. When the cymbal sounds normal, that's the
right spot. 

It's easy to get this clipping with guitar distortion effects too, so be
careful not to turn the input level up too much in that case too. You
basically want to set the gain with the worst case sound, which is the one
with the most high frequency stuff.

If you do the gain mods on the web page, it's much easier to deal with. The
original version had way too much input gain and not enough output, in my
opinion. The gain mods are a great improvement.

I think Oberheim has made these changes in production now, so I think the
newer units should be a lot easier to handle. Of course, I can never be 
real
sure what they are and are not doing, so if the input seems too sensitive
and the output too wimpy, get out yer soldering iron....

kim

_______________________________________________________
Kim Flint                       408-752-9284
Mpact Systems Engineering       kflint@chromatic.com
Chromatic Research              http://www.chromatic.com