15 Apr
2008
15 Apr
'08
3:39 a.m.
You're recording the output of a 5150?? You mean the original IBM PC? Have
we forgotten how the PC generated audio, until real audio cards were
generated? They just toggled a line connected to the speaker-- thereby
pumped square waves at it. Squarewaves are rich in harmonics, and sound
very harsh. They were reliant on the fixed response of the speak to smooth
it out mechanically. Recording that properly will be pretty hard. You need
an RC filter to try and smooth that out, and I'm not sure what that would
look like. Probably a capacitor from the hot lead to ground, with resistor
in the hot lead before the cap. It's still going to be harsh.
It will be too "hot" certainly, and you will have to pad it down with a
series resistor followed by one to ground. But as much of the distortion is
coming from trying to record squarewaves as from overdriving the recorder's
input stage.
Series resistors or pots won't help enough. You need a T or Pi network with
a loading resistor across the speaker.
-T
At 09:54 PM 2008-04-14 -0500, Randy Dawson wrote:
Connect a series cap to BOTH leads, this will isolate
your input from any
ground fault.
Connect a pot across the input to your RCA, 1K to start, if nothing
happens untill you twist to the bottom end, use a 500 ohm pot, this is
your attenuator so you are not overdriving your input
Randy
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Date: Mon, 14 Apr 2008 21:47:00 -0400
From: rtellason at verizon.net
To: cctalk at classiccmp.org
Subject: Re: Beginner's capacitor question
On Monday 14 April 2008 21:03, Jim Leonard wrote:
all
I have a need to record the output of a (5150)
speaker. Although I
thought that I could just alligator-clip a positive lead to one speaker
terminal and the negative to the case/ground, the output was decidedly
"buzzy" (I assumed it was too "hot" and overmodulating). I routed
it
into a mixer and turned it down (speaker is 5v, not sure what line input
is) but it still didn't sound right.
"Line input" is typically no more than a volt or so...
The output being buzzy -- do you mean while the speaker was sounding or the time? It's quite possible that the mixer
was picking up other
stuff from
inside the case. Or if it's only when
sounding then you're probably
overdriving the mixer input which could have all sorts of effects
depending
on the circuitry there.
> I found some old directions on hooking up a PC speaker to a line input,
> and was confused by the use of a capacitor -- I would have thought that
> a resistor would have been more appropriate, to limit the signal
> perhaps? In any case, here are the instructions, followed by my
question:
Parts required:
- 6' to 12' shielded cable with RCA plug (male) on one end
- Two alligator clips
- One 4.7 uf capacitor
1. Connect one alligator clip to the shielded portion of the cable.
2. Connect the (-) minus side of the capacitor to the center conductor
of the cable and then connect the (+) side of the capacitor to the
second alligator clip.
3. Attach the clip with the capacitor to one of the wires going to your
computer's speaker. Attach the other clip to the metal case (ground)
somewhere (such as a screw or bolt connection).
4. Connect the RCA plug to the auxiliary input on your stereo system or
boom box.
While I have read the wikipedia entry on capacitors, I'm missing
something obvious. My question: Why the 4.7uf capacitor? Does it
serve to limit the signal? Reduce it's voltage? (or increase it?)
Filter the signal in some way?
That's a rather odd way of doing things. I had thought iniially when I this that you were looking at perhaps blocking DC
with a capacitor, which
would probably be a good thing anyhow.
I could be mistaken about this, but my recollection is that they drove
that
speaker with one wire tied to +5V and the other
coupled to ground
through an
open-collector device such as one section of a
7405 inverter, or
similar.
What I would suggest is that you use both
resistance and capacitance
there,
you want to attenuate that signal downwards some
(probably by at least a
factor of 5-10) and you also likely want to block DC from the input of
your
mixer, though there may possibly already be a DC
blocking capacitor at
that
input.
I'd try say a 10K resistor connected to the speaker, a 1K resistor
connected
to ground, and the mixer connected to the common
junction of the
two. Maybe
even make that 1K a trimpot, if you want to be
able to adjust the level
downward further.
Is there some reason you don't want to just use a microphone?
--
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ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies.
--James
M Dakin
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