All,
I think this discussion will end up being very interesting, but I think
we need to quantify all of the parameters so we are playing with apples
and apples.
Discriminator audio is not always high impedance; many radios, like the
Motorola Micor and GE Mastr II use a buffer to make the discriminator
audio stiff and robust. This is necessary to drive the Volume and
Squelch Pots, along with other control circuitry as in a Station
configuration. While this audio isn't what is directly from the
demodulator, it IS discriminator audio. It isn't advisable to connect
anything directly to a radios discriminator, as it will likely upset the
operation of the radio set, especially the squelch. This is why the
discriminator is followed by a buffer in most of the equipment we use in
repeater service.
That being said, it does matter what the source impedance is. It won't
matter as far as the 6 dB per octave slope is concerned, but it will
matter where the knee of the circuit will occur, and how much audio is
available after it.
So, how about we use a source impedance of 100 ohms, and an audio range
of 50 to 5000 cycles.
The slope of the De/Pre-Emphasis isn't what is important here, as that
won't change in a 2 component circuit, what is important is where the
circuit starts (or finishes) working; which will affect the equalization
of the transferred audio.
Kevin Custer
Bill Hudson wrote:
Discriminator is always high impedance so high impedance into the circuit.
De-emphasized audio is typically used for microphone input, which
would typically be low, but you can hang whatever impedance you want
because in this case, "it's only audio". The output impedance isn't
going to matter much for audio EQ, except really low impedance is
going to "horse down" the level into the transmitter (circuit output).
In this case, you can just about choose your "output" or "transmitter
input" impedance. EQ wont change enough, to make a difference.
------------------------------------------------------------------------
*From:* [email protected]
[mailto:[EMAIL PROTECTED] *On Behalf Of *Doug Zastrow
*Sent:* Saturday, December 30, 2006 6:23 PM
*To:* [email protected]
*Subject:* Re: [Repeater-Builder] Re: audio equalizers for repeater
audio-circuit correct??
Bill,
What source and load impedance are you looking at for this test?
Doug
----- Original Message -----
*From:* Bill Hudson <mailto:[EMAIL PROTECTED]>
*To:* [email protected]
<mailto:[email protected]>
*Sent:* Saturday, December 30, 2006 8:12 PM
*Subject:* RE: [Repeater-Builder] Re: audio equalizers for
repeater audio-circuit correct??
To: Ken
Cc: Laryn
Tell you what Ken. I'll make you a deal.
You have the facilities to do it, and you have the test equipment
to prove it. I'll let you do the work.
The time constant equation is the same, if you put the capacitor
in front or behind the resistor. In fact: You could put a 7.5K
in front of, and a 7.5K behind the capacitor to ground, and you
should get the same time constant, roll off, de-emphasis.
I have never tried it. Here's the deal. You build it and test
it, and if you can make the de-emphasis different by placing the
resistor before, after, or splitting them with 7.5K resistors on
both sides, I will send you a crisp one hundred dollar bill ($100)
in the mail.
I've never done it -- but where I went to school, it shouldn't matter.
Let us all know what you find. If you find a difference, I'll
send you $100. Of course, I'm going to be out $100, but then I'm
going to go build it myself and make sure you're not fibbing! J
Bill Hudson
W6CBS
