[Icom] Icom 756-Pro II audio/mic input alternatives

Sat, 01 Jun 2002 11:23:01 -0700

Larry,

the low-end audio response is lower at ACC1 Pin 4 (80 ~ 100 Hz at -6 dB) than at
the MIC input (250 ~ 300 Hz at -6 dB), but the high end is hard-coded in the DSP
firmware. From a look at the speech amplifier (IC451 on the MAIN UNIT board),
there is an RC feedback network in the first stage (AMP). This network looks
like a high-pass filter, which would account for the steeper low-frequency
roll-off at the MIC input compared to the ACC1 input. The second stage (VCA,
voltage-controlled amplifier) appears to have a flat response. The design intent
is probably to cut off low-frequency components of the microphone output (below
250~300 Hz). These components contribute virtually nothing to articulation at
the receiver, but rob transmitter power which can more usefully employed to
transmit the midband and upper frequency subbands which determine the
articulation index at the receiver.

If a bass-boost is applied to the mic audio fed to the front-panel mic socket,
IC451 will filter out all components below 250 ~ 300 Hz.

The AMOD (ACC1 Pin 4) input bypasses IC451 entirely. The outputs of IC451 and
the AMOD buffer are fed to the ADC via the DTAF line. The design intent here is
probably to pass AFSK signals without distorting the frequency transitions,
which have a low-frequency component.

Icom's engineers probably felt that this analogue solution would be more
cost-effective and less risky (in terms of potential bugs) than separate,
selectable DSP modulation algorithms for voice and data transmission.

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Note new e-mail address:
mailto:[email protected]

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Larry Benko
Sent: Saturday, June 01, 2002 10:20
To: [email protected]
Subject: Re: [Icom] Icom 756-Pro II audio/mic input alternatives

Larry,

Using the ACC1 input does lose the VOX  function but does NOT lose the
PTT
function.  The SEND line (pin 3 in the ACC1 jack) is the PTT line.

73,
Larry, W0QE

[email protected] wrote:
>
> I believe that this topic may have been covered many times, so if you wish,
> please email directly as to not clutter up the reflector.
>
> Is there another way to feed audio into a pro 2 other than the standard mic
> jack/pin config on the front, so that after eq'ing the mic/audio, the pro 2
> won't flatten the eq'ed audio and canceling the eq effect. I heard something
> about using the acc1 audio input, but there seems to  be some conflicting
> information in the manual.
>
> Also, If i feed the acc1 with audio, I know I will lose ptt, but will I also
> lose VOX.
>
> thank you all again..
>
> vy 73.. Larry.. N2LH
>