[Icom] 756 Pro audio question

Sun, 23 Feb 2003 10:33:26 -0600

I need to make myself stop posting to this same topic, but it seems to keep
coming up and I can't keep quiet.   <:}

Unfortunately, the analog internal audio sections of the PRO and PRO2 are
not up to the standard of the rest of the radio. The mic input circuitry up
to the A/D conversion and the receive audio circuitry from the D/A
conversion out to the speaker collectively are noisy, add distortion to the
signal and have limited dynamic range. One result is that they do not sound,
to some people, as good as expected. This is a relative matter. On an
absolute basis, internal PRO audio is "good" - but it can be much better.

My solution, and that of many others, is to avoid using any of the internal
analog audio circuitry of the PROs by introducing transmit audio into and
taking receive audio from ACC1 on the rear panel. The signals there are at
line level and cleaner than what is obtained using the mic and speaker
connectors. Of course,  the receive audio output requires outboard
amplification to drive a speaker, and a microphone will not drive the
transmit audio input there, but the return for using appropriate outboard
equipment is outstandingly clean audio. The measured response of my PROs on
the low end goes down to around 33 Hz at the -6 dB point using outboard
audio equipment. The upper end is flat  up to about 3300 Hz in the 3600 Hz
SSB filter setting. These figures come from measured spectral responses
taken of the IF and audio sections of the receiver. These spectra can be
seen and compared with comparable ones from the IC-765 for all common
bandwidths on Adam Farson's (VA7OJ) website

						http://www.qsl.net/ab4oj/

PRO receive audio quality can be evaluated in either or both of two ways:

1. Tune in a high quality AM broadcast "quality" music station and compare
the audio from the PRO to that from a good broadcast receiver or tuner,
preferably using the same speaker system.

2. As above, compare the audio from a speaker system connected to the
External Speaker connector with that from the same system driven from ACC1 -
Pin 5.

The audio capabilities and deficiencies of the PROs become clear when a
relatively uniform and high-quality signal is used for comparison. Using
amateur SSB signals is unreliable due to the great variation in signal
quality.

On the sound of CW signals, the PROs can sound the same, better or worse
than a conventional analog receiver with IF crystal filters. The low shape
factor of the IF DSP filters allows the operator to literally dissect a CW
signal and thereby to separate the main tone signal from the keyed waveform
sideband artifacts. I have found that any "good" clean CW signal sounds
better to my ears coming from the PRO than the same signal coming from my
IC-765 with its Icom crystal filters, using the same external audio system.

With the PRO, the great configurability of the passband characteristics made
possible by the IF DSP architecture permits the operator to hear various
combinations of the keyed tone and its artifacts that can impart an unusual
quality to the sound of the signal. For some operators, this is a negative
and they may not like the CW signal sound. For others, the added
capabilities for adjusting the passband and dial tuning to produce various
presentations of the signal are a bonus. And, to the point, there are a lot
of CW signals on the bands these days that are not "clean" and that sound
accordingly on any quality receiver.

After nearly three years of operating a PRO and now a PRO2, and nearly six
decades in all of tuning most amateur radios, I maintain that the PROs are
among the best sounding receivers that I have ever used. There is not much
the operator can do with a conventional receiver except to tune in a signal,
adjust the volume and listen. The parameters that affect overall signal
"sound" are largely fixed and not available for operator adjustment. But,
with the PROs you have ample opportunity to improve the received sound
greatly over a conventional receiver as well as exceptional opportunity to
reduce a good signal to trash. A PRO is not your Father's radio . . .

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Jim Cox
Sent: Sunday, February 23, 2003 8:22 AM
To: [email protected]
Subject: Re: [Icom] 756 Pro audio question

I think you must have a "problem" radio there as I have owned 2 of the
original Pros and now own one of the Pro IIs and the audio is superb on all
of them.  No fuzziness at all, a little different sound than you get from
the Kenwoods but certainly not fuzzy.   I notice you said the radio was a
repack, does this mean it was a "return"?  Perhaps the audio problem was the
reason it was returned?
I had a friend who purchased a returned pro from a dealer which had a
definite audio problem, evidentally the dealer had just put it back in the
box and resold it again.   Jim K4JAF

----- Original Message -----
From: "Kelly Jones" <[email protected]>
To: <[email protected]>
Sent: Saturday, February 22, 2003 11:17 PM
Subject: [Icom] 756 Pro audio question

> Hello List,
>
> Excuse me if this has been discussed before, but I am new to Icom
> radios.  For years and years I've owned Kenwood radios, but recently
> purchased an Icom 756 Pro (not II) Repack.  It is a 4000+ serial number,
so
> I'm guessing it's late model.  (Can anybody provide a manufacture date)
>
> My question.  To me, some of the audio sounds "raspy" or "fuzzy".  I
> realize this is a DSP rig where my TS850 is analog.  However, I've seen
> lots of rave reviews about the audio in the 'Pro.  Is this fuzziness
> normal?  It is especially noticeable on CW signals with the 500 & 250 hz
> filter in.  And some signals tend to have more "fuzz" than others.  This
> could also be entirely user error as there are so many settings on the
> Pro.  One thing I have noticed is that using the NR fuction tends to
> "smooth" out the fuzziness, although not completely eliminate it.
>
> I found it interesting that Icom themselves point out the old "vinyl vs
CD"
> argument.  They actually say in the document that if you're a "vinyl"
> person, perhaps a DSP radio is not for you.  I wouldn't consider myself a
> "vinyl" person by any means, but the "fuzzy" audio really bugs
> me.  Personally, I think the audio in my old Kenwood is superior to what
> I'm getting from the Pro.
>
> Any suggestions would be great.  I've been playing with the rig for the
> past couple of days and I'm luke warm as to whether or not I like this
radio.
>
> Thanks,
> Kelly - KE9KD
> http://www.dx-central.com