[TenTec] 3.1 KHz IF Filter for 2nd IF in Omni 6

Mon Jan 11 12:10:29 EST 2016

Early Omni 6 radios did not have DSP noise reduction. The plain Omni 6 did have an analog audio Notch Filter function via U5 on the IF/AF board.

The original Omni 6 did have DSP auto notch in SSB as well as programmable CW offset and a CW digital lowpass filter. No mention is made of any audio low pass filtering in SSB.

So yes, the original DID have some DSP functions but not very much (sorry for saying “no DSP”, I should have said “limited DSP”). From my personal listening experience, they did not do any low pass filtering of the audio in SSB in the first version.

An early Omni 6 had a sound very similar to that of an Omni 5, to my ears. Not perfectly “natural” but not terrible.

73,

Gary

> On Jan 11, 2016, at 10:30 AM, Lee <lee at wa3fiy.com> wrote:
> 
> I thought all Omni VI's had DSP.   Later versions only added to or modified the DSP functions.
> 
> 73,
> 
> -Lee-
> 
> WA3FIY
> 
> ------ Original Message ------
> From: "Gary J FollettDukes HiFi" <dukeshifi at comcast.net>
> To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
> Sent: 1/11/2016 10:45:22 AM
> Subject: Re: [TenTec] 3.1 KHz IF Filter for 2nd IF in Omni 6
> 
>> I don’t disagree with the things you have said about the role of the DSP in restricting the Omni 6 SSB sound. It is a major limitation.
>> 
>> However, I had experienced significant audio frequency restriction in previous tests I had done with plain Omni 6 radios that had no DSP. Perhaps that was the result of some of the audio filtering that was present in those radios.
>> 
>> However, I believe I am probably the only person who has had access to this filter pair as they were very costly one-up devices.
>> 
>> The result was that, with the DSP and all on-board audio circuitry bypassed in this very late Omni 6+ that I have, the use of the 3.1 KHz filters in both IF’s made a very dramatic change in the audio quality, when compared against the stock 2.4 KHz filters. No amount of operating the PBT with the 2.4 KHz filters in place could produce the sound quality I hear with the 3.1 KHz filters.
>> 
>> In addition, Inrad does offer a 2.8 KHz filter for the Eagle and the Orion 2. Neither of these passes the CW signal through the IF roofing filter. Why would they offer that?
>> 
>> I  think there is a need to define the term “restricted response” in terms of a ham receiver. 2.4 KHz is plenty of bandwidth for any receiver to produce acceptable communications quality audio and, as you state correctly, the Omni 6 (any version) provides “acceptable communications quality audio” with the stock 2.4 KHz filters in place and the DSP bypassed or not present). But when I set any Omni 6 beside an Orion or an Icom Pro series radio, the sound from the Omni is fatiguing because it does not sound natural. I’m not looking for hyper-bass, just sound that is natural (to me), as close as possible to that which I would hear with the person in the room with me. With the 3.1 KHz filters in place, and the DSP bypassed, this Omni 6+ sounds VERY natural.
>> 
>> I have been a music audiophile for probably 40 years, and have built output transformer less vacuum tube power amplifiers from scratch in order to give me the sound I wanted with some significant bass (but not to excess). Therefore I am pretty experienced in knowing what to look for when I make changes to an audio product, which is essentially what we are talking about here.
>> 
>> 73,
>> 
>> Gary
>> W0DVN
>> 
>> 
>> 
>>> On Jan 11, 2016, at 7:09 AM, Barry N1EU <barry.n1eu at gmail.com> wrote:
>>> 
>>> I just wanted to post a fuller follow-on comment, now that I'm no longer
>>> dependent on a cell phone to post.
>>> 
>>> People are free to believe whatever they want to believe, but in actuality,
>>> there is ONE and only one reason for the restricted ssb rx audio passband
>>> in the Omni 6, assuming you don't have narrow xtal filters installed and
>>> assuming you have the BFO oscillators properly aligned.  Actually a single
>>> picture tells the whole story:
>>> http://omni6.wdfiles.com/local--files/rxaudio/o6spectra_text.jpg  - with
>>> the dsp processor out of the signal path (top trace), the response is only
>>> determined by the filter bandwidth.
>>> 
>>> All the Omni 6 rx audio goes through the dsp processor.  The dsp processor
>>> has several principal functions, and you can't simply bypass it without
>>> there being issues.  But one obvious dsp effect is rolling off the rx audio
>>> spectrum at the bottom and at the top, by design.  Ten-Tec designed the
>>> Omni 6 from the beginning to use 2400hz filters and optimized the dsp
>>> processing for those filters.  It only becomes an issue if you want fuller
>>> fidelity (especially on the low end).
>>> 
>>> The Inrad 2800hz 9MHz filter was introduced NOT to improve ssb fidelity,
>>> but to produce a better sounding cw transmit signal.  You can search the
>>> reflector archives and find all the information from ~15 years ago.
>>> 
>>> Over the years I've done A LOT of experimentation on improving the ssb
>>> receive response, but it's not a simple matter.  I succeeded in the end,
>>> but I also came to realize that the Omni 6 is essentially/inherently a
>>> superb cw radio (perhaps the best ever) and the Orion is the much more
>>> suitable radio if you want better ssb receive fidelity.
>>> 
>>> I also found that the Omni 6 carrier null level is marginal with the Inrad
>>> 2800Hz 9MHz filter installed, even after re-aligning as Inrad recommends.
>>> I've done this on several Omni 6's and the carrier null is shallow and I've
>>> always thought the amount of carrier still being transmitted was enough to
>>> make me a bit uncomfortable using the rig in ssb with that filter installed.
>>> 
>>> 73, Barry N1EU
>>> 
>>> 
>>> 
> 
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