Omni 6 Receive Audio Mods

Inrad Audio Mod

Inrad offers an inexpensive mod kit for the Omni VI/VI+ to improve ssb receive audio.

I've installed the mod along with some careful measurements. The mod accomplishes two improvements:
- Without the mod, the minimum notch filter frequency setting is 240hz ("Notch" knob fully ccw). Because there's no way to completely shut off the notch filter, there is always a 240hz "suckout" in the audio in the stock Omni 6. The mod lowers this to 50hz, below the ssb rx passband, by replacing a notch filter IC resistor. With the mod, the highest notch frequency ("Notch" knob fully cw) is now 2500hz.
- Without the mod, the ssb rx passband tilts down with 2700hz level approx -10dB relative to 500hz level. The mod flattens the passband by replacing two capacitors at the product detector output.

The mod involves removing the IF/AF Board and replacing three components. I've only done the mod with leaded (not SMT) components. It is an easy job for those with a bit of solder rework experience. The board removes easily - 5 screws along with numerous well marked connectors. Start to finish, it takes about 30 minutes.

HOWEVER (and a big however), you should bear in mind that the receive audio always goes through the Omni 6 dsp circuit. The dsp circuit has two big effects on the ssb audio passband:
- passband is rolled off by -25dB above 3400hz (filters out high frequency noise, i.e. "hiss")
- passband is steeply rolled off below 260hz; this factor makes the first part of the Inrad mod of questionable utility unless you can somehow bypass the Omni 6 dsp and extend the low frequency response. However, if you simply bypass the dsp, the rx audio will have an unacceptably high level of "hiss" without the dsp 3400hz low pass filter. And even if you both bypass the dsp and add a low pass audio filter to take out the hiss, you are left with the difficult task of finding a workable solution for smooth cw T/R switching and sidetone injection which is accomplished in the Omni 6 dsp. The solution I went with was to bypass dsp with added 3KHz LPF on ssb and use the dsp audio for CW, although I added 1KHz LPF on cw as well to clean up objectionable artifacts.

Realize that Ten-Tec did in fact have a clear vision in mind when they carefully designed the stock ssb audio to behave the way it does. With the stock 2400hz filter fitted and the BFO aligned per spec, the audio passband is 250-2650hz. So the stock notch filter behavior and stock dsp high pass filtering makes sense in light of that plus the fact that Ten-Tec engineers were doing their best to cut out as much noise as they could from the passband. They were battling with a level of internal rx noise that had been slowly increasing since the Omni A days and had tried other audio noise filtering tricks previously with the Corsair II and Omni V.

But for many of us, a ssb rx passband lower cutoff of 250Hz is just too high. Read on.

DSP Bypass Experiments by N1EU

Although the Omni 6 DSP was a laudable effort in bringing new signal processing technology to an amateur hf xcvr, it removes all audio below 260hz. Some operators won't be bothered by this but many of us are. It's not just an issue of diminished fidelity. For example, when running narrow ssb receive bandwidths, I find it desirable to shift the receive passband toward the low end of the spectrum (i.e., 150-200Hz). There is only one way to get more low frequency ssb rx audio from the Omni 6: bypass the audio dsp loop. TMP mini-coax cables 54 and 56 send the audio between the IF/AF board and the DSP CODEC on the Logic Board. Bypassing this loop will restore audio below 260hz. Because of the difficulty in achieving smooth T/R audio transitions and sidetone injection with cw, I will limit this discussion to dsp bypassing for ssb rx audio only while continuing to use dsp rx audio (with its smooth T/R transitions and sidetone) for cw. In designing a ssb rx audio dsp bypass solution, the main requirement is to implement low pass filtering (~ 3KHz) to remove high frequency hiss.

Basically, this is what you want to do in your bypass circuit, added to IF/AF board:

  1. Bring both pre-DSP and post-DSP audio into the circuit; for pre-DSP audio, directly tap the product detector output through a 1uF cap instead of using the J54 TMP jack audio - this will also totally bypass the notch filter circuit which considerably degrades the stock Omni6 audio and injects a constant low level (but audible) 5KHz tone into the audio.
  2. Run pre-DSP audio through 3KHz low pass filter for ssb (no DSP is used in ssb receive)
  3. Run post-DSP audio through 1KHz low pass filter for cw
  4. Switch between the pre-DSP filtered ssb receive audio and the post-DSP filtered cw receive audio via a relay triggered by the ssb or cw mode signal and send the output audio signal back to IF/AF board J56

There was an unforseen benefit realized by the dsp bypass. The stock Omni 6 has a high amount of low-level, clearly audible, digital "grundge" in the rx audio - various tones/buzzes/encoder tuning ("zipper") noise, etc - and these were totally removed.

Omni VI+ SSB RX Audio Spectra Compared - Before and after DSP Bypass Mod

o6spectra_text.jpg

Coupling the dsp bypass mod with installing dual Inrad 2.8khz filters (and carefully aligning BFO oscillators and carrier null) will transform the Omni 6 ssb rx audio and remove all the frustrating grundge from the Omni 6 cw rx audio. Following is a short clip of the resulting rx audio, fed from my Omni 6+ headphone jack into my soundcard. This is with BFO oscillators aligned per factory spec.

Click here to play a sound clip of W3OZ and KA7GKN through the dsp-bypassed Omni 6 rx

The Corsair II is actually a much easier radio to modify for exceptional audio over the Omni 6. The need to selectively bypass the dsp in the Omni 6 makes it an unusually difficult job. But the CAT port in the Omni 6 is an undeniably huge advantage.

Just to reiterate, careful carrier null alignment is a necessity - even with careful adjustment, the LSB/USB carrier suppression is barely acceptable with the 2.8Khz filter installed in the 9MHz i.f.