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563ssbaud [2012/12/10 14:46]
n1eu [Omni 6 DSP Bypass Mod]
563ssbaud [2015/02/26 07:34] (current)
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 ====== Omni VI/Omni VI Plus SSB RX Audio: Inrad Audio Mod & DSP Bypass Mod ====== ====== Omni VI/Omni VI Plus SSB RX Audio: Inrad Audio Mod & DSP Bypass Mod ======
-==== Inrad Audio Mod ==== 
-Inrad offers an [[http://www.inrad.net/product.php?​productid=209&​cat=90&​page=1|inexpensive mod kit]] for the Omni VI/VI+ to improve ssb receive audio. +Information is now located ​[[http://omni6.wikidot.com/rxaudio ​here]]
- +
-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 plan on modifying your Omni 6 to bypass its 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. +
- +
-==== Omni 6 DSP Bypass Mod ==== +
- +
-The key to getting good sounding ssb receive audio on the Omni 6 is to bypass the dsp.  I routinely bypass both the dsp and the notch filter. ​ However, you need to add some low pass filtering if you do this or the audio will contain an inordinate amount of hiss.  Tap the product detector output (U3 pin 6) through a 1uF cap and then apply low pass filtering. ​ I've done this both inside the Omni 6 (with homebrew 3Khz low pass filter) and outside the Omni 6 (with DSP-599ZX or Behringer equalizers). ​ Inside the Omni 6, you can then send the output of the low pass filter to the AF Gain control and then to the audio amp by splicing into connector 55.  This way, you can leave the dsp sidetone circuit intact. ​ The result is a TOTAL transformation of the Omni 6 ssb receive audio. +
- +
-A side benefit of this filtering technique is that all the digital "​grunge",​ encoder noise, and other annoying artifacts are totally removed from the rx audio. ​  +
- +
-Coupling the dsp bypass mod with installing dual Inrad 2.8khz filters will transform the Omni 6 ssb rx audio from mediocre to sublime. ​ Although rx audio is the topic at hand, I've also optimized my tx audio by feeding line level (i.e., externally pre-amped) tx audio into the balanced modulator U4-5 through a 1uF cap and a 10K resistor. ​ This mod leaves the existing Omni 6 tx audio chain intact. +
- +
-- Barry N1EU+

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