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563inradaud [2011/09/02 16:27]
n1eu
563inradaud [2013/02/24 23:25]
n1eu
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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 ====+This page has moved:
  
-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. +  *[[http://omni6.wikidot.com/rxaudio|Improving ​Omni 6 SSB RX 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** +
-  - 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 ==== +
- +
-To get around the 260hz high pass filtering of the Omni 6 dsp and still provide the needed low pass hiss filtering, I have done the following with great success: tap the Omni 6 audio right at the product detector output (pin 6) through a 1uF cap and bring it via shielded cable (you could also tap the audio after the notch filter) into an appropriate low pass filter to remove hiss. Below is a list of some ideas for implementing the low pass filter. ​ I've personally tested methods 1-3 with good results, although I tend to favor analog filtering. +
-  1. Timewave DSP-599ZX high Z input B set to 0.0dBV, filtering set for 100-2800hz ​  OR +
-  2. Behringer PEQ 2200 analog parametric equalizer set for 85khz lo cut, 2.5khz hi cut, band5 eq -15dB 10khz 1.2 octaves ​  OR +
-  3. 4Khz Low Pass filter circuit designed by Elecraft for use in the K3  OR +
-  4. Low Pass filter circuit (fig. 3.50) from Experimental Methods in RF Design, with each capacitor value scaled down by 50% +
-   +
-The advantage to filter methods 3 & 4 is that they can easily be incorporated internal to the Omni6. +
- +
-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. ​  +
- +
-The existing audio chain can be left intact as well, in case you ever want to listen through the Omni 6 headphone jack or speaker. ​ Another option for the experimenter is to re-inject the externally filtered audio back into the Omni 6 which would be easily accomplished via the DSP return connector 54/56 on the IF/AF board. +
- +
-Coupling the dsp bypass mod with installing dual Inrad 2.8khz filters will transform the Omni 6 ssb rx audio from mediocre to superb. ​ Although rx audio is the topic at hand, you may want to consider feeding the tx audio directly into the Omni 6 balanced modulator (U4-5 on TX Audio board) via a 1uF capacitor if you are already using an external mic preamp and can adjust its output to a fairly low level. +
- +
-- Barry N1EU+
  
  

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