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563ssbaud [2011/09/12 12:46] n1eu |
563ssbaud [2012/12/10 19:46] n1eu [Omni 6 DSP Bypass Mod] |
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| **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: | **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 **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. | - 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 ==== | ==== 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. I've personally tested methods 1-3 below with good results: | + | 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. |
| - | 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 (optionally scaling resistor values up 1.25-1.33X to lower the cutoff to 3-3.2khz) | + | |
| - | 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. | 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 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. |
| - | + | ||
| - | 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, tx audio suffers from the Omni 6 mic preamp circuit and would benefit from a little attention in that area as well. I've been successful feeding the tx audio into the balanced modulator U4-5 through a 1uF cap and a 1.5K resistor. One last project for the future is to implement a ssb monitor by feeding some of the tx audio through the unused analog switch U7D in parallel with the cw sidetone audio. | + | |
| - Barry N1EU | - Barry N1EU | ||
