AGC-? for RTTY

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  • Updated 8 months ago
Hi, 

I read that for FT8 (in DIGU mode) I should turn off the AGC. Is that the case for all digital modes, esp. RTTY?

Thanks
73
Andy
KU7T
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Andy - KU7T

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Posted 8 months ago

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Jay / NO5J

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Andy

It's not always the case. Flexradios are different. Always isn't always, Always.

     SDRgadgets

#FlexRadio IRC Chat

   73, Jay - NO5J

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Andy - KU7T

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So, what is the right setting? I am a digital beginner. All I know is CW=AGC-F, SSB=AGC-M, FT8=AGC Off, RTTY=AGC-??.  Can you explain what to use for RTTY and why?

Andy
KU7T
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Jay / NO5J

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Andy
It kind of depends on the band conditions, and also what works best for you.
I'm not trying to side step your question. In general I use AGC-T fast for digital, (including RTTY) and CW. and AGC-T slow or medium for voice. There is no problem trying each to see which improves the copy. Sometimes AGC-T off works better.

     SDRgadgets

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   73, Jay - NO5J

(Edited)
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Andy - KU7T

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Ok, will try AGC-F and see how it works. 

Thanks,
Andy
KU7T
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Bob Wright, N7ZO

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Andy,

One of the beauties of the Flex Radio architecture is that you can put multiple slices on the same signal (each slice set to a different DAX channel), run multiple copies of your decoding software, and do direct real-time comparisons of how the different settings on each slice affect the decoding.

Since you asked especially about RTTY:  When I am doing casual RTTY I typically use Fldigi, but I find simultaneously running a second different decode program (such as MMTTY) on the same slice greatly increases the probability of a successful decode during marginal conditions.

73, Bob, N7ZO
(Edited)
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KI4P

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Wow! The possibilitys of the flex are amazing! Never thought if that way to use it, I struggle on how to do a lot of configuration on a lot of things, I really love my Flex!
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Michael Walker, Employee

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Hi Andy

AGC-F should help.  I always adjust AGC-T correctly (start from 0 and work up to the point the noise starts to increase).  

The Flex SDR works radically different than a SuperHet radio.    Read below and try different things.  YMMV.

Mike va3mw

13.1 AGC THRESHOLD

Automatic Gain Control (AGC) is a feature which automatically adjusts the Slice Receiver’s audiogain (volume) based on the strength of signals in the receiver’s passband filter. The goal of AGC isto amplify weak signals and attenuate strong signals so that they all lie within a comfortablelistening range.

The receiver Automatic Gain Control Threshold (AGC-T) can be adjusted for optimum performancein noisy or quiet environments. The AGC-T sets the maximum gain applied under anycircumstances. Since the noise floor is relatively constant on a given band at a given time, the AGCcan be adjusted using the threshold control so that the AGC never applies gain to noise, but it willapply gain to signals just out of the noise. In doing so, the AGC can reduce the level of noise youhear and help signals pop out of the noise.

The AGC system in SmartSDR is a dual track system, meaning that it can track both slow and fastincreases in signal strength, making appropriate gain correction decisions in the presence of each.The speed of the AGC (FAST, MED, SLOW) determines how quickly or slowly the AGC recovers afterattenuating a strong signal. You can easily hear this by tuning to a CW signal and going through thethree settings. On FAST with a strong signal you can hear the gain pump up and down while onSLOW it recovers after a longer pause once the signal stops. When the signal stops, you will hearthe noise floor increase as the gain returns.

How to Set AGC for Different Operating Conditions

If you are listening to a loud voice signal, AGC SLOW will resist increasing the gain between syllablesand therefore reject most of the noise which is at a level far below the signal. FAST and MEDprovide faster levels of recovery for situations when you want the system to more closely followthe dominant signal in the passband. Any time you have a very strong signal that causes the gain inthe AGC to be reduced, you could experience a loss of gain to a weak signal you are listening to.The filter passband edges, which are continuously adjustable, and TNFs can be used to eliminatesignals that might interfere with AGC operation.

The operator might prefer to use SLOW settings when rag chewing in a high signal to noiseenvironment where there isn't much QRN and the noise floor is stable. This keeps the gain at moreof a constant level that is less distracting. If the operator is trying to pull a weak CW signal out ofthe noise, they may prefer to use FAST mode to quickly ensure that the long-term average of thenoise floor doesn't overcome the signal and prevent it from being heard. MEDium is a reasonablecompromise.

When AGC is set to OFF, a fixed amount of gain determined by the AGC-T setting will be applied toboth fast and slow signals regardless of their level. The more you adjust the AGC-T to the right, themore gain is applied and the louder the signal and noise will be. The benefits of increasing SNRwith AGC are lost in this mode. Another disadvantage of turning AGC off is that the operator mustadjust the 'RF Gain' manually to avoid distortion due to overload by strong signals.

Operating with AGC turned OFF may desired by operators who want to avoid having a strong signaldrive a weak nearby signal into the noise floor resulting from AGC audio attenuation, such as whenoperating digital modes.

To adjust the AGC Threshold, tune to a quiet spot between stations. Starting with the AGC-T at ahigh value adjust the AGC-T slider to the left (lower gain values) until the background noise justbegins to decrease. This is the AGC-T "sweet spot" or the "knee" of the AGC algorithm. Dependingon band conditions, if the AGC-T is set below 50, you may have to compensate for the loss in audiogain (volume) by increasing the Slice or master AF volume to a higher value. When you get theknee and AF volume adjustment correct for the band conditions, it will keep the volume of strongsignals constant which will allow weaker signals to be heard even with AGC in FAST mode. ThusAGC-T is one of the most important adjustments, and often overlooked, to achieve the maximumweak signal receive performance out of the FLEX- 6000 series SDRs.

The Automatic Gain Control Threshold (AGC-T) adjustments are located with the slice audiocontrols; it is the bottom control in the audio panel. Each Slice Receiver can have its own settingsfor the AGC timing and threshold.