Gamma Dog - Rate-to-Tone conversion and Audio Frequency Modifier

One of Gamma Dog's unique features is the continuously variable tone representing the detected count rate by changing its frequency.

The approach is fairly straight forward - the detected count rate in CPS (Counts-per-second) coming out of the detector and into the MCU is converted into an audio tone with the same frequency - i.e. 200 CPS will produce 200Hz tone and 1000 CPS will produce 1 kHz tone. 

Due to a lucky coincidence, the detectors we use, especially the 63mm NaI(Tl) crystals produce around 180 - 230 CPS as a background level which is a really good starting point and the  overall detector sensitivity and rate response work pretty well with such direct conversion.

As the count rate increases though, the tone frequency will increase (as expected) and this could become a problem at some point when the count rate gets really high (above 7000 - 8000 CPS) - nobody likes these very high pitch audio frequencies (certainly not the dogs and the mine bats :-) - such high pitch is not the most pleasant thing to listen to. Not to mention it becomes quite difficult to hear small variations in the frequency within this high frequency range.

To combat this problem, the "classic" version of the Gamma Dog always starts the frequency generation at the set squelch level - this way if Squelch is set to 7000 CPS, and detected rate is around 7000 CPS the tone frequency will be low - less than a hundred Hertz (whatever difference is needed to break through the Squelch Threshold) , opposed to a 7 kHz tone.

In my Gamma Dog+, the system for rate-to-tone conversion is further improved and offers the user a toolset of numerous conversion options. These options afford greater control over how the Frequency Audio Response to the Count Rate conversion is taking place, customizing it for different applications and listening preferences. 

The "xL" indicates Logarithmic Scale conversion (selectable when in Continuously Open Squelch Mode / "#" by using the Soft Key) 

Changes in the conversion model are done via a user-selectable option that can be assigned to the Soft-Key button. It is called Audio Frequency Modifier or AFM for short. 

In a nutshell, AFM (in some of the options) is a Multiplication Factor that is applied to the count rate while converted to tone:

There is a total of 8 options: x0.5, x1 (default), x1.25, x1.5, x2, Auto, Exp and Log

The first few options are self-explanatory - if x0.5 is selected, the rate is divided by 2 before it determines the frequency of the tone - i.e. 200 CPS rate will produce 100Hz tone, 210 CPS will produce 105 Hz and so forth. It halves the base frequency but also the steps between changes. On the other end, with x2 selected, 200 CPS will produce a 400 Hz tone and 210 CPS will produce a 420 Hz tone. The default value of x1 is the direct 1:1 conversion.

The x0.5 option for example, is useful with very large detectors to keep the audio frequency output low against the natural high-count rate of the detector, while x2 is useful with smaller detectors which natively produce fewer counts, and the option allows to keep the tone frequency higher than a direct 1:1 conversion in this case.

In Auto mode the multiplication factor is based on the Detected Rate increase over the Squelch Threshold Level. The audio frequency modifier is dynamically adjusted in 4 steps based on the delta between the two rates.

If Current Rate exceeds Squelch Rate by more than 175% - Frequency Multiplier x2.5 is used.

If Current Rate exceeds Squelch Rate by more than 150% - Frequency Multiplier x2 is used.

If Current Rate exceeds Squelch Rate by more than 125% - Frequency Multiplier x1.5 is used.

If Current Rate exceeds Squelch Rate by less than 125% - Frequency Multiplier x1 .25 is used.

This feature will change automatically through different multiplier levels while using the squelch level as control of where the "step-ups" should take place.

This graph plots how the frequency conversion steps through the multiplication range using the difference between detected rate and the set squelch rate. As the squelch opens and the rate continues to climb, the multiplier will start stepping up, increasing the audio frequency.

There are also two non-linear conversion modes available - Exponential and Logarithmic.

Exponential Mode – The audio tone frequency will increase in an exponential manner, with a scale factor of 0.0033 and base frequency of 100Hz using (e) Euler's Number.

This feature is independent of the Squelch Level - the squelch just needs to open but otherwise has no effect on the conversion. 

It is usable with absolute rates up to 1400 CPS.  Beyond 1400 CPS the audio frequency will exceed 10kHz!

Exponential mode is useful to detect very small increases in the count rate when the absolute rate is also very low. For example, in very low activity areas where small rate changes need to be detected - the audio tone frequency is exponentially increased, exaggerating the tiny rate variations by using higher pitch tones.

Logarithmic Mode – The audio tone frequency will change on a Logarithmic scale – the range of 40 CPS to 10K CPS will be "compressed" and mapped by using a logarithmic curve to 40Hz – 3kHz audio range.

This feature, just like Exp Mode is independent of the Squelch Level from Rate-To-Frequency standpoint, and it provides very good audio resolution for lower count rates (<2000 CPS), while still capable of handling very high count-rates (2K to10K+ CPS) - all within a manageable 3kHz audio range.

 

The range between 2000 CPS and 10000 CPS is allocated within less than 1kHz audio frequency response (from ~2200 kHz to 3000 kHz) which is useful when the instrument is used with both, very low and very high-count rates. This comes at the expense of audio resolution in the high-count region of the curve and overall higher frequency tone at the mid-low count range.

In the Gamma Dog's menu system, there is a config item (#10) which allows the user to select a startup mode for the Audio Frequency Modifier, but the mode can always be changed later, during operation, if "Multiplier" is assigned to the Soft Key button in Menu Item #8.

(The other assignable function to the Soft Key button is control for the Audio Conversion Hysteresis Level responsible for how closely variations of the rate are followed before converted to audio tone)