Hi all
I'm so happy. That's why I am writing this. CESSB works! I have recorded a 1-minute YouTube demo (Churchill's speech still) showing a QMX receiving QMX+ CESSB transmissions, more on this below.?
CESSB (Controlled Envelope Single Sideband) reminder: the place to read about it is the original ARRL QEX article published in November 2014 by David L. Hershberger, W9GR:
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However that article is focused on generating SSB by a conventional SSB exciter. I struggled with thinking about how to apply that to the rather special case in QMX where we are generating SSB by EER (Envelope Elimination and Restoration) which is a rather different process, though also involving the inevitable Hilbert Transform.?
In the end I decided I did not NEED to do it exactly the same way it's done in the article: in QMX SSB via EER, we have separated the signal into phase and amplitude components. I realized that this fact can be used to ADVANTAGE. I developed a completely different way of generating CESSB to the one in the article. I guess I shall have to write down all the details carefully to document all this.?
I am still using as my audio sample (I'll call it track from now on), Winston Churchill's "Be Ye Men of Valour" speech from 1940 ?
Note that for these experiments I am running the QMX+ powered with 12V but with the gain set such that the RF output is 1.0W PEP. When using full power about a 6dB improvement in dynamic range would be expected.
In my attached photograph "CESSB Off.jpg" I photographed the same first syllable in which he says "I". The audio recording has an amplitude which does not exceed the range?+/- 1; in other words it reaches full scale but it does not exceed it. Yet you can clearly see that at RF the envelope overshoots "full scale", the dotted blue cursor lines on the 'scope screen, which are determined using a steady 1kHz test tone. In this case the overshoot is 32%, or 2.4 dB. You would need to turn down your gain by 2.4 dB in order to avoid clipping and splatter (or, if using an amplifier with ALC, the ALC would do that for you).?
"CESSB On.jpg" I photographed the exact same first syllable but now CESSB is switched ON. Peaks of the RF envelope which do not exceed "full scale" indicated by the dotted blue lines are left alone, they do not trigger the CESSB algorithm. But a peak which exceeds the allowed maximum envelope appears cleanly reduced without altering its shape.?
The best thing about my new method for generating CESSB is that it is inherently clean (does not increase bandwidth) and I do not need to run any subsequent filtering, so there is no need for a computationally expensive linear phase FIR filter. I am constantly reminded that although I chose a powerful processor for QMX, a 32-bit ARM Cortex M4 at 168 MHz, it is certainly NOT anywhere near the kind of powerful CPU that is being used in the very few top-end amateur radio manufacturers where CESSB is being used (Elecraft K4, Flex SDR, Apache Anan). So when doing DSP it is critical to keep remembering that CPU cycles are still very precious. It was therefore all the more satisfying, finding a way to do this that involves only a few simple arithmetic operations per DSP audio sample.?
For testing the SSB reception I used an 80-20m QMX having the current latest production firmware version 1_00_027, in "digi" mode, which has a filter bandwidth? of 150-3200 Hz. This is wider than the SSB filter bandwidth used in the QMX SSB firmware which is 300-2800 Hz. I connected the BNC RF port of my QMX+ (the transmitter) via 80dB of inline 50-ohm BNC attenuators to the BNC RF port of my 80-20m QMX (the receiver). Both the QMX+ (transmitter) and QMX (receiver) are connected to my PC and appear as USB Sound cards there. I played the Churchill quote on VLC Media Player to the QMX+ (transmitter); and recorded the QMX (receiver) using Audacity software.?
I have two versions of the audio clip. One of them is the original one, scaled so that its peaks hit (but do not exceed) full scale audio resolution. The other one was amplified by 3dB (voltage) using Audacity, allowing clipping at full scale audio. The clipping therefore introduces harmonic content, and increases the average power level by 3dB. In the attached chart these are labelled #1 and #2, being the original and 3dB amplified versions respectively.?
I used the Audacity Spectrum analysis function (48ksps, 8192 point, Hanning window) exporting to a CSV and loading it into the spreadsheet for graphing. In the QMX Lab screen (attached "lab.png") I can switch on or off CESSB and also measure the peak and average envelope amplitude over a 2.4 million sample duration (200 seconds).?
The attached chart "spectrum.png" shows all four spectrum lines. Audio track #1 is colored blue (CESSB off) and purple (CESSB on); track #2 is colored red (CESSB Off) and orange (CESSB On). You can see that there is about a 3dB difference between audio tracks #1 and #2 which is expected because the gain was increased on track #2. In each track, there is NO discernible difference in spectrum at all, between whether CESSB is switched Off or On.?
Therefore QMX CESSB is, as it should be, limiting the envelope without causing any distortion and without widening the bandwidth. It simply increases the average to peak power without clipping or splatter or distortion.?
In my track #1 the envelope overshoot is 29% corresponding to 2.2dB. In track #2 the envelope overshoot is 65% which is 4.3dB. This is also expected (see the CESSB article) because when audio is amplified and clips, at the clipping point the waveform is somewhat trapezoidal which starts to approach a squarewave. As the article explains, squarewaves behave rather badly in the Hilbert transform: the zeroes of a series of squarewave harmonics line up, but when shifted 90-degrees (the Hilbert transform) then the PEAKS all line up at the same time. Which is the whole yuckology of the SSB situation.?
So the use of CESSB provides an increase in average power, the exact amount depends on the nature of the speech being transmitted; in my two example tracks the benefit was 2.2dB and 4.3dB. Taking the latter number for example, it means that a 5W transmitter has the average power equivalent to a 13W transmitter, yet without adding any splatter or distortion.?
Here is my 1 minute YouTube demo video:
The SSB sounds pretty good, if you compare it to the original Churchill speech you will certainly say it sounds rather similar. Note that the video and audio of the video aren't synchronized; I took the video then I overlayed the sound from the MP3 file I had recorded on my PC. So don't worry that what's shown on the 'scope doesn't quite match the sound.?
CESSB is not exactly the same thing as compression or pre-emphasis though you could say they are all quite closely related. I am aware too that low frequency tones in SSB don't do much for intelligibility but DO waste a lot of power unnecessarily so the advice for well set-up SSB is to reduce the bass tones somewhat. So compression and tone adjustments to what comes out of an actual microphone, are still on the ToDo list.?
WV0H
Chris / W2BPL
Daniel Conklin
Gust ON6KE
Willem VA2WLM