Hi Peter
Thanks for the nice recordings. I will check with my CW speakers. I learned a
lot of new things playing the 9M6NA CW signal recordings. Just for sharing my
notes, long notes, sorry for that.
1- SDR radio D/A converter also introduce noise, intermodulation and the signal
to noise ratio is proportional to the price you pay. Some called them DAC.
Resolution is important, like 192K/32 bits has more resolution than 96K/16
bits. Well, price talks, the DAC used on our popular radios are not the same.
My main radio is an IC-7851, there are 3 main audio output, each one uses a
different DAC. the phone output on the front panel, the DAC has near 100 db
SNR and 120 DNR, and the DAC for the speaker output on the back has only 80 db
SNR and 100db DNR, DNR is dynamic range, The IC7850 has both DAC with 120db DNR
and was much pricier!
Price talks the ICOM save some money with a not so good DAC for the speaker
output. Another important fact is the THD is also different. The best way to
get a better signal to noise ratio out of the IC-7851 is the S/PDIF output. The
digital signal can be converted using a good external DAC with 140db DNR. My
audio panel has a DAC with S/PDIF input and, 32K-192KHz SNR: ＞120DB, Dynamic
Range: ＞120DB and THD + N : -107DB. Here you can spend U$50 to U$ 5000 on
audiophile DAC, I paid U$20 few years ago :
https://www.ebay.com/itm/353414734553
These are good enough. Imagine you situation working a weak signal (-130 dBm)
covered by the noise and intermodulation from the DAC. Remember noise is basic
temperature noise, db is just a number, simple math does not apply here. The
audio intermodulation noise plus weak signal is another issue.
2- Using a high-quality DAC like the USB external Sound BlasterX G6 sound card
on my PC. I noticed the problem with audio intermodulation. The G6 has an
excellent performance for the price. 32-Bit / 384 kHz 130 dB DNR. Here the
tests results.
I started to test the CW speaker playing the HPSDR RF recording using a good
quality PC speaker. The weak signal was Q5 on the cw speaker and just noise on
the PC speaker. I recorded the experiment using my iPhone. Playing the video, I
learned that the DSP on iPhone notch off the CW signal, like removing eco or
noise, the signal from the CW speaker just was not recorded. The same happened
during 3 club meeting presentation, the local guys heard the cw from the CW
speaker but not the remote guys on ZOOM meeting App. The DSP was optimized for
voice, and not a single frequency. It is just noise for the DSP on the ZOOM
app.
Here is the main discovery. I have a HiFi amplifier and HiFi speaker to listen
to Jazz connected to the G6 DAC, all that jazz!. I played the same recording on
the HiFi system and for my surprise, I was able to copy much better the same
recording on the HiFi system. Yes, the noise was the same, but the signal was
copyable. The CW speaker was better SNR than the HiFi, and the HiFi was better
than the PC speakers. Conclusion the noise and TDH on the audio amplifier does
impact the copy of the weak signal. PC speaker system amplifier/speaker have
very limited performance.
3- Audio intermodulation is something I don’t understand very well, it is not
only THD, when you have the signal below noise level the audio quality is not
the same when the signal is above noise level. May be Jim can help me here.
Audio amplifier with low noise at the low power level we need for CW copy is a
challenge. Performance at high power is not the same for low power, in the
same amplifier. One good amplifier you can found here. Ultra-low distortion
amplifier at low cost, at low power < 1W.
https://www.kaltecs.com/32w-ultra-low-distortion-kit/
Difficult to understand but you can copy the weak signal using one audio
amplifier and do not copy the same recording on a different audio amplifier,
using the same speakers.
Same thing with our loved headphone! The difference here is unbelievable. Last
year I bought a Beyerdynamic DT 880 600 ohms. I have a Beyerdynamic DT 770 PRO
250 Ohm
I was having fun working a SSB contest on 160m, and I heard a PY3 very weak on
SSB using the 600 Ohms headphones, then I switched to the 250 ohms, and I could
not understand the same station, the difference between understand and not, is
it a question about clarity ? Jim, can you help me here to understand why so
much difference between to good headphones. The same results hearing he 9M6NA
CW recording, clear difference using the 600 ohms headphones from the same
maker.
4- Psychoacoustic is real. Psychoacoustics is the scientific study of how
humans perceive sound, including the psychological responses to different
sounds like noise, speech, and music. It combines elements from psychology and
acoustics to understand how sound waves are interpreted by our auditory system
and how they affect our emotions and experiences. Each of us are different and
alike at the same time.
However electronic circuits are not the same at all, even with the same names.
I like to experiment with audio and build my own speakers since I and a
teenager.
I know FT8 does not have all the above, or it does, I don’t work FT8 so I
don’t, know.
73's
N4IS
JC
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