[RTTY] RTTY SO2R "leakage"?
g3yyd at btinternet.com
Sat Sep 5 07:00:57 EDT 2015
The amplitude range that 2Tone will work over is enormous. It samples at
48KHz at 16 bits and decimates that to a low sample rate. This alone
increases the 90dB of the sound card by 30dB. Then there is the 45.45Hz
filter bandwidth as well adding additional 10s of dB.
A small amount of cross talk could easily result in a decode. If it bothers
you then make sure the leads going to each radio are well screened and
correctly terminated at the radio plug end and run individual leads from
each radio to the plug that goes into the PC. However the crosstalk is most
likely to be internal to the PC/laptop. If you are using a cheap $5 sound
card then buy another around $25 or so. I tested a $5 one here and threw it
in the bin! Its performance was terrible in virtually all aspects.
I looked up the spec for a chip used in a good quality sound card and its
cross talk for the A/D converter was 90dB. That would be for an optimally
designed PCB. However I would think most PCBs are non-optimal so it will be
worse than that.
If it really bothers you then the answer is simple set the radios to use
different tone sets. 2Tone has unlike MMTTY the ability to use the sound
card with left or right channel for TX so that the RX and TX tone
frequencies for the each radio will be correct.
73 David G3YYD
From: RTTY [mailto:rtty-bounces at contesting.com] On Behalf Of Tim Shoppa
Sent: 04 September 2015 16:47
To: rtty at contesting.com
Subject: [RTTY] RTTY SO2R "leakage"?
I am a bit mystified by some observed behavior in my SO2R RTTY setup.
I feed the left radio into the left audio channel, and feed the right radio
into the right audio channel.
I have one MMTTY and one 2Tone per radio, each set up to only look at the
appropriate left or right audio channel.
Doing this all in SO2R seems to work. But every so often a really loud
signal on radio 1, will be decoded and printed in the decoder for radio 2,
especially if the signal on radio 1 is pinning the S-meter and radio 2 is
looking at a signal in the noise. Same things happen with 1 and 2 reverse.
The first time this happened I was stumped, but then figured out that I had
left a microphone plugged in to the sound card at same time. Audio was
escaping from my headphone, finding its way to the microphone, and then
being fed into both channels! (Evidently microphones are treated as mono
But now I have the microphone unplugged, and still the leakage is still
continuing at a lower level. I don't see any signs of decoder leakage in the
decoders' waterfalls when just looking at band noise and regular signals but
we all know in a contest there are some really loud signals.
I'm trying to figure out if this "leakage" between channels is in my PC
hardware, in the audio cabling, or in the windows driver, maybe a
microphonic effect in the PC case, or some common coupling through ground or
a power supply, or what. Or maybe there is some AGC function being applied
by the sound card ADC in response to a loud signal, such that a loud signal
on one channel shows up very weakly in the other channel. Any advice on
tracking this down?
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