[RTTY] RTTY SO2R "leakage"?

[Joe Subich, W4TV]

> 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.

Or use separate sound cards for each radio.

73,

   ... Joe, W4TV


On 9/5/2015 7:00 AM, David G3YYD wrote:
> Tim
>
> 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
>
> -----Original Message-----
> 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
> devices.)
>
> 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?
>
> Tim N3QE
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