[sdiy] How to use an analog time-domain multiplexer?

cheater cheater cheater00social at gmail.com
Fri Jun 4 15:59:57 CEST 2021


On Fri, Jun 4, 2021 at 3:53 PM Richard Wentk <richard at wentk.com> wrote:
>
> If you’re sampling at 100k and max f is (say) 20k the filtering won’t need to be complicated.
>
> You will need S&Hs, however.
>
> Which is one of many reasons why this won’t work, because the hard part is building a super-low-distortion audio rate S&H with minimal jitter and no switching noise.
>
> That’s not going to be clean and transparent unless you spend a lot of time and money on it.
>
> And that still won’t help, because you’re effectively trying to do Pulse Amplitude Modulation at RF rates down unterminated plain wire.
>
> This guarantees all kinds of time smearing and reflections, so the odds are good that what comes out the other end will be a mess no matter how much filtering you do at each end.
>
> You might just about get PAM working for a single audio channel with a sample rate of 44k, and if you try really really hard you can possibly push that to 100K or so for stereo. It probably won’t sound great, but it has the potential to more or less work.
>
> But any form of PAM at RF rates without RF connectors and RF design techniques is pure fantasy. And even with them, the cost, complication, and likely low fidelity make it unworkable.
>
> There are good reasons this kind of thing is usually done digitally.
>
> Richard

Explain one thing to me... how come I can send NTSC over minijack?
That's not RF designed, 50-ohm terminated, or even shielded. It's just
a minijack. And the ground is shared with two audio channels, none the
less!



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