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I have a board that outputs an i2s signal (mclk at 49Mhz) using normal fliflops at output. Works perfectly when connected with short connections.

If I try connecting a cable at 10cm or more, I have some noise comming into the system.

My question is, should I use a buffer after the FFs to correct this ?

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    \$\begingroup\$ You should give us a bit more details. What type of FFs/driver is this? What is connected at the receiver side of the cable? What type of cable (coax)? How many signal lines? ... \$\endgroup\$ – Stefan Wyss Apr 27 '18 at 10:47
  • \$\begingroup\$ This is what S/PDIF especially over an optical link is for; implemented with careful impedance matching the coaxial form might also work, but there are lots of implementations that cheat on the driver/receiver circuitry, temptation to use random audio cables rather than those with the right impedance, etc. \$\endgroup\$ – Chris Stratton Apr 27 '18 at 13:50
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At 49MHz, a 10 cm cable becomes a long line and needs to be terminated to avoid signal reflections. The problem is that termination works well on point-to-point connections, and not so well on buses.

If your connection is unidirectional P2P, I would try to source-terminate all signals at the transmitter's side. Otherwise, you'll have to look at your signals with a scope to see what kind of noise you're getting exactly and what could be done to deal with it.

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