I noticed a mention of some latency issues in another thread on this forum.
Rob Sherwood has made some latency measurements of a selection of transceivers on receive at this page:
My own oscilloscope and pulsed audio testing, both on transmit and receive, give the following results on the two transceivers I have in the shack and which incidentally both work fine on VarAC.
Icom IC-7300 RX measurements using Data mode:
3 KHz bandwidth, 3.3 mS
2 KHz bandwidth, 3.5 mS
Icom IC-7300 TX measurements using Data mode:
Elecraft K4D RX measurements using Data mode:
Any bandwidth, 32 mS
Elecraft K4D TX measurements using Data mode:
The above figures are the pure delay timing through the respective transceiver chain, not including any additional TX/RX switching delays.
As you will see from the above, the humble Icom IC-7300 has significantly less latency than some other full SDR transceivers and indeed my IC-7300 works "out of the box" without any issues on Amtor using an SCS or Kantronics KAM, which is much more critical on timing and latency than VarAC.
I have an IC-7300 and I love it. Very good all around radio and on demanding digital modes such as PACTOR and VarAC I think it is the best! 73 de Demetre
In the early days of VarAC we struggled with pure SDR to SDR connections, specifically Flex radios to SunSDR. But not all pure SDR's seemed to have issues, my Anan 7000DLE seemed to be fine but my Flex 6500 had issues. My IC-7300 had no issues.
We did a lot of testing with a special version of VARA HF that Jose made that measured Latency and we collected a lot of good info that Jose put to good use and minimized the effects of this latency.
In ARQ modes like VARA, latency can be a link killer.