Latency measurement in remote (EAX, USB) and local modes and comparison with Flex radio (DAX v2)

Internal latency measurement of the ERMAK-TT transceiver

• Transmit mode (microphone input -> RF output): 45ms

• Receive mode (RF modulator output -> headphone output): 52ms

Internal latency measurement of FLEX radio

• https://community.flexradio.com/discussion/8027536/signal-in-to-audio-out-rf-to-audio-latency
• https://www.n1kdo.com/sdr-delay-measured/
• https://community.flexradio.com/discussion/6865953/

Thanks to Mike-VA3MW and W7NGA

Total latency measurement of the ERMAK-TT transceiver + EAX (USB)

Measurements were performed on a Windows 11 PC Host + VAC by Eugene Muzychenko. The original VAC from ERMAK can also be used. They provide slightly lower latency due to optimized buffers.

Important note - the original ERMAK VAC drivers are signed with a self-signed certificate. Therefore, driver signature verification must be disabled on the PC host.

1. EAX audio control - release history
2. EAX VAC Windows 10
3. Installing and configuring EAX on WINDOWS
4. Installing and configuring EAX on LINUX

• Receive mode (RF modulator output -> EAX -> PC audio output): 197ms

• Receive mode (RF modulator output -> USB audio -> PC audio output): 220ms

Total latency measurement of the ERMAK-TT transceiver + EAX (USB) and Flex radio in remote mode

Measurement program

• ERMAK-TT — local connection to PC + EAX
• FlexRadio — remote connection (~5000 km), RTT ~100 ms, DAX v2, "long" filters. Propagation speed ≈ 3×10⁸ m/s → latency over 5000 km (one-way) ≈ 16.7 ms; network RTT 100 ms → one-way ≈ 50 ms; total ≈ 67-70 ms margin

Measurement subject:
Reception of one-second pulses at 4999 kHz

• Red ERMAK-TT
• Blue - FLEX radio. "Fast" filters (51ms)

Result: 104ms-70ms=30ms,  ERMAK-TT provides lower latency

• Red ERMAK-TT
• Blue - FLEX radio. "Fast" filters (167ms)

Result: 40ms+70ms=110ms latency difference, FLEX radio provides lower latency

Conclusions

• When using short filters and DAX v1, the latency of FlexRadio and ERMAK-TT(Phoenix)  + EAX) is approximately comparable.
• The main contribution to latency in DAX v1 comes from the audio stack buffers and the VAC driver; in DAX v2 this contribution has been substantially reduced thanks to the redesigned VAC.
• In DAX v1, the original VAC — Virtual Audio Cable by Eugene Muzychenko was used. (https://community.flexradio.com/discussion/8027376/smartsdr-2-8-1-and-bsod)

• In DAX v2, a solution based on Thin Audio Gateway (the same author) is used, which provides smaller buffers and lower latency.
• DAX v2 delivers a latency improvement of approximately ~100 ms compared to DAX v1.
• We plan to migrate to a similarly architected VAC/low-latency audio engine by autumn 2026.