Why so much delay?

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Why so much delay?

Post by KE2KB » Tue Dec 06, 2016 7:44 pm

I was listening to a repeater on my Yaesu FT60R, and at the same time listening to the same repeater and its input on SDRPlay RSP1 with SDRUno 1.1.
There was about 1/2 seconds delay on the SDR compared to the repeater direct on the Yaesu HT.
I tried switching to ASIO drivers, but that didn't change the delay, so I assume its not my sound card's drivers.
The computer is a core i5 2500 @ 3.6Ghz. The sound card is a Realtek Hi-Def audio onboard the MSI P67A mobo.
There weren't any other major apps running while I had SDRUno running, and I tried setting both normal and high priority.

Any ideas?

Frank - KE2KB

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Re: Why so much delay?

Post by NN4F » Tue Dec 06, 2016 8:03 pm

Every SDR I've used has some sort of delay over a regular radio, you have the lag of the system decoding the IQ stream and extracting the audio...same on HF when using as a second rx for my ftdx3000 there is a slight delay from the 3000 to the sdr...

Paul - NN4F

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Paul Jones - NN4F, Charleston SC

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Re: Why so much delay?

Post by DK7OB » Wed Dec 07, 2016 6:44 am

Half a second is quite a lot of delay.

I am using the SDRPlay on Linux, mostly with gqrx but I also tested CubicSDR. I also see quite a lot of delay and did some measurements (triggering a dual channel digital oscilloscope with the RF pulse at the SDRplay antenna input and looking at the demodulated audio on the other channel).

There is a delay of ~200ms from antenna input to AF output. Some of this delay is due to the audio buffers of the operating system, but that is not that much. The signal processing also introduces some delay, but that is not that much either.

My suspicion is the buffering in the USB driver and API software, but I did not find the time to verify it.

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Re: Why so much delay?

Post by DaveB » Thu Dec 08, 2016 11:31 am

The same problem occurs when listening to DAB. Analogue FM to FM via SDR#/SDRPlay - less than a second difference - more like an echo. Analogue FM to DAB (DABPlayer+RTL dongle) - DAB is 3 seconds behind.

The reason is that it takes time to digitally process a signal. In the first example above there is a slight delay because of the digital procesing of the analogue signal. In the second example you have the processing delay in the transmission side, plus the processing delay in the SDR chain.

If you ever notice lip-sync problems in a TV broadcast it is because the audio takes less time to process than the video - and the necessary compensation hasn't worked properly.

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