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Author Topic: SDR - where to start from?  (Read 516 times)

N2DTS

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Re: SDR - where to start from?
« Reply #15 on: October 31, 2022, 05:31:43 PM »

There are no analog sdr receivers since sDr is Digital.
Receiver latency comes from two or maybe three sources, a major source is the USB interface some radios use. USB was a polled port with buffers and while throughput could be ok, every computer I ever used had at least 500ms of latency through it, no matter how fast the computer was.
USB-3 may be much better, but I have not had any radios that came with a usb3 port, only usb2.
Firewire was a Sony interface I think, for Music processing I think that had a very good throughput and low latency and worked very well with the old Flex 3000 and 5000 radios.
All the Flex radios after that use Ethernet. Anan uses Ethernet.
Some radios have the processor in the radio like the Icom 7300 and 7610, the Flex M series, the new Yeasu SDR rigs, the Elad FDM DUO, and others.
Latency can be very low (Icom, Yaesu) or a bit higher (flex M series and the Elad).
On a fair (not great) computer the old Flex 3000 and 5000 could be almost real time, I could listen to my audio going out with just a trace of delay.

I have an MChf radio that converts the RF to I and Q using a sound card chip and processes the data in the radio and it has very low latency, same as the Xiegu G90 I think.
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N3KXZ

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Re: SDR - where to start from?
« Reply #16 on: November 01, 2022, 07:41:37 AM »

Can you talk a little about why latency makes a difference with SDR radios? I guess it depends on what you want to do? Which functions that one uses an SDR for would latency make, or not make, a significant difference?
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G8FXC

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Re: SDR - where to start from?
« Reply #17 on: November 02, 2022, 07:14:10 AM »

Can you talk a little about why latency makes a difference with SDR radios? I guess it depends on what you want to do? Which functions that one uses an SDR for would latency make, or not make, a significant difference?

A lot depends on how constant that latency is. If the audio arrives a constant 100mS late, nothing happens. But running digital modes can be seriously impacted by variable latencies - for example on 50 baud RTTY, each bit is of the order of 20mS long - a latency varying by even a few mS can make it almost impossible to decode. The same is true of most digital modes - even moderately fast CW can be impacted - at 25wpm, each dit is just a few tens of mS long...

With the newer clock synched digital modes like FT8, a long, constant latency could be a problem, but it would have to be a significant fraction of a second...

Martin (G8FXC)
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N2DTS

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Re: SDR - where to start from?
« Reply #18 on: November 02, 2022, 05:31:33 PM »

On asll the setups I have had the latency is constant.
Over the web would vary a lot...
USB is about 250 to 500 Ms, if you want to listen to your voice going out you can not do it with that much delay.
Some radios had issue with the sidetone being delayed on CW but the work around was doing it seperate from the actual signal going out.
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K6JH

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Re: SDR - where to start from?
« Reply #19 on: November 02, 2022, 05:51:00 PM »

I imagine latency can be an issue also if one runs full break-in CW.
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Jim K6JH

N6YWU

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Re: SDR - where to start from?
« Reply #20 on: November 06, 2022, 12:14:08 PM »

Variable latency can be a huge problem if you are trying to transmit. 

Variable latency can either cause chopped up SDR transmit signals, where the Rx/Tx relays might even starting clicking/clattering if any packets of transmit data arrive too late compared to the usual arrival rate.  Or variable latency requires buffering up enough data ahead of time before transmitting to cover the worse case time jitter, which can cause the receiving station or full break-in CW ops (who often don't wait) to think they weren't heard and then call someone else. 

Variable latency is mainly a problem when using network routers, especially WiFi and cellular network routers.
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AK4YA

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Re: SDR - where to start from?
« Reply #21 on: November 07, 2022, 10:52:24 AM »

I've requested ApacheLabs to get me on a notification/waitlist for the next run of Mercury boards.  I've also asked the same of MakerFabs for their HermesLite2.
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