[hpsdr] HPSDR and Beamforming

[Murray Lang]

Hi Jeff

I don't think you're clicking "Reply All" because hpsdr at hpsdr.org is not 
appearing in your headers. I suspect I'm the only one receiving your posts.
More comments below.

At 04:10 AM 14/09/2006, jeff millar wrote:
>>Now I'm wondering about synchronisation of sound cards if phase 
>>adjustments are performed by PC software and there is a separate sound 
>>card for each Tx/Rx. I suspect that they would have jitter relative to 
>>each other due to task switching in the device drivers and to their own 
>>clocks being unsynchronised. I suppose you would have to go for a 
>>professional sound card with more inputs/outputs (at least 4 stereo 
>>channels in each direction?). Maybe this is a candidate for an HPSDR board.
>>
>Hmmm...As the frequency drops from RF to IF to Audio, the amount of jitter 
>tolerance goes up (as a percentage of the frequency).

But this contradicts what has been said earlier about the phase shift at 
I/Q scaling proportionately to RF.

>Assume each audio A/D channel can DMA a continuous stream into into 
>memory. Now we have to assume that each audio A/D runs from the same clock 
>reference. I don't know how the HPSDR project arranges that across audio cards.

This basically proscribes using multiple sound cards unless they have 
external clock inputs (which I'm not aware that any do). Even if they could 
be synchronised, I'm not convinced that the data sent to them would be 
synchronised unless maybe a single device driver drove all of them. Sounds 
way too messy.

Perhaps it would be better if all the audio channels were delivered by USB. 
I think this is relevant to the HPSDR project because any audio data packet 
format will need to be designed to allow for more than two channels.

Murray
VK6HL







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