[USRP-users] b200 Overflows

Marcus D. Leech mleech at ripnet.com
Sat Sep 27 15:35:50 EDT 2014


On 09/27/2014 03:15 PM, Simon Brown via USRP-users wrote:
>
> Hi,
>
> Thanks, that's invaluable information. I was wondering whether some of 
> the Ettus radios maybe supported 24-bit resolution, apparently not.
>
> I'll try sc12 tomorrow and see what happens, currently running backups 
> for the rest of the evening.
>
> Simon Brown G4ELI
> http://v2.sdr-radio.com
>
ADCs with that many bits and 10s-of-Msps don't, as far as I know, exist.

Low-bandwidth SDRs very-often use audio codecs, which are available in 
24-bit formats. But as you go up in sample-rate, you find that 16-bits
   is about as much as you will find, and the price goes up 
precipitously as the Msps increases.  Above a few hundred Msps sampling 
rate,
   even 8-bit ADCs are rather pricey, and 10-12 bit ADCs are 
eye-watering....


The ADC in the AD9361 chip in the B200 is 12 bits, the DSP inside the 
B200 normally scales this up to 16 bits as part of the decimation and
   filtering that goes on, and 16 bits is more "natural" than 12 bits 
over the wire.  With 12 bits over-the-wire, you end up with a 
not-wonderfully-fast
   "unpacking" that you have to do, but if bus bandwidth is more 
precious than CPU usage, 12 bits makes sense if you want to preserve dynamic
   range.


> *From:*Marcus D. Leech [mailto:mleech at ripnet.com]
> *Sent:* 27 September 2014 19:41
> *To:* Simon Brown
> *Cc:* usrp-users at lists.ettus.com
> *Subject:* Re: [USRP-users] b200 Overflows
>
> On 09/27/2014 02:30 PM, Simon Brown via USRP-users wrote:
>
>     Hi Marcus,
>
>     I'll try sc12 tomorrow, possibly later this evening. I'm currently
>     preparing food and adding some diagnostics.
>
>     It's probably in the manual, but is there a way to determine the
>     most efficient format for a device without losing resolution, for
>     example sc12 for b200, (maybe) sc16 or fc21 for the N210 etc.? I
>     would like to reduce any network / bus traffic where possible.
>
>     Simon Brown G4ELI
>     http://v2.sdr-radio.com
>
> The number of over-the-wire-formats is strictly-limited---it's not 
> open-ended.
>
> For all products other than B200, the only available formats are sc16 
> and sc8.   B200 has the additional sc12 over-the-wire format.
>
> These "wire formats" are then converted by the driver into one of a 
> few host-side formats, the most natural for a lot of work being
>   fc32.
>
> The idea behind "wire formats" is to preserve a strictly-limited 
> resource, namely, over-the-wire bandwidth. No amount of "buying the 
> very best"
>   1GiGe controller, for example, will get you beyond 1Gigabit of 
> bandwidth over that medium.  Which is why to support 50Msps on the 
> N2xx, you
>   have to use 8-bit wire format.    On the B200, you can reduce USB 
> bus bandwidth, but preserve ADC/DAC dynamic range by using SC12.  I think
>   that only really "plays out" (normally) over USB-2.0. Over USB-3.0, 
> you *should* have plenty of bandwidth available, at least over the USB-3.0
>   bus and inside the controller.   But outside the controller, there 
> may be host-bus limitations that may drive on to using more 
> byte-per-second-conserving
>   formats.
>
>
>
> *From:*USRP-users [mailto:usrp-users-bounces at lists.ettus.com] *On 
> Behalf Of *Marcus D. Leech via USRP-users
> *Sent:* 27 September 2014 19:11
> *To:* usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>
> *Subject:* Re: [USRP-users] b200 Overflows
>
> On 09/27/2014 02:00 PM, Simon Brown via USRP-users wrote:
>
> Thanks,
>
> I've tried changing these, still get overruns with sample rates of 
> 8MS/s or higher.
>
> Simon Brown G4ELI
> http://v2.sdr-radio.com
>
> Are you sure that your USB-3.0 interface is actually dealing with the 
> device as a USB-3.0 and not a USB-2.0 device?
>
> If you specify a wire-format of sc8 or sc12, do the overruns go away?  
> This will help distinguish between cases involving CPU exhaustion, and 
> interior
>   bus deficiencies  (I found this on one of my embedded systems--could 
> sustain only 6.4Msps with full-width samples, but was perfectly happy to
>   stream 12.8Msps with 8-bit samples).
>
>
>
>
> *From:*USRP-users [mailto:usrp-users-bounces at lists.ettus.com] *On 
> Behalf Of *Marcus D. Leech via USRP-users
> *Sent:* 26 September 2014 20:03
> *To:* usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>
> *Subject:* Re: [USRP-users] b200 Overflows
>
> On 09/26/2014 02:18 PM, Simon Brown via USRP-users wrote:
>
>     Hi,
>
>     Windows 64-bit: I'm streaming nicely, none of my threads indicate
>     CPU problems (plenty of headroom). At 8MB/s and higher I'm getting
>     fastpath Overrun messages every second or so even though I'm
>     pulling data from the b200 as fast as it's available, I am not CPU
>     limited in any way. Using Intel USB 3.
>
>     FWIW I don't see any way to tune the uhd::rx_streamer -- bigger /
>     more buffers, also I don't see any way to flush either.
>
>     Interestingly if I get my CUDA card working harder (more work on
>     the bus) the Overrun messages appear more frequently. I7 4770k,
>     good motherboard (can't remember what).
>
>     Any suggestions?
>
>     Simon Brown G4ELI
>     http://v2.sdr-radio.com
>
>
>
>
>
>
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>
> The USB transport parameters can be tweaked:
>
> http://files.ettus.com/manual/page_transport.html#transport_usb_params
>
>
>
>   
>   
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>
>
>
> -- 
> Marcus Leech
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> http://www.sbrac.org
>
>
>
>
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