Re: [Discuss-gnuradio] ADC tied directly to CPU?

[mgray]

A video ADC will have a number of draw backs.  First, the sampling rate
will be tied to the video bandwidth, so expect something like 14.318MHz
(4x color burst) which is then comb filtered to exact the Y and C
components.  Second, the board will have a DC level shifter to setup the
ADC input to only digitize the video portion of the signal.  The sync
signal are stripped off and processed away from the ADC.  If you digitize
a band of signals, this circuit will be fooled and the level shifter will
move around.  This is similiar to digitizing a macro vision encoded video
source.  The video signal "brightness" moves up and down and in some cases
the capture card loose sync.

The USRP is about the lowest priced digitizer you'll find unless you want 
to make your own.  By the time you do that, you probably could have bought 
a couple USRPs.  If only it was Gig-Ethernet instead of USB. :)




On Fri, 21 Jan 2005, Hew How Chee wrote:

> Hi,
> 
> I am also looking for a cheap alternative to get the
> ADC data into the PC. Was thinking about the Bt878 or
> the CX2388x PCI video decoder (used on many low cost
> TV cards). They have high speed ADC but alas, the chip
> processed the sampled data first before it is DMA'ed
> to the PC memory. How I wish Conexant add in a
> register to bypass the processing. BT878 however, have
> a lower speed ADC from where you can get the sampled
> data.
> 
> USRP is a good option but will need an upgrade on my
> PC (USB 1.2 only)
> 
> My question is have anyone use USRP through a USB2.0
> PCI interface card instead going direct to the onboard
> USB 2.0 interface ?
> 
> Does anyone know how many people are using the USRP
> cards now ? For development of GnuRadio or for any
> other purpose ?
> 
> Best regards,
> Hew
> 
> 
> --- Eric Blossom <address@hidden> wrote:
> 
> > On Fri, Jan 21, 2005 at 05:39:27PM -0500, Bryan E.
> > Chafy wrote:
> > > How feasable would it be to directly interface an
> > AD to a pentium (1,2,or 3)
> > > class CPU?
> > 
> > You'd hang it off some standard bus, e.g., PCI, PCI
> > express.
> > 
> > > For raw AD sampling say a satellite IF signal, the
> > USB and even PCI is marginal
> > > at best.
> > 
> > How fast do you want to sample?
> > 64-bit 66 MHz PCI is pretty fast  (~500 MB/sec)
> > 
> > > Also, the protcols are not simple (nothing is easy
> > and synchronous,
> > > always a burst, always a delay).  PCI/USB make
> > FPGA's almost a requirement.
> > 
> > The FPGA isn't a requirement for PCI, however you
> > might want one
> > anyway.  They are much, much faster than the host
> > processor for
> > certain classes of problems.
> > 
> > > PCI Express looks nice and fast, but it's far away
> > from cheap or practical at
> > > this time.
> > > 
> > > I was thinking something like Fig 9-11, or 9-12 in
> > the following page:
> > >
> >
> http://216.239.63.104/search?q=cache:b630kpcbVcoJ:www.ee.polyu.edu.hk/staff/eenc
> > > heun/WebSubject2/chapter9.htm
> > > It appears to look straightforward enough, but
> > would the same concept
> > > work with a modern cpu/mobo combination?
> > 
> > It was straight-forward because the microprocessor
> > it was hooked to
> > was running at something on the order of 1 MHz, not
> > 1 GHz, and had a
> > very simple bus.
> > 
> > > I dont mean to create a new circuit board, but
> > simply homebrew
> > > solder an AD converter directly to the CPU pins
> > underneath an ordirary
> > > motherboard.  Doing an "in" at some hardwired
> > address would read the contents.
> > 
> > I think you'll find it's a bit of a challenge.
> > 
> > For any given processor, there will be a hardware
> > manual will explain
> > the organization of the bus.  One of the reason PCI
> > exists is so that
> > there would be a standard bus to plug peripherals
> > into.  What goes on
> > between the CPU / memory controller / north bridge /
> > south bridge is
> > *highly* dependent on exactly which CPU chip you've
> > got.  Grab one of
> > the pentium hardware manuals and take a look.
> > 
> > http://developer.intel.com/
> > 
> > 
> > > Since the FSB of a P3 class is 100 or 133mhz, a
> > highspeed AD could be used
> > > (or slower if the clock is divided down).
> > > If successful, I know starvation is a problem at
> > these rates, but if more
> > > than one motherboard were wired up in this way,
> > perhaps some
> > > parallelism/staggering could solve that.?
> > > 
> > > Just a thought on highspeed AD on the cheap.
> > 
> > We had the same goal: getting high speed analog data
> > in and out of the
> > PC for a reasonable price.  The USRP was the result.
> > 
> > Eric
> > 
> > 
> > _______________________________________________
> > Discuss-gnuradio mailing list
> > address@hidden
> >
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> > 
> 
> 
> 
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