Re: USRP Custom Filter Taps

Matt E. wrote:

Yes, you can change the filter taps in the FPGA. See the halfband (HB)
verilog code.

Thanks! I’ll take a look.

I read on a previous thread that it is possible to invert the spectrum of the sampled signal so that the tuned frequency would be the FM carrier. The end result being a stream from the USRP where 0 Hz is the center frequency of the audio carrier the the video carrier is at 4.5 MHz. Is this possible? The advantage here is that the base-band audio does not need to be shifted before FM demodulation; also the video carrier does not need to be shifted before demodulation (AM).

Not sure what you’re looking for here.

After thinking about it, I’m not sure either. I figured I can to tune to
FM carrier to the center frequency (0Hz), set the decimation to 4 (16
MS) and the video carrier is at -4.5MHz. Unfortunately, my USB/computer
can’t handle can’t handle the 16 MS rate. I was hoping to avoid
translating the signal by placing the FM signal at 0Hz. Since my PC
can’t handle that data rate, I’m just going to concentrate on the NTSC.

-Daniel

As an aside: It would be nice to have something like the GC4014 on-board
the USRP. In my application it could be used to tune the video and audio
signals on separate receiver channels without using up resources on the
PC (and also reduce the need for a faster interface in some cases). Not
sure if there’s enough room on the FPGA though.

As an aside: It would be nice to have something like the GC4014
on-board the USRP. In my application it could be used to tune the
video and audio signals on separate receiver channels without using
up resources on the PC (and also reduce the need for a faster
interface in some cases). Not sure if there’s enough room on the FPGA
though.

That’s essentially what is in the FPGA. There are 2 DDCs in the
standard build, and 4 in the RX-only build. Each can be tuned to
different ADCs, or to the same ADC to get separate bands from the same
stream.

Matt

On Thu, Oct 05, 2006 at 05:51:49PM -0700, Daniel G. wrote:

Matt E. wrote:

Yes, you can change the filter taps in the FPGA. See the halfband (HB)
verilog code.

Thanks! I’ll take a look.

I read on a previous thread that it is possible to invert the spectrum of the sampled signal so that the tuned frequency would be the FM carrier. The end result being a stream from the USRP where 0 Hz is the center frequency of the audio carrier the the video carrier is at 4.5 MHz. Is this possible? The advantage here is that the base-band audio does not need to be shifted before FM demodulation; also the video carrier does not need to be shifted before demodulation (AM).

Not sure what you’re looking for here.

After thinking about it, I’m not sure either. I figured I can to
tune to FM carrier to the center frequency (0Hz), set the decimation
to 4 (16 MS) and the video carrier is at -4.5MHz. Unfortunately, my
USB/computer can’t handle can’t handle the 16 MS rate. I was hoping
to avoid translating the signal by placing the FM signal at
0Hz. Since my PC can’t handle that data rate, I’m just going to
concentrate on the NTSC.

-Daniel

As an aside: It would be nice to have something like the GC4014
on-board the USRP. In my application it could be used to tune the
video and audio signals on separate receiver channels without using
up resources on the PC (and also reduce the need for a faster
interface in some cases). Not sure if there’s enough room on the
FPGA though.

The cost of extracting and decimating the audio portion of the full
signal in s/w is pretty small.

Have you tried just setting decimation to 8 giving 8MS/s complex? Why
are you concerned about anything at 16MS/s? The 8 MS/s signal will have
both the audio and video in it. A small amount of filtering would
separate the two streams in software.

Eric

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