Re: [Discuss-gnuradio] QPSK Receiver and Processing Power

[Ed Criscuolo]

On 2/18/15, 6:45 PM, Richard Bell wrote:
> Hello all,
>
> I am testing a QPSK Tx/Rx I made in grc across two USRP N210's. I can
> demodulate and decode the data fine at the receiver to produce the bits
> I transmitted. The problem is, when watching the frequency sink at the
> output of the USRP source (picture attached), there is typically 4
> seconds of latency between making a change at the transmitter and seeing
> the change at the frequency sink block. I'm worried this is a bad sign
> of things to come.

Richard,

  The lag is not due to processing power, but instead due to buffering
latency.  Every flow between blocks has a default buffer size of 64k
bytes.  This means that the LOWER sample rates will take longer to
traverse the buffer.

The latest versions of GRC include a block parameter to restrict the
buffer size.  This should be set to a much lower value than the default
for the portions of the flowgraph that are dealing with packed or
unpacked data bits, as these are at a MUCH lower rate than the I/Q
samples going into the demodulator.

@(^.^)@  Ed