Index: gr-usrp/src/usrp1_source_base.cc
===================================================================
--- gr-usrp/src/usrp1_source_base.cc	(revision 3982)
+++ gr-usrp/src/usrp1_source_base.cc	(working copy)
@@ -29,7 +29,6 @@
 #include <usrp_standard.h>
 #include <assert.h>
 
-static const int OUTPUT_MULTIPLE_BYTES = 4 * 1024;
 
 usrp1_source_base::usrp1_source_base (const std::string &name,
 				      gr_io_signature_sptr output_signature,
@@ -58,9 +57,9 @@
     throw std::runtime_error ("can't open usrp1");
 
   // All calls to d_usrp->read must be multiples of 512 bytes.
-  // We jack this up to 4k to reduce overhead.
+  // We set this to match the fusb block size
 
-  set_output_multiple (OUTPUT_MULTIPLE_BYTES / output_signature->sizeof_stream_item (0));
+  set_output_multiple (d_usrp->block_size() / output_signature->sizeof_stream_item(0));
 }
 
 usrp1_source_base::~usrp1_source_base ()
@@ -91,13 +90,19 @@
 			 gr_vector_const_void_star &input_items,
 			 gr_vector_void_star &output_items)
 {
-  static const int BUFSIZE = 4 * OUTPUT_MULTIPLE_BYTES;
+  static const int BUFSIZE = 64 * 1024;
   unsigned char buf[BUFSIZE];
   int output_index = 0;
   int output_items_produced;
   int bytes_read;
   bool overrun;
 
+  // To minimize rx latency, never return more than 1 fusb block_size worth of samples.
+  // FIXME: A better fix would be to return the maximum we could without blocking,
+  // but the current interface doesn't allow us to do that.
+
+  noutput_items = std::max(output_multiple(), noutput_items);
+
   while (output_index < noutput_items){
     int nbytes = ninput_bytes_reqd_for_noutput_items (noutput_items - output_index);
     nbytes = std::min (nbytes, BUFSIZE);