Latency out of Stream Mux block with unequal input lengths

[Gannon, Adam M. (GRC-LCI0)]

Hi List,

I'm trying to understand the latency behavior of the Stream Mux block when two 
streams of unequal lengths are being multiplexed together. Attached is a 
minimal flow graph that illustrates the point. 

In the example, two streams are multiplexed: one Vector Source of length 50 and 
one of length 1 whose value is adjusted while running. It takes several seconds 
at 1Msps for the change in the value of the shorter (L=1) stream to be 
reflected in the muxed stream. The reverse is not true - a change in the longer 
stream (L=50) is immediately reflected in the muxed stream. I believe this is 
because both Vector Sources are producing samples at the same rate but are 
being muxed at a ratio of 50:1. So when the value of the L=1 stream changes it 
has to empty the existing samples from its output buffer before the new samples 
start to be muxed. 

In my application, I am formatting data from an external source into the header 
of my packet. There are several stream multiplexers after that to form the 
packet (adding payload, trailer, etc). I'd like to minimize the latency before 
new data is reflected in the entire formatted packet. I was considering writing 
one block to create the entire packet, but wanted to ask for advice on if there 
is a better / more modular solution. 


Much appreciated, 
Adam

latency_test.PNG
Description: latency_test.PNG

latency_test.grc
Description: latency_test.grc