What's the difference between Signal Processing Toolbox and DSP System Toolbox? Why is the DSP System Toolbox required for generating code of Signal Processing Toolbox functions using MATLAB Coder?
Signal Processing Toolbox works in MATLAB and in general only deals with double precesion signals. On the other hand, DSP System Toolbox provides more sophisticated filter design capability, can work with single and fixed point signals, and has a Simulink interface too.
AFAIK, DSP System Toolbox (which was introduced in R2011a) simply replaces the Signal Processing Blockset™ and Filter Design Toolbox™ products - in other words, it combines functions from both toolboxes. Perhaps your code contains Filter Design functions?