Ehlers tends to brush over most of the DSP theory and in general focuses on applications to trading. But I do think the theory he does present is enough. I don't think you need to do a DSP course to understand and use the concepts he presents. I started doing a DSP course via coursera.org, but quickly realised that it is not necessary to have that level of detailed knowledge in order to use DSP techniques in your trading. If you post specific questions about what you don't understand, I might be able to help.