3–4 students can split the cost of one legal eBook or physical copy and share access (e.g., reading on a tablet one at a time). Not ideal, but legal.

If you just need DSP content, use these legal PDFs from open-access or author-posted sources:

| Book | Link / Source | |------|---------------| | Digital Signal Processing – Proakis & Manolakis (4th ed) – older edition PDF legally free? No, but used copies cheap. | Not free, but Pearson offers some chapters as samples. | | Understanding Digital Signal Processing – Richard Lyons (free 1st ed legally?) | No – but 3rd ed available via library. | | Think DSP – Allen B. Downey | Green Tea Press (free PDF under CC BY-NC) | | Introduction to Digital Signal Processing – Bob Meddins (free from Elsevier’s open archive? No) | Instead use OpenStax – none yet for DSP. |

Actually useful free legally:

For those interested in DSP, there are numerous resources available:

No. He has not released any free full PDF. The only official versions are paid through OUP. Some sample chapters are free on OUP’s website.