What is the burden of establishing unavailability and due diligence in a criminal case?

"The burden of establishing unavailability and due diligence . . . so as to render admissible [the statement] of a declarant who is unavailable as a witness, is on the proponent of the evidence, and the ruling of the trial judge on the factual question of whether the burden has been met will not be disturbed unless there has been an abuse of discretion. In criminal cases, the guidelines for determining whether the prosecution has met its burden of establishing 'unavailability' and 'due diligence' are as follows: First, the prosecution must show that reasonable diligence has been exercised in an effort to procure the witnesses' attendance by the court's process. Secondly, where the witness cannot, for whatever reason, be obtained by the court's process, the prosecution must make the additional showing of a good-faith effort and reasonable diligence to procure the witnesses' voluntary attendance." (People v. Masters (1982) 134 Cal.App.3d 509, 513.)