What is the burden of proving due diligence in the prosecution's search for a witness?

"Evidence Code section 1291, subdivision(a)(2), provides that former testimony is not rendered inadmissible as hearsay if the declarant is 'unavailable as a witness,' and '[t]he party against whom the former testimony is offered was a party to the action or proceeding in which the testimony was given and had the right and opportunity to cross-examine the declarant with an interest and motive similar to that which he has at the hearing.' In turn, Evidence Code section 240, subdivision(a)(5), states a declarant is 'unavailable as a witness' if the declarant is '[a]bsent from the hearing and the proponent of his or her statement has exercised reasonable diligence but has been unable to procure his or her attendance by the court's process." (People v. Wilson, supra, 36 Cal.4th at p. 341.)

The courts have not divined a mechanical definition for reasonable or due diligence. It does connote " 'preserving application, untiring efforts in good earnest, efforts of a substantial character.' " (People v. Cromer (2001) 24 Cal.4th 889, 904.) "Considerations relevant to the due diligence inquiry 'include the timeliness of the search, the importance of the proffered testimony, and whether leads of the witness's possible location were competently explored.' " (People v. Herrera (2010) 49 Cal.4th 613, 622.) The prosecution bears the burden of proving due diligence in attempting to secure a witness's attendance at trial. (People v. Roldan (2012) 205 Cal.App.4th 969, 979.)