"The confrontation clauses of both the federal and state Constitutions guarantee a criminal defendant the right to confront the prosecution's witnesses. (U.S. Const., 6th Amend.; Cal. Const. art. I, 15.) That right is not absolute, however. An exception exists when a witness is unavailable and, at a previous court proceeding against the same defendant, has given testimony that was subject to cross-examination. Under federal constitutional law, such testimony is admissible if the prosecution shows it made 'a good-faith effort' to obtain the presence of the witness at trial. (Barber v. Page (1968) 390 U.S. 719, 725; accord, Ohio v. Roberts (1980) 448 U.S. 56, 74, reversed on other grounds in Crawford v. Washington (2004) 541 U.S. 36, 62.) California allows introduction of the witness's prior recorded testimony if the prosecution has used 'reasonable diligence' (often referred to as due diligence) in its unsuccessful efforts to locate the missing witness. (Evid. Code, 240, subd. (a)(5) . . . )." (People v. Cromer (2001) 24 Cal.4th 889, 892 (Cromer).) Generally, "'what constitutes due diligence to secure the presence of a witness depends upon the facts of the individual case. The term is incapable of a mechanical definition. It has been said that the word "diligence" connotes persevering application, untiring efforts in good earnest, efforts of a substantial character. The totality of efforts of the proponent to achieve the presence of the witness must be considered by the court. Prior decisions have taken into consideration not only the character of the proponent's affirmative efforts but such matters as whether he reasonably believed prior to trial that the witness would appear willingly and therefore did not subpoena him when he was available , whether the search was timely begun, and whether the witness would have been produced if reasonable diligence had been exercised .' " (People v. Sanders (1995) 11 Cal.4th 475, 523.) The trial court's ruling regarding due diligence presents a mixed question of fact and law. (Cromer, supra, 24 Cal.4th at p. 893.) To the extent the trial court resolved conflicts in the evidence regarding historical fact, we review the trial court's findings for the existence of substantial evidence. (Id. at pp. 894, 900.) To the extent the trial court concluded that the historical facts "amounted to due diligence by the prosecution," we review the determination de novo. (Id. at p. 900.) The law is well established that "a criminal defendant states a violation of the Confrontation Clause by showing that he was prohibited from engaging in otherwise appropriate cross-examination designed to show a prototypical form of bias on the part of the witness, and thereby, 'to expose to the jury the facts from which jurors . . . could appropriately draw inferences relating to the reliability of the witness.' " (Delaware v. Van Arsdall (1986) 475 U.S. 673, 680 (Van Arsdall).) "However, not every restriction on a defendant's desired method of cross-examination is a constitutional violation. Within the confines of the confrontation clause, the trial court retains wide latitude in restricting cross-examination that is repetitive, prejudicial, confusing of the issues, or of marginal relevance. Thus, unless the defendant can show that the prohibited cross-examination would have produced 'a significantly different impression of the witnesses' credibility' , the trial court's exercise of its discretion in this regard does not violate the Sixth Amendment. " (People v. Frye (1998) 18 Cal.4th 894, 946 (Frye).)