In Barber v. Page, 390 U.S. 719 (1968), a witness whose testimony was sought in a state trial court was incarcerated in a federal prison in a different state. The witness testified at the defendant's preliminary hearing, but he was in a federal penitentiary in Texas when the defendant's trial began in Oklahoma, and the prosecution made no effort to procure the witness's presence. The trial court permitted the witness's preliminary hearing testimony to be read to the jury on the ground that the witness was unavailable to testify since he was outside the jurisdiction. (Barber, supra, 390 U.S. at p. 720.) Barber reversed the judgment and held the defendant was deprived of his Sixth Amendment right to confrontation by the introduction of the witness's former testimony. Barber held that in light of the adoption of the Uniform Act by multiple states, the mere absence of a witness from the jurisdiction is insufficient to establish due diligence when the state knows of the witness's whereabouts. (Barber, supra, 390 U.S. at p. 724 & fn. 4.) Barber held that "a witness is not 'unavailable' for purposes of the foregoing exception to the confrontation requirement unless the prosecutorial authorities have made a good-faith effort to obtain his presence at trial. The State made no such effort here, and, so far as this record reveals, the sole reason why [the witness] was not present to testify in person was because the State did not attempt to seek his presence. The right of confrontation may not be dispensed with so lightly." (Id. at pp. 724-725.) In holding that the prosecution was required to make a good faith effort to obtain the attendance of the witness, the court noted that the ability to obtain such attendance depended on the discretionary cooperation of federal authorities. It concluded, however, that "'the possibility of a refusal is not the equivalent of asking and receiving a rebuff.' " (Barber v. Page, supra, 390 U.S. at p. 724.) The Court recognized the state court did not have the power to compel the attendance of a witness incarcerated in federal prison in a different state. Federal statute, however, gave "federal courts the power to issue writs of habeas corpus ad testificandum at the request of state prosecutorial authorities." (390 U.S. at p. 724 88 S. Ct. at p. 1321.) Furthermore, it was the policy of the federal prisons "to permit federal prisoners to testify in state court criminal proceedings pursuant to writs of habeas corpus ad testificandum issued out of state courts." (Ibid.) The Barber court concluded the defendant's right to confrontation had been violated because the prosecution did not make a good faith effort to obtain the attendance of the witness at trial. (Id. at p. 725 88 S. Ct. at p. 1322.) In Barber, in 1968, the high court, relying on then recent changes in cooperation between states to make witnesses available at trials, rejected earlier pronouncements that a witness is unavailable simply because that witness was not present within the jurisdiction of the court. (Barber v. Page, supra, 390 U.S. at pp. 723-725 88 S. Ct. at pp. 1321-1322.) The Mancusi court held, in 1972, that the United States had not yet made agreements with foreign countries similar to the interstate agreements found in Barber. (Mancusi v. Stubbs, supra, 408 U.S. at p. 212 92 S. Ct. at pp. 2312-2313.) In Roberts, in 1980, the court declared that the "basic litmus of Sixth Amendment unavailability" is whether the prosecution made a good faith effort to obtain the witness's presence (Ohio v. Roberts, supra, 448 U.S. at p. 74 100 S. Ct. at p. 2543), thus implicitly rejecting the notion that a witness could be found unavailable without considering any change in the conditions that might allow the prosecutor to secure the attendance of the witness at trial. The Roberts court warned that "if there is a possibility, albeit remote, that affirmative measures might produce the declarant, the obligation of good faith may demand their effectuation." (Ibid.)