What is the test for determining the voluntariness of a confession in a criminal case?

"[A]ny factual findings by the trial court as to the circumstances surrounding an admission or confession, including ' "the characteristics of the accused and the details of the interrogation" [citation],' are subject to review under the deferential substantial evidence standard." (People v. Williams, supra, 16 Cal.4th at p. 660.) However, " ' "[w]hen, as here, the interview was tape-recorded, the facts surrounding the giving of the statement are undisputed, and the appellate court may independently review the trial court's determination of voluntariness." ' " (Peoples, supra, 62 Cal.4th at p. 740.)