What is the current state of the law on the issue of the voluntariness of a confession?

The issue of voluntariness presents a mixed question of law and fact that is nevertheless predominantly legal.... [Citation.] Hence [o]n appeal, the determination of a trial court as to the ultimate issue of voluntariness of a confession is reviewed independently.... [] The trial court's determinations concerning whether coercive police activity was present, whether certain conduct constituted a promise and, if so, whether it operated as an inducement, are apparently subject to independent review as well. [Citation.] However, the trial court's findings as to the circumstances surrounding the confessionincluding the characteristics of the accused and the details of the interrogation [citation]are clearly subject to review for substantial evidence.... ' [Citation.] (People v. Jones (1998) 17 Cal.4th 279, 296, 70 Cal.Rptr.2d 793, 949 P.2d 890.)