What is the test for a valid waiver of the Miranda rights?

3 The court also states: "We have concluded that without proper safeguards the process of in-custody interrogation of persons suspected or accused of crime contains inherently compelling pressures which work to undermine the individual's will to resist and to compel him to speak where he would not otherwise do so freely." (Miranda v. State of Arizona, supra, 384 U.S. 436, 467, 86 S.Ct. 1602, 1624.) Under earlier cases, confessions obtained without a valid waiver of Miranda rights were deemed to have been coerced. (People v. Murtishaw (1981) 29 Cal.3d 733, 753, 175 Cal.Rptr. 738, 631 P.2d 446.)