Article I, section 14 of the California Constitution gives the defendant a right to an interpreter throughout the proceedings against him. This right has several aspects: (1) an interpreter assists questioning of a non-English-speaking witness; (2) an interpreter enables the non-English-speaking defendant to comprehend the proceedings; (3) an interpreter allows the non-English-speaking defendant to communicate with counsel. (People v. Aguilar (1984) 35 Cal.3d 785, 790.) The presence of an interpreter is necessary so the defendant can understand and fully participate in the proceedings. (People v. Rodriguez (1986) 42 Cal.3d 1005, 1010.) "At moments crucial to the defense - when evidentiary rulings and jury instructions are given by the court, when damaging testimony is being introduced - the non-English-speaking defendant who is denied the assistance of an interpreter, is unable to understand and participate in the proceedings which hold the key to freedom." (People v. Aguilar, supra, at pp. 790-791.) A stipulation by counsel cannot serve as a valid waiver of this right. (Id. at p. 794.) If no interpreter is provided, the defendant must show that he was prejudiced by the omission. We employ the "'harmless beyond a reasonable doubt'" standard of review in examining whether the deprivation requires reversal. (People v. Rodriguez, supra, 42 Cal.3d at pp. 1011-1012; Chapman v. California (1967) 386 U.S. 18, 24.) Even where some deprivation is shown, the absence of a personal interpreter may be found harmless because the proceedings which took place while the interpreter was absent may be unsubstantial or concern matters which are not possibly prejudicial to the defendant. (People v. Rodriguez, supra, at p. 1011.)