Is a simple directive to "testify" to an experienced police officer sufficient to impress the witness with the duty to tell the truth?

It is questionable whether a simple directive to "testify," even to an experienced police officer, is adequate to impress the witness's mind with the duty to tell the truth. However, any procedural deficiency in failing to administer a formal oath was waived by the failure to object at the hearing. (People v. Duffy (1930) 110 Cal.App. 631, 635-636.) A timely objection would have alerted the trial court to the oversight, and allowed correction. It is now too late to raise the matter. Where "`a witness is permitted to testify without having been previously sworn and that fact is known at the time, the defect must be taken advantage of at once and a failure to do so is such acquiescence in the testimony as will preclude objection after verdict.'" (Ibid.)