We went live in New York and Detroit with a Wheat processor and Belar’s ADC algorithm to determine the success of HD radio and analog signal alignment in blended areas.
Tests so far look promising, indicating a consistent and seamless HD blend to analog whenever HD Radio coverage is less than robust.
"We've had the system on since the beginning of May and the delay between the analog and HD has been within one sample. It was pretty close before, but now it's spot on,” said engineer Brian Kerklan with WMUZ-FM in Detroit, a market chosen for its ties with the auto industry. Belar’s FMHD-1 continuously measures FM/HD time alignment and transmits closed-loop diversity delay corrections back to the Wheatstone on-air processor via our ACI interface. (See related article in this edition of Wheat:News: ACI: It’s Wheatstone’s DNA Needle and Thread).
Initial Findings
Belar's FMHD-1 has a +/- 375mS correction window, so if alignment is within that error band, it can measure it and tell the processor what correction to make. Our processors can adjust over their entire diversity delay range. ”Because the correction scheme is 'closed loop', there are normally no large jumps in delay as correction is applied. With a closed loop system, delay correction is typically on the order of a few samples at most, once the target delay value is met. If I were to go and purposely enter a 300ms offset at the processor, the Belar would see it, order the processor back to the correct value, and then keep it there - probably in one or two or perhaps three correction jumps from what I have observed,” explained Jeff Keith, Wheatstone’s Senior Product Development Engineer for the Vorsis line.
Also being field tested is a new Wheatstone “ramp delay correction” feature that if the time offset error is larger than "x", the correction ramps the delay towards zero offset in user-definable small increments steps over "x" time; both variables are user-definable.
These new developments will eliminate the need for yet another box in the air chain to correct for diversity delay errors between the HD digital and FM analog signals. Wheatstone processors AirAuraX3, FM-531HD, VP-8ip and FM-55 support the automatic diversity delay correction.