Feedback from our article about KRPE-LD’s conversion to hybrid analog FM digital TV:
I read your article in TV Technology covering the Channel 6 ATSC 3.0 with analog FM STA that is being implemented by Venture Technologies. In the article, you mentioned that Venture needed to shift the FM carrier frequency back to the old NTSC aural location at 87.75 MHz rather than using 87.7 MHz to accommodate automotive receivers.
Let me give you a bit of history/perspective from someone who was a Radio design engineer and engineering manager for Delphi/Delco Electronics designing radios for most of the GM and some Toyota and Honda vehicles.
Back in the days before ETR’s (synthesized Electronically Tunable Receivers) with digital readout the continuous tuning receivers with analog dials had enough margin at the bottom of the band to reach 87.75 or close enough to get the AFC to lock.
With the advent of ETR’s with 200 kHz tuning steps, the low end of the display became 87.7MHz. What was not commonly known is that the synthesizer was actually tuning 87.75 while displaying 87.7 (for that frequency only).
Another issue with the analog receivers was low recovered audio due to the lower deviation of the NTSC aural signal. When tuned to 87.75 some of the newer receiver designs (beginning in the mid to late 1990s) would increase the gain of the radio’s audio stage by about 6dB to bring the Channel 6 audio closer to the level of the FM stations without having to adjust the volume control radically upward.
Since 2008 when I left Delphi and retired, designs may have changed but it is highly likely that they hadn’t since “Franken FMs” were still alive even after the switch from NTSC to ATSC. My best guess is that they didn’t change since there was still something to hear on 87.75 in several markets.
Hopefully, this will provide a bit of closure to Daniel Bissett’s questions about why the automotive receivers distorted so badly before the shift back to 87.75. It’s not obvious to the casual listener in this world of digital tuners with 200 kHz display resolution. The underlying receivers are considerably more complex.
PD Technologies LLC