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12 Aug 2016

Towards a 'TiVo for radio'

The work I’ve been doing with Tvheadend to record and time-shift ATSC broadcast television got me thinking about my pile of old NTSC tuner cards, leftover from my MythTV system designed for recording analog cable TV. These NTSC cards aren’t worth much, now that both OTA broadcast and most cable systems have shifted completely over to ATSC and QAM digital modulation schemes, except in one regard: they ought to be able to still receive FM broadcasts.

Since the audio component of NTSC TV transmissions is basically just FM, and the NTSC TV bands completely surround the FM broadcast band on both sides, any analog TV reciever should have the ability to receive FM audio as well – at least in mono (FM stereo and NTSC stereo were implemented differently, the latter with a system called MTS). But of course whether this is actually possible depends on the tuner card’s implementation.

I haven’t plugged in one of my old Hauppage PCI tuner cards yet, although they may not work because they contain an onboard MPEG-2 hardware encoder – a feature I paid dearly for, a decade ago, because it reduces the demand on the host system’s processor for video encoding significantly – and it wouldn’t surprise me if the encoder failed to work on an audio-only signal. My guess is that the newer cards which basically just grab a chunk of spectrum and digitize it, leaving all (or most) of the demodulation to the host computer, will be a lot more useful.

I’m not the first person to think that having a ‘TiVo for radio’ would be a neat idea, although Googling for anything in that vein gets you a lot of resources devoted to recording Internet “radio” streams (which I hate referring to as “radio” at all). There have even been dedicated hardware gadgets sold from time to time, designed to allow FM radio timeshifting and archiving.

• Linux based Radio Timeshifting is a very nice article, written back in 2003, by Yan-Fa Li. Some of the information in it is dated now, and of course modern hardware doesn’t even break a sweat doing MP3 encoding in real time. But it’s still a decent overview of the problem.
• This Slashdot article on radio timeshifting, also from 2003 (why was 2003 such a high-water-mark for interest in radio recording?), still has some useful information in it as well.
• The /drivers/media/radio tree in the Linux kernel contains drivers for various varieties of FM tuners. Some of the supported devices are quite old (hello, ISA bus!) while some of them are reasonably new and not hard to find on eBay.

Since I have both a bunch of old WinTV PCI cards and a newer RTL2832U SDR dongle, I’m going to try to investigate both approaches: seeing if I can use the NTSC tuner as an over-engineered FM reciever, and if that fails maybe I’ll play around with RTL-SDR and see if I can get that to receive FM broadcast at reasonable quality.

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