Hmm that's a good point, that you can use the ultrasonic bandwidth of the LP, which is useless for analog audio, to increase the bitrate. I'd say it's a bit generous to give an LP a bandwidth of 50 kHz, especially in the inner tracks though. I think what I was getting at would apply where the bandwidth of the analog signal you want to encode is as wide as the bandwidth of your media though. Take compact cassettes, let's say they have a bandwidth of 18 kHz and a dynamic range of 60 dB. If you want to encode an arbitrary audio signal with a bandwidth of 18 kHz and with 60 dB dynamic range, you could only do that in the usual analog way. But if you wanted to encode a digital telephone signal (say at 8 kHz bandwidth, 30 dB dynamic range) maybe you could use the 18 kHz cassette bandwidth to encode that signal digitally.