5 ms·

AM radio's band isn't suitable for modern high-speed communications. The most you can extract out of it is ~1200 bps: https://www.diffen.com/difference/AM_vs_

AM radio's band isn't suitable for modern high-speed communications. The most you can extract out of it is ~1200 bps: https://www.diffen.com/difference/AM_vs_FM https://www.diffen.com/difference/AM_vs_FM Even if you multiplexed all possible amplitudes into one it still wouldn't be worth it for (two-way) digital communications.

hotpotamus 3y ago
That seems unintuitive given that we had 56K modems running over telephone systems also designed to carry human-audible audio.
- ianburrell 3y ago
  The radio bands are small. Each AM HD station uses 30 kHz which gives 40-60 kbit. That isn't enough for full fidelity music. And that is using more than one frequency since analog AM is 10kHz stations. AM band uses a large frequency range, 530-1700 kHz, but the total bandwidth is small, slightly more than 1MHz. Telephone is bad compared to AM and really bad compared to FM or CDs. Human voice fits in much smaller bandwidth than music.
  - cf100clunk 3y ago
    > Each AM HD station uses 30 kHz which gives 40-60 kbit. That isn't enough for full fidelity music That's part and parcel of station management's programming decisions for HD Radio subchannels on the AM band, hence the focus on ''talk''.
- MrRadar 3y ago
  Indeed. The US "HD Radio" system has AM support in addition to FM and on AM it can broadcast up to 60 kbps (though most stations chose a 40 kbps mode with additional error correction to improve reception)[1]. Digital Radio Mondiale [2] (with the unfortunate acronym DRM) is another competing digital system system that can be used in the AM and shortwave broadcast bands, supports similar bitrates. [1] https://en.wikipedia.org/wiki/HD_Radio#AM https://en.wikipedia.org/wiki/HD_Radio#AM [2] https://en.wikipedia.org/wiki/Digital_Radio_Mondiale https://en.wikipedia.org/wiki/Digital_Radio_Mondiale
- poo-yie 3y ago
  One is going over the air and the other was going over copper lines. I don't know if that makes a huge difference, but I suspect it makes some difference (especially in inclement weather).
- jalk 3y ago
  I guess that figure is set by some ancient standard for transmitting bits semi reliable on AM, which doesn't employ the same encoding tricks that modems did. POTS lines had a max baud rate of 2400, and the higher speeds were accomplished by encoding more bits pr baud. Parts of the modem noises was to establish which encoding scheme to uses, based on capabilities of the two devices and the line quality. AM broadscasting can't really make such handshakes, so my guess is that 1200 bps was some lowest common denominator. edit: wasn't too clear that I meant to say that the 1200 bps must be from an old standard
- icehawk 3y ago
  The SNR from an AM radio station at about ~30mi is 13dB and if you plug that into the Shannon capacity calculation for regular 15kHz AM radio (not HD AM) you get about 57kBps at ~100mi, that drops to 4kB/sec. 56K modems also had the advantage that the noise floor for the downstream direction was much lower than the upstream direction.

hotpotamus 3y ago

That seems unintuitive given that we had 56K modems running over telephone systems also designed to carry human-audible audio.

ianburrell 3y ago
The radio bands are small. Each AM HD station uses 30 kHz which gives 40-60 kbit. That isn't enough for full fidelity music. And that is using more than one frequency since analog AM is 10kHz stations. AM band uses a large frequency range, 530-1700 kHz, but the total bandwidth is small, slightly more than 1MHz. Telephone is bad compared to AM and really bad compared to FM or CDs. Human voice fits in much smaller bandwidth than music.
- cf100clunk 3y ago
  > Each AM HD station uses 30 kHz which gives 40-60 kbit. That isn't enough for full fidelity music That's part and parcel of station management's programming decisions for HD Radio subchannels on the AM band, hence the focus on ''talk''.
MrRadar 3y ago
Indeed. The US "HD Radio" system has AM support in addition to FM and on AM it can broadcast up to 60 kbps (though most stations chose a 40 kbps mode with additional error correction to improve reception)[1]. Digital Radio Mondiale [2] (with the unfortunate acronym DRM) is another competing digital system system that can be used in the AM and shortwave broadcast bands, supports similar bitrates. [1] https://en.wikipedia.org/wiki/HD_Radio#AM https://en.wikipedia.org/wiki/HD_Radio#AM [2] https://en.wikipedia.org/wiki/Digital_Radio_Mondiale https://en.wikipedia.org/wiki/Digital_Radio_Mondiale
poo-yie 3y ago
One is going over the air and the other was going over copper lines. I don't know if that makes a huge difference, but I suspect it makes some difference (especially in inclement weather).
jalk 3y ago
I guess that figure is set by some ancient standard for transmitting bits semi reliable on AM, which doesn't employ the same encoding tricks that modems did. POTS lines had a max baud rate of 2400, and the higher speeds were accomplished by encoding more bits pr baud. Parts of the modem noises was to establish which encoding scheme to uses, based on capabilities of the two devices and the line quality. AM broadscasting can't really make such handshakes, so my guess is that 1200 bps was some lowest common denominator. edit: wasn't too clear that I meant to say that the 1200 bps must be from an old standard
icehawk 3y ago
The SNR from an AM radio station at about ~30mi is 13dB and if you plug that into the Shannon capacity calculation for regular 15kHz AM radio (not HD AM) you get about 57kBps at ~100mi, that drops to 4kB/sec. 56K modems also had the advantage that the noise floor for the downstream direction was much lower than the upstream direction.