Re:Akin's Laws of Spacecraft Design [pdf]
by mlyle on Thursday January 01, 2026 @10:31PM
(#46461097)
> An analog channel with the bandwidth and SNR characteristics of a landline phone line has (IIRC) a Shannon capacity of 30-something kbit/s,
The problem is that the ADC in the telephone exchange is not clocked coherently to your modem. You're paying various companding, quantization (time and level), etc, losses-- it is not a nice linear channel.
An end-to-end analog connection could be capable of significantly more-- while there's noise, there's no "8KHz cliff." A 45dB SNR for 0-4KHz has a >60kbps Shannon bandwidth, and there's not exactly a 4KHz cliff even with loading coils present.
The problem is that the ADC in the telephone exchange is not clocked coherently to your modem. You're paying various companding, quantization (time and level), etc, losses-- it is not a nice linear channel.
An end-to-end analog connection could be capable of significantly more-- while there's noise, there's no "8KHz cliff." A 45dB SNR for 0-4KHz has a >60kbps Shannon bandwidth, and there's not exactly a 4KHz cliff even with loading coils present.