Fred (FL) wrote:
The SSB bandwidth 2007 realities, sound familiar.
Yes, this sort of false reasoning is common in marketing. They rely on
a perception that anything new to the market must be better, and that
the general public doesn't understand the true reasons for limitations.
In 1978 - max bandwith over telephonic modems,
and conditioned AT&T lines was like 6250 baud.
The maximum baud rate over telephone lines with the standard analogue
telephony bandwith is about 2400, and still is.
A "limit" everyone agreed.
Then some clever soul or group, came up with
quadrature modulation - and rather quietly,
6250 bps is not possible without at least the use of quadrature
modulation. I think the limit for that is 4800 (although it might just
be 2400). I guess, if you pushed the bandwidth to the limit, you might
get 3125 baud, and therefore 6250 bps with quadrature modulation.
modems into the hot-copper telephone lines
The limits were not set by the copper, but by the SSB carrier systems
used on the trunks between exchanges (central offices). Their bandwidth
was set to the minimum needed for the general public to consider the
quality acceptable for speech.
of 52kbaud became a reality.
What actually happened is that it became possible to manufacture
digital signal processors cheaply enough to use them in telephone
modems, and that made it possible to use echo cancellation and advanced
equalisation algorithms. In addition, it required that the transmission
path from the local office to the service provider be all digital. The
modems now attempt to select every possible quantisation level in every
sample on the digital bearer - this relies on the remote end not having
an analogue connection (they are only acting as a modem over the local
loop in the downlink direction)! The actual 56kbps limit is set by the
characteristics of the PCM network, including US robbed bit signalling
and the need to avoid putting too much power into the equivalent
analogue signal, thus not achieving the absolute limit of 64kbps. The
baud rate is actually 8k, but one gets away with that because the local
loop analogue connections are not bandwidth limited.
Actually, because PCM phone connections are companded, some of the steps
between quantisation levels are much smaller than others, so 56k modems
don't actually take full advantage of the signal to
noise ratio. Assuming the telephone SNR and that equalisation doesn't
impose too much of a problem, an analogue radio channel should be able
to achieve a lot more than 56kbps in 2.7kHz.
Also, a considerable time before this sort of modem became possible
(and, I think, significantly before the enabling technology of PCM
bearers became common) it became possible to transmit communications
quality speech over a 2400 bps connection. That suggests that the true
technical advance would be the transmission of amateur radio speech in
about a thirtieth of the 2.7kHz, SSB, bandwidth, assuming landline
telephone signal to noise ratios.
By comparison, there is really no new technology at all in using laxer
SSB filters.
rethink its SSB modulation approach, to allow wider
more naturally sounding voice comms, and still
not take up unnecessary bandwidth, beyone which
supposedly 2.7khz now consumes. (when do we
It's the essence of SSB that it takes up the same bandwidth as the
baseband signal! To get more natural speech in less bandwidth, you have
to basically treat the channel as being a digital one, and send a signal
that takes advantage of the actual nature of speech signals (generally
these use some sort of voice tract model).
--
David Woolley
Emails are not formal business letters, whatever businesses may want.
RFC1855 says there should be an address here, but, in a world of spam,
that is no longer good advice, as archive address hiding may not work.
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