24 Jan
2014
24 Jan
'14
1:33 p.m.
On Jan 24, 2014, at 1:14 PM, Al Kossow <aek at bitsavers.org> wrote:
On 1/24/14 9:36 AM, John Wilson wrote:
Manchester encoding is an excellent example of a modulation scheme that provides bit clock
as part of the demodulation process. Given the required bandwidth, it?s a good LAN (or
tape, e.g., DECtape) scheme, not so much for long distance connections.
Some other simple modulation schemes are not clocked; FSK is a good example (110 baud, 300
baud, and Bell 202 modems). But many others, and in particular any of the faster phone
modem schemes (1200 bps and up) use modulation where the demodulator recovers the bit
clock as part of (or in order to do) the demodulation. PSK is an example, and certainly
anything harder like QAM inherently depends on having a good receiver clock.
paul
Almost definitely showing my own ignorance
(I'm very new to serial comms),
but my impression is that in the old days, synchronous ports *always* used
a modem-supplied (etc.) external clock signal, and it's only newer fancy-pants
ports like the Zilog Z85(2)30 that try to be cute about using a PLL to derive
a clock from transitions in the bit stream.
Manchester-encoded transmission schemes have embedded clocking. These go back well
before the days of the SCC. The on-wire encoding was normally hidden by whatever
modem was handling what was on the wire, so all you would see would be clock and
data. Think of the distinction today between the PHY and MAC in an Ethernet interface.