KindOfBlue skrev:
Bare et innspill, i forhold til bruken av ordet signal. Det handler om en puls - enten er den av, eller så er den på.
Man forenkler digital signaltransmisjon til det banale ved kun å se på tilstandene: 0 og 1. (Volt på, Volt av). Hva om pulsene ikke var 4-kanter, men hadde ulike stige og fall tider som skyldes påvirkninger fra klokking, re-klokking og transmisjon. Man beskriver digitalteknikk her inne som om det er i en ideel verden uten påvirkninger.
Bare for å få en ide over noen av faktorene som kan påvirke digital signaltrasmisjon kan vi se hva Peter Daniel ytrer seg om ulike lengder på digitalkabel mellom DAC og drev. Lang digitalkabel er bedre enn kort.....
I later found the info about cable length and reflections, and this seems to be more critical than termination itself:
When a transition is launched into the transmission line, it takes a period of time
to propagate or transit to the other end. This propagation time is somewhat slower
than the speed of light, usually around 2 nanoseconds per foot, but can be longer
When the transition reaches the end of the transmission line (in the DAC), a reflection
can occur that propagates back to the driver in the transport. Small reflections can occur
in even well matched systems. When the reflection reaches the driver, it can again
be reflected back towards the DAC. This ping-pong effect can sustain itself for several
bounces depending on the losses in the cable. It is not unusual to see 3 to 5 of these reflections
before they finally decay away. So, how does this affect the jitter? When
the first reflection comes back to the DAC, if the transition already in process at the
receiver has not completed, the reflection voltage will superimpose itself on the transition
voltage, causing the transition to shift in time. The DAC will sample the transition
in this time-shifted state and there you have jitter.
If the rise-time is 25 nanoseconds and the cable length is 3 feet, then the propagation
time is about 6 nanoseconds. Once the transition has arrived at the receiver, the reflection
propagates back to the driver (6 nanoseconds) and then the driver reflects this back to the
receiver (6 nanoseconds) = 12 nanoseconds). So, as seen at the receiver, 12 nanoseconds
after the 25 nanosecond transition started, we have a reflection superimposing on the
transition. This is right about the time that the receiver will try to sample the transition,
right around 0 volts DC. Not good. Now if the cable had been 1.5 metres, the reflection
would have arrived 18 nanoseconds after the 25 nanosecond transition started at
the receiver. This is much better because the receiver has likely already sampled the
transition by this time."