Homework 1 is now on Sakai. It is due Friday, July 17.
Suppose you want to rehearse music with someone at a
remote location. Here, 100 ms of one-way delay is generally a significant
problem. Sound moves through air at about 1 foot/ms, so that would be like
playing in sync while spaced 100 feet apart. Large orchestras do sometimes
have issues with this, and professional musicians do sometimes play varying
degrees behind the conductor's beat. The audience in the back of the theater
might see the conductor appear to be ahead of the musicians by a
quarter-second (250 feet).
If the tempo is ♩ = 150, then that quarter-note takes 400 ms, and a sixteenth-note
takes 100 ms.
Classic circuit-switching (including digital trunk lines) can achieve
store-and-forward delays of well under 10 ms, though there are still the
propagation delays. Across 5000 km, the one-way propagation delay in wire is
25 ms (200 km/ms).
Packet switching has a very hard time keeping delay under 100 ms, though
sometimes one gets lucky. A more serious problem is the very large jitter
buffer of some VoIP-like systems. For music, one would have to keep the
jitter buffer at a minimum, in effect trading a larger loss rate for a
minimum of latency.
All VoIP solutions contend with packet
switched networks that were not designed to transmit real-time media
streams. Packet latency and packet loss are the principal manifestations
of this reality, and the jitter
buffer is their canonical solution.
They then describe an adaptive-jitter-buffer algorithm, where the buffer can
change size. That's a bit of a trick, as that means slowing down or speeding
up the audio. In order to do this at all, one must know that
sometimes voice can be stretched (eg by pausing), or shortened. For voice,
the resultant distortion is generally acceptable; for music, it is likely
not.
Notes:
Go over homework
Räisänen recap
clock resynchronization
Shannon-Hartley limit revisited
modulation_transmission.html
Digital data / digital signal: B8ZS, 4b/5b: done
Analog data/ digital signal: PCM (done), µ-law (still to
do)
Analog data/ analog signal: SSB, FM v PM
Digital data / analog signal: BFSK v MFSK, PSK, QAM, ...
OFDM