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The software for the C64 real-time streaming audio server runs on
an unexpanded 1 MHz 6510 CPU with 64k RAM. The software is based
around an extended version of the uIP TCP/IP stack that
previously has been used for another C64
webserver, TCP/IP enabled
Lego Mindstorms as well as various other projects. The software is
freely avaliable for download and use.
Web server
The web server is a simple HTTP server running on top of uIP, but with
a small twist that protects from memory overload. The web server
includes a simple virtual file system which stores the web pages in
RAM. We have the web server running at TCP port 6510 and have devoted
ports 80, 81, 82, 83, and 84 to the overload protection optimized web
server described below.
Overload protection
Because of the limitations in processing speed and memory resources,
the regular web server application is not able to handle heavy
load. Each web client uses a small amount of memory, and each TCP
packet sent by the web clients has to be processed and parsed. With
many simultaneous web clients browsing the server, memory and CPU
cycles run out quickly. When the memory is exhausted any attempts to
connect to the server are discarded, thus making it hard to reach the
web pages on the server.
In order to prevent service degradation under heavy load, we have
employed a TCP optimization for TCP ports 80-84, while we keep port
6510 open for the normal web server application. The optimization
encodes connection state information in the TCP sequence numbers and
keeps no connection state in the server, thus requiring no
per-connection memory. This only works in cases where the web pages
are small enough to fit into one TCP segment, and it is used only for
the frame-set, the menu frame (including the pictures), and the main
page. All other pages are server by the regular web server application
running on top of uIP.
RTSP/RTP server
The RTSP (Real-Time Streaming Protocol) is a standard (RFC2326) session
initiation/maintenance protocol that is used by RealPlayer, QuickTime,
and many
other real-time audio and video players. RTP (Real-Time Protocol)
is a standard (RFC1889) protocol used
for transmission of real-time data such as audio and video. RTSP uses
TCP as its primary transport layer, and RTP runs over UDP.
The RTSP server
The RTSP server is extremely simple and basically follows the steps
outlined in Appendix D.2 in RFC2326. The server only supports the
OPTIONS, DESCRIBE, SETUP, PLAY, and TEARDOWN commands. The
implementation has not been extensively tested or verified and there
may be incompatibilities with RTSP clients other than RealPlayer 8
Basic.
The RTP sender
RTP packets are sent in UDP datagrams which has the checksum turned
off. While the 1 MHz 6510 CPU probably could support as many as four
or five simultaneous listeners, there would not be many cycles left
for the web server. Therefore, we only allow one listener at a time.
The audio sampler
A timer IRQ fires 2000 times every second and samples the input pin of
the casette interface. The 1-bit value is written into two buffers
that are used circularly. Once a buffer is full, an UDP/RTP header is
prepended and the packet is sent out. The IP header for each listener
is preconstructed and sent directly to the Ethernet interface without
modifications.
Download
The full source code for the C64 real-time streaming audio server can
be downloaded here. The
code is written in C with the most CPU intensive parts such as the
CS8900a device driver and the sampling IRQ written in 6502 assembler.
$Date: 2004/05/01 20:37:51 $
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