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NAME
radio_xmit - radio broadcast utility
SYNOPSIS
radio_xmit [ -C ] [OPTION=value ] ...
DISCLAIMER
This program is furnished on an AS IS basis as a demonstra-
tion of audio applications programming.
DESCRIPTION
radio_xmit is the Radio Free Ethernet (RFE) broadcast util-
ity. (For an overview of Radio Free Ethernet, refer to the
radio(6) manual page.) Though radio_xmit is normally ini-
tiated directly by the window-based xmit(6) program, it may
also be used directly as a command-line program, or in con-
junction with a user-written window-based tool.
In Normal Mode (the default case), radio_xmit is initiated
with a set of broadcast options specified on the command
line. The program immediately begins transmitting audio
data over the network according to the currently selected
options. The program will terminate on receipt of a SIGINT
(<CTRL-C>). If an input file is specified, the program will
exit when the entire file has been broadcast.
If -C is specified on the command line, radio_xmit operates
in Command Mode. In this mode, the program reads and
processes command options from stdin and writes error and
status information to stderr. Window-based tools use Com-
mand Mode to control the broadcast program and receive
status information for display. When operating in Command
Mode, radio_xmit requires that a station name and input file
be specified explicitly, and the START command must be
issued to initiate broadcasting.
OPTIONS
The broadcast options are described below. If an option
requires a value, the option name must be followed by an
equal sign (=) and the value. If an option takes an ON/OFF
value, the default is ON if the value is missing or poorly
constructed.
Station=[call letters]
This option specifies the station call letters of your
radio station. A maximum of four characters may be
specified. In Normal Mode, the station name defaults
to the first four letters of the current workstation
hostname. However, it is preferable to supply a Sta-
tion name on the command line.
Input=[audio file or device name]
This option specifies the audio input source for radio
broadcast. If a file is specified, it must be a legi-
timate audio file (i.e., with a standard Sun audio
file header), sampled at 8 kHz. In Normal Mode, the
audio input defaults to /dev/audio.
Squelch=[ON/OFF]
This option controls the behavior of the transmitter
when the audio input data is determined to be silent.
When Squelch is enabled, silent data is not transmit-
ted over the network. This reduces the network traffic
for stations that are broadcasting with no audio
input. Note that the Station Identification packet
continues to be transmitted every 5 seconds, even if
the data broadcast is squelched.
When Squelch is disabled, all audio input data is
always transmitted. Squelching only occurs for audio
device input; audio files are not squelched, even if
they contain silence. The Squelch option is enabled by
default.
Format=[COMPRESSED/UNCOMPRESSED]
This option controls the format for audio data that is
broadcast. The uncompressed format broadcasts 8000
bytes of audio data each second. The compressed format
sends only 4000 bytes per second, but requires more
computation on both the transmitter and receiver.
Data format conversions only occur for audio device
input; audio files are transmitted in the format in
which they were stored. By default, audio data is
broadcast uncompressed.
Agc=[ON/OFF]
This option controls the behavior of an Automatic Gain
Control feature. When Agc is enabled, the audio
recording volume is adjusted automatically to keep the
input signal within reasonable bounds. If the input
signal is too loud, the record volume is decreased.
If the input signal is determined to be present but
too soft, the record volume is increased.
Automatic Gain Control adjustments only occur for
audio device input; audio files are broadcasted at the
level at which they were recorded. The Agc option is
enabled by default.
Autostop=[ON/OFF]
This option controls the behavior of the transmitter
when the audio input data has been silent for a length
of time. When Autostop is enabled, the transmitter
will sign off the station and stop broadcasting if the
audio input has been silent for 60 seconds. When
Autostop is disabled, the station continues to remain
active. This option does not interact with the
Squelch option, although the same criteria are used
for determining whether or not the audio input is
silent.
Autostop is only processed for audio device input;
broadcasting always stops when an end-of-file has been
reached on audio files. The Autostop option is dis-
abled by default.
Address=[hostname|address]
This option is used to configure the IP Multicast base
address that the radio_xmit program uses for broad-
casting data. The value may either be a hostname
(which is translated into an IP Multicast address by
looking up the name in the NIS hosts map) or a
specific numeric IP Multicast address.
Normally, the address should end in 0 or 255, in which
case an id-address and data-address are constructed as
follows: the id-address consists of the address with
the last byte set to 255; the data-address consists of
the base address with a randomly-selected last byte
(0-254). Station identification packets are broadcast
to the id-address, and normal audio data packets are
broadcast to the data-address. This convention allows
RFE receivers to register interest only in the id-
address, so that audio data packets need not be repli-
cated over network gateways until a receiver actually
tunes in to the station.
If the IP Multicast address ends in a byte that is in
the range 1-254, then the address remains unmodified,
and both station and identification packets are broad-
cast to that address. This convention allows a
specific multicast address to be used for multi-party
audio conferences.
The default IP Multicast address is designated by the
hostname RadioFreeEthernet. The special hostname
BROADCAST may be used to force the program to transmit
UDP Broadcast packets (which will not be relayed over
a network gateway). The radio(6) manual page contains
more information on the RFE network implementation.
Range=[hopcount]
This option is used to configure the number of gate-
ways over which a radio broadcast may pass. If the
value is set to one, the broadcast will be restricted
to the current subnet. The default Range is 8.
Service=[service|port number]
This option is used to configure the IP protocol port
number used to identify Radio Free Ethernet network
packets. The value may be either a service name
(which is translated into a port number by looking up
the name in the NIS services map) or a specific
numeric port number.
The default port number is designated by the service
name radio. The radio(6) manual page contains more
information on configuring the port number.
Report=[ON/OFF]
This option enables the reporting, to stderr, of all
changes in the transmitter status. It is normally used
only in conjunction with Command Mode to allow the
controlling program to maintain state.
Buffer=[input buffer size in bytes]
In normal operation, the RFE tools are not concerned
with the delay between the audio input at the
transmitter and the output of audio data at the
receiving end. Following the model of radio broad-
casting, the delay is not important as long as the
transmitted data arrives in sequence and on time. For
this reason, and to ensure a consistent, uninterrupted
flow of audio data, the radio_xmit program normally
uses a buffer size of around 8000 bytes to collect
audio input data before broadcasting. This results in
an end-to-end delay of approximately one second (or
longer if the receiver's audio buffer backs up).
For use with real-time audio conferencing, shorter
delays may be desired. For experimental purposes, the
Buffer command is provided to alter the input delay.
The buffer size is specified in bytes. For real-time
experimentation, a reasonable value is 256
(corresponding to approximately 1/32 of a second).
Notice that a side-effect of lowering the input buffer
size is to lower the transmitted packet size, result-
ing in an increased number of broadcast packets.
Start This command is used to initiate broadcasting accord-
ing to the current parameter settings. It is provided
for Command Mode; the transmitter is enabled by
default in Normal Mode.
Stop This command is used to sign off the current station
and cease broadcasting. It is provided for Command
Mode operation.
Quit This command causes the radio_xmit program to exit. It
is provided for Command Mode operation.
EXAMPLES
Example 1: Broadcasting over a network
Let's say that you have a radio at work that you always keep
tuned to your favorite station (though you might shut off
your speakers when you don't want to listen to it). If you
connect its output to your workstation audio input, you can
broadcast the station over the network by using the follow-
ing command (the Autostop option will cause the program to
sign-off and exit if you turn your radio off for a minute or
longer):
example% radio_xmit Station=KPFA Input=/dev/audio Autostop=on
Example 2: Rescheduling a broadcast
The following shell script may be scheduled to run every day
at 6:00 AM to record a half-hour of news and rebroadcast it
at a more sensible time:
#!/bin/sh
# Record from /dev/audio for 30 minutes (around 14 Mbytes).
file=/bigdisk/sound/news
audiorecord -t 30:00 $file
#
# If this runs at 6am, the news is over at 6:30. Wait 3 hours til 9:30.
sleep 10800
#
# Now broadcast the news on the network and remove the file when finished.
radio_xmit Station=KPFA Input=$file
rm $file
SEE ALSO
audiorecord(1), radio(6), radio_recv(6), xmit(6)
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